Attachment 347982 Details for Bug 186416 – Patch

[patch] Patch

bug-186416-20180823180632.patch (text/plain), 343.56 KB, created by Simon Fraser (smfr) on 2018-08-23 18:06:32 PDT

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Creator: Simon Fraser (smfr)

Created: 2018-08-23 18:06:32 PDT

Size: 343.56 KB

patch

obsolete

>Subversion Revision: 235242
>diff --git a/Source/JavaScriptCore/ChangeLog b/Source/JavaScriptCore/ChangeLog
>index a53407616ebf05a2fa792bb0bf22a9221d526e74..1b64432979a32d850a78c490842b6d1c6499f422 100644
>--- a/Source/JavaScriptCore/ChangeLog
>+++ b/Source/JavaScriptCore/ChangeLog
>@@ -1,3 +1,77 @@
>+2018-08-23  Simon Fraser  <simon.fraser@apple.com>
>+
>+        Add support for dumping GC heap snapshots, and a viewer
>+        https://bugs.webkit.org/show_bug.cgi?id=186416
>+
>+        Reviewed by Joseph Pecoraro.
>+
>+        Make a way to dump information about the GC heap that is useful for looking for leaked
>+        or abandoned objects. This dump is obtained (on Apple platforms) via:
>+            notifyutil -p com.apple.WebKit.dumpGCHeap
>+        which writes a JSON file to /tmp which can then be loaded into the viewer in Tools/GCHeapInspector.
>+        
>+        This leverages the heap snapshot used by Web Inspector, adding an alternate format for
>+        the snapshot JSON that adds additional data about objects and why they are GC roots.
>+
>+        SlotVisitor maintains a RootMarkReason (via SetRootMarkReasonScope) that allows
>+        the HeapSnapshotBuilder to keep track of why a JSCell was treated as a GC root. For
>+        objects visited via opaque roots, we record the reason why via a new out param to
>+        isReachableFromOpaqueRoots().
>+
>+        HeapSnapshotBuilder is enhanced to produce GCDebuggingSnapshot JSON output. This contains
>+        additional information including the address of the JSCell* and the wrapped object (for
>+        JSDOMWrappers), the root reasons, and for some objects like JSDocument a label which can
>+        be the document URL.
>+
>+        GCDebuggingSnapshots are always full snapshots (previous snapshots are not kept around).
>+
>+        * API/JSAPIWrapperObject.mm:
>+        (JSAPIWrapperObjectHandleOwner::isReachableFromOpaqueRoots):
>+        * API/JSManagedValue.mm:
>+        (JSManagedValueHandleOwner::isReachableFromOpaqueRoots):
>+        * API/glib/JSAPIWrapperObjectGLib.cpp:
>+        (JSAPIWrapperObjectHandleOwner::isReachableFromOpaqueRoots):
>+        * CMakeLists.txt:
>+        * heap/ConservativeRoots.h:
>+        (JSC::ConservativeRoots::size const):
>+        (JSC::ConservativeRoots::size): Deleted.
>+        * heap/Heap.cpp:
>+        (JSC::Heap::addCoreConstraints):
>+        * heap/HeapSnapshotBuilder.cpp:
>+        (JSC::HeapSnapshotBuilder::getNextObjectIdentifier):
>+        (JSC::HeapSnapshotBuilder::HeapSnapshotBuilder):
>+        (JSC::HeapSnapshotBuilder::~HeapSnapshotBuilder):
>+        (JSC::HeapSnapshotBuilder::buildSnapshot):
>+        (JSC::HeapSnapshotBuilder::appendNode):
>+        (JSC::HeapSnapshotBuilder::appendEdge):
>+        (JSC::HeapSnapshotBuilder::setOpaqueRootReachabilityReasonForCell):
>+        (JSC::HeapSnapshotBuilder::setWrappedObjectForCell):
>+        (JSC::HeapSnapshotBuilder::previousSnapshotHasNodeForCell):
>+        (JSC::snapshotTypeToString):
>+        (JSC::rootTypeToString):
>+        (JSC::HeapSnapshotBuilder::setLabelForCell):
>+        (JSC::HeapSnapshotBuilder::descriptionForCell const):
>+        (JSC::HeapSnapshotBuilder::json):
>+        (JSC::HeapSnapshotBuilder::hasExistingNodeForCell): Deleted.
>+        * heap/HeapSnapshotBuilder.h:
>+        * heap/SlotVisitor.cpp:
>+        (JSC::SlotVisitor::appendSlow):
>+        * heap/SlotVisitor.h:
>+        (JSC::SlotVisitor::heapSnapshotBuilder const):
>+        (JSC::SlotVisitor::rootMarkReason const):
>+        (JSC::SlotVisitor::setRootMarkReason):
>+        (JSC::SetRootMarkReasonScope::SetRootMarkReasonScope):
>+        (JSC::SetRootMarkReasonScope::~SetRootMarkReasonScope):
>+        * heap/WeakBlock.cpp:
>+        (JSC::WeakBlock::specializedVisit):
>+        * heap/WeakHandleOwner.cpp:
>+        (JSC::WeakHandleOwner::isReachableFromOpaqueRoots):
>+        * heap/WeakHandleOwner.h:
>+        * runtime/SimpleTypedArrayController.cpp:
>+        (JSC::SimpleTypedArrayController::JSArrayBufferOwner::isReachableFromOpaqueRoots):
>+        * runtime/SimpleTypedArrayController.h:
>+        * tools/JSDollarVM.cpp:
>+
> 2018-08-23  Michael Saboff  <msaboff@apple.com>
> 
>         YARR: Need to JIT compile a RegExp before using containsNestedSubpatterns flag
>diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
>index d300016200038a04943585e6946c1e04e75f85e3..4619a9b204707eb7a1a84d0608c64e1769be37ad 100644
>--- a/Source/WebCore/ChangeLog
>+++ b/Source/WebCore/ChangeLog
>@@ -1,3 +1,314 @@
>+2018-08-23  Simon Fraser  <simon.fraser@apple.com>
>+
>+        Add support for dumping GC heap snapshots, and a viewer
>+        https://bugs.webkit.org/show_bug.cgi?id=186416
>+
>+        Reviewed by Joseph Pecoraro.
>+
>+        Make a way to dump information about the GC heap that is useful for looking for leaked
>+        or abandoned objects. This dump is obtained (on Apple platforms) via:
>+            notifyutil -p com.apple.WebKit.dumpGCHeap
>+        which writes a JSON file to /tmp which can then be loaded into the viewer in Tools/GCHeapInspector.
>+
>+        This leverages the heap snapshot used by Web Inspector, adding an alternate format for
>+        the snapshot JSON that adds additional data about objects and why they are GC roots.
>+        
>+        The generated bindings code is changed to include the output root reason from isReachableFromOpaqueRoots(),
>+        and to implement heapSnapshot() which provides the address of the wrapped object. A new IDL attribute,
>+        CustomHeapSnapshot, is used to allow custom heapSnapshot() implementations for classes like JSDocument
>+        that need to decorate the heap snapshot cell data with things like the document URL.
>+
>+        GCController registers a notifyutil callback which gathers the debug heap snapshot, and dumps it
>+        to a file in /tmp. The file path is printed out to the system log.
>+
>+        * bindings/js/DOMGCOutputConstraint.cpp:
>+        (WebCore::DOMGCOutputConstraint::executeImpl):
>+        * bindings/js/GCController.cpp:
>+        (WebCore::GCController::GCController):
>+        (WebCore::GCController::dumpHeap):
>+        * bindings/js/GCController.h:
>+        * bindings/js/JSCSSRuleListCustom.cpp:
>+        (WebCore::JSCSSRuleListOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSCallbackData.cpp:
>+        (WebCore::JSCallbackDataWeak::WeakOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSCallbackData.h:
>+        * bindings/js/JSCanvasRenderingContext2DCustom.cpp:
>+        (WebCore::JSCanvasRenderingContext2DOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSDOMWindowCustom.cpp:
>+        (WebCore::JSDOMWindow::getOwnPropertySlot):
>+        (WebCore::JSDOMWindow::heapSnapshot):
>+        * bindings/js/JSDeprecatedCSSOMValueCustom.cpp:
>+        (WebCore::JSDeprecatedCSSOMValueOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSDocumentCustom.cpp:
>+        (WebCore::JSDocument::heapSnapshot):
>+        * bindings/js/JSMicrotaskCallback.h:
>+        (WebCore::JSMicrotaskCallback::call):
>+        * bindings/js/JSMutationObserverCustom.cpp:
>+        (WebCore::JSMutationObserverOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSNavigatorCustom.cpp:
>+        (WebCore::JSNavigator::visitAdditionalChildren):
>+        * bindings/js/JSNodeCustom.cpp:
>+        (WebCore::isReachableFromDOM):
>+        (WebCore::JSNodeOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSNodeListCustom.cpp:
>+        (WebCore::JSNodeListOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSOffscreenCanvasRenderingContext2DCustom.cpp:
>+        (WebCore::JSOffscreenCanvasRenderingContext2DOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSPerformanceObserverCustom.cpp:
>+        (WebCore::JSPerformanceObserverOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/JSPopStateEventCustom.cpp:
>+        * bindings/js/JSTextTrackCueCustom.cpp:
>+        (WebCore::JSTextTrackCueOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/WebCoreTypedArrayController.cpp:
>+        (WebCore::WebCoreTypedArrayController::JSArrayBufferOwner::isReachableFromOpaqueRoots):
>+        * bindings/js/WebCoreTypedArrayController.h:
>+        * bindings/scripts/CodeGeneratorJS.pm:
>+        (GenerateHeader):
>+        (GenerateImplementation):
>+        * bindings/scripts/IDLAttributes.json:
>+        * bindings/scripts/test/JS/JSInterfaceName.cpp:
>+        (WebCore::JSInterfaceName::heapSnapshot):
>+        (WebCore::JSInterfaceNameOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSInterfaceName.h:
>+        * bindings/scripts/test/JS/JSMapLike.cpp:
>+        (WebCore::JSMapLike::heapSnapshot):
>+        (WebCore::JSMapLikeOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSMapLike.h:
>+        * bindings/scripts/test/JS/JSReadOnlyMapLike.cpp:
>+        (WebCore::JSReadOnlyMapLike::heapSnapshot):
>+        (WebCore::JSReadOnlyMapLikeOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSReadOnlyMapLike.h:
>+        * bindings/scripts/test/JS/JSTestActiveDOMObject.cpp:
>+        (WebCore::JSTestActiveDOMObject::heapSnapshot):
>+        (WebCore::JSTestActiveDOMObjectOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestActiveDOMObject.h:
>+        * bindings/scripts/test/JS/JSTestCEReactions.cpp:
>+        (WebCore::JSTestCEReactions::heapSnapshot):
>+        (WebCore::JSTestCEReactionsOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestCEReactions.h:
>+        * bindings/scripts/test/JS/JSTestCEReactionsStringifier.cpp:
>+        (WebCore::JSTestCEReactionsStringifier::heapSnapshot):
>+        (WebCore::JSTestCEReactionsStringifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestCEReactionsStringifier.h:
>+        * bindings/scripts/test/JS/JSTestCallTracer.cpp:
>+        (WebCore::JSTestCallTracer::heapSnapshot):
>+        (WebCore::JSTestCallTracerOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestCallTracer.h:
>+        * bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.cpp:
>+        (WebCore::JSTestClassWithJSBuiltinConstructor::heapSnapshot):
>+        (WebCore::JSTestClassWithJSBuiltinConstructorOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.h:
>+        * bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.cpp:
>+        (WebCore::JSTestCustomConstructorWithNoInterfaceObject::heapSnapshot):
>+        (WebCore::JSTestCustomConstructorWithNoInterfaceObjectOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.h:
>+        * bindings/scripts/test/JS/JSTestDOMJIT.cpp:
>+        (WebCore::JSTestDOMJIT::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestDOMJIT.h:
>+        * bindings/scripts/test/JS/JSTestEnabledBySetting.cpp:
>+        (WebCore::JSTestEnabledBySetting::heapSnapshot):
>+        (WebCore::JSTestEnabledBySettingOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestEnabledBySetting.h:
>+        * bindings/scripts/test/JS/JSTestEventConstructor.cpp:
>+        (WebCore::JSTestEventConstructor::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestEventConstructor.h:
>+        * bindings/scripts/test/JS/JSTestEventTarget.cpp:
>+        (WebCore::JSTestEventTarget::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestEventTarget.h:
>+        * bindings/scripts/test/JS/JSTestException.cpp:
>+        (WebCore::JSTestException::heapSnapshot):
>+        (WebCore::JSTestExceptionOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestException.h:
>+        * bindings/scripts/test/JS/JSTestGenerateIsReachable.cpp:
>+        (WebCore::JSTestGenerateIsReachable::heapSnapshot):
>+        (WebCore::JSTestGenerateIsReachableOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestGenerateIsReachable.h:
>+        * bindings/scripts/test/JS/JSTestGlobalObject.cpp:
>+        (WebCore::JSTestGlobalObject::heapSnapshot):
>+        (WebCore::JSTestGlobalObjectOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestGlobalObject.h:
>+        * bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.cpp:
>+        (WebCore::JSTestIndexedSetterNoIdentifier::heapSnapshot):
>+        (WebCore::JSTestIndexedSetterNoIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.cpp:
>+        (WebCore::JSTestIndexedSetterThrowingException::heapSnapshot):
>+        (WebCore::JSTestIndexedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.h:
>+        * bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.cpp:
>+        (WebCore::JSTestIndexedSetterWithIdentifier::heapSnapshot):
>+        (WebCore::JSTestIndexedSetterWithIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestInterface.cpp:
>+        (WebCore::JSTestInterface::heapSnapshot):
>+        (WebCore::JSTestInterfaceOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestInterface.h:
>+        * bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.cpp:
>+        (WebCore::JSTestInterfaceLeadingUnderscore::heapSnapshot):
>+        (WebCore::JSTestInterfaceLeadingUnderscoreOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.h:
>+        * bindings/scripts/test/JS/JSTestIterable.cpp:
>+        (WebCore::JSTestIterable::heapSnapshot):
>+        (WebCore::JSTestIterableOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestIterable.h:
>+        * bindings/scripts/test/JS/JSTestMediaQueryListListener.cpp:
>+        (WebCore::JSTestMediaQueryListListener::heapSnapshot):
>+        (WebCore::JSTestMediaQueryListListenerOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestMediaQueryListListener.h:
>+        * bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.cpp:
>+        (WebCore::JSTestNamedAndIndexedSetterNoIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedAndIndexedSetterNoIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.cpp:
>+        (WebCore::JSTestNamedAndIndexedSetterThrowingException::heapSnapshot):
>+        (WebCore::JSTestNamedAndIndexedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.h:
>+        * bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.cpp:
>+        (WebCore::JSTestNamedAndIndexedSetterWithIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedAndIndexedSetterWithIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedConstructor.cpp:
>+        (WebCore::JSTestNamedConstructor::heapSnapshot):
>+        (WebCore::JSTestNamedConstructorOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedConstructor.h:
>+        * bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.cpp:
>+        (WebCore::JSTestNamedDeleterNoIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedDeleterNoIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.cpp:
>+        (WebCore::JSTestNamedDeleterThrowingException::heapSnapshot):
>+        (WebCore::JSTestNamedDeleterThrowingExceptionOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.h:
>+        * bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.cpp:
>+        (WebCore::JSTestNamedDeleterWithIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedDeleterWithIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.cpp:
>+        (WebCore::JSTestNamedDeleterWithIndexedGetter::heapSnapshot):
>+        (WebCore::JSTestNamedDeleterWithIndexedGetterOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.h:
>+        * bindings/scripts/test/JS/JSTestNamedGetterCallWith.cpp:
>+        (WebCore::JSTestNamedGetterCallWith::heapSnapshot):
>+        (WebCore::JSTestNamedGetterCallWithOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedGetterCallWith.h:
>+        * bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.cpp:
>+        (WebCore::JSTestNamedGetterNoIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedGetterNoIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.cpp:
>+        (WebCore::JSTestNamedGetterWithIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedGetterWithIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.cpp:
>+        (WebCore::JSTestNamedSetterNoIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedSetterNoIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterThrowingException.cpp:
>+        (WebCore::JSTestNamedSetterThrowingException::heapSnapshot):
>+        (WebCore::JSTestNamedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterThrowingException.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.cpp:
>+        (WebCore::JSTestNamedSetterWithIdentifier::heapSnapshot):
>+        (WebCore::JSTestNamedSetterWithIdentifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.cpp:
>+        (WebCore::JSTestNamedSetterWithIndexedGetter::heapSnapshot):
>+        (WebCore::JSTestNamedSetterWithIndexedGetterOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.cpp:
>+        (WebCore::JSTestNamedSetterWithIndexedGetterAndSetter::heapSnapshot):
>+        (WebCore::JSTestNamedSetterWithIndexedGetterAndSetterOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.cpp:
>+        (WebCore::JSTestNamedSetterWithOverrideBuiltins::heapSnapshot):
>+        (WebCore::JSTestNamedSetterWithOverrideBuiltinsOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.cpp:
>+        (WebCore::JSTestNamedSetterWithUnforgableProperties::heapSnapshot):
>+        (WebCore::JSTestNamedSetterWithUnforgablePropertiesOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.h:
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.cpp:
>+        (WebCore::JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins::heapSnapshot):
>+        (WebCore::JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltinsOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.h:
>+        * bindings/scripts/test/JS/JSTestNode.cpp:
>+        (WebCore::JSTestNode::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestNode.h:
>+        * bindings/scripts/test/JS/JSTestObj.cpp:
>+        (WebCore::JSTestObj::heapSnapshot):
>+        (WebCore::JSTestObjOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestObj.h:
>+        * bindings/scripts/test/JS/JSTestOverloadedConstructors.cpp:
>+        (WebCore::JSTestOverloadedConstructors::heapSnapshot):
>+        (WebCore::JSTestOverloadedConstructorsOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestOverloadedConstructors.h:
>+        * bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.cpp:
>+        (WebCore::JSTestOverloadedConstructorsWithSequence::heapSnapshot):
>+        (WebCore::JSTestOverloadedConstructorsWithSequenceOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.h:
>+        * bindings/scripts/test/JS/JSTestOverrideBuiltins.cpp:
>+        (WebCore::JSTestOverrideBuiltins::heapSnapshot):
>+        (WebCore::JSTestOverrideBuiltinsOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestOverrideBuiltins.h:
>+        * bindings/scripts/test/JS/JSTestPluginInterface.cpp:
>+        (WebCore::JSTestPluginInterface::heapSnapshot):
>+        (WebCore::JSTestPluginInterfaceOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestPluginInterface.h:
>+        * bindings/scripts/test/JS/JSTestPromiseRejectionEvent.cpp:
>+        (WebCore::JSTestPromiseRejectionEvent::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestPromiseRejectionEvent.h:
>+        * bindings/scripts/test/JS/JSTestSerialization.cpp:
>+        (WebCore::JSTestSerialization::heapSnapshot):
>+        (WebCore::JSTestSerializationOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestSerialization.h:
>+        * bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.cpp:
>+        (WebCore::JSTestSerializationIndirectInheritance::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.h:
>+        * bindings/scripts/test/JS/JSTestSerializationInherit.cpp:
>+        (WebCore::JSTestSerializationInherit::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestSerializationInherit.h:
>+        * bindings/scripts/test/JS/JSTestSerializationInheritFinal.cpp:
>+        (WebCore::JSTestSerializationInheritFinal::heapSnapshot):
>+        * bindings/scripts/test/JS/JSTestSerializationInheritFinal.h:
>+        * bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.cpp:
>+        (WebCore::JSTestSerializedScriptValueInterface::heapSnapshot):
>+        (WebCore::JSTestSerializedScriptValueInterfaceOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.h:
>+        * bindings/scripts/test/JS/JSTestStringifier.cpp:
>+        (WebCore::JSTestStringifier::heapSnapshot):
>+        (WebCore::JSTestStringifierOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestStringifier.h:
>+        * bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.cpp:
>+        (WebCore::JSTestStringifierAnonymousOperation::heapSnapshot):
>+        (WebCore::JSTestStringifierAnonymousOperationOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.h:
>+        * bindings/scripts/test/JS/JSTestStringifierNamedOperation.cpp:
>+        (WebCore::JSTestStringifierNamedOperation::heapSnapshot):
>+        (WebCore::JSTestStringifierNamedOperationOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestStringifierNamedOperation.h:
>+        * bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.cpp:
>+        (WebCore::JSTestStringifierOperationImplementedAs::heapSnapshot):
>+        (WebCore::JSTestStringifierOperationImplementedAsOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.h:
>+        * bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.cpp:
>+        (WebCore::JSTestStringifierOperationNamedToString::heapSnapshot):
>+        (WebCore::JSTestStringifierOperationNamedToStringOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.h:
>+        * bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.cpp:
>+        (WebCore::JSTestStringifierReadOnlyAttribute::heapSnapshot):
>+        (WebCore::JSTestStringifierReadOnlyAttributeOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.h:
>+        * bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.cpp:
>+        (WebCore::JSTestStringifierReadWriteAttribute::heapSnapshot):
>+        (WebCore::JSTestStringifierReadWriteAttributeOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.h:
>+        * bindings/scripts/test/JS/JSTestTypedefs.cpp:
>+        (WebCore::JSTestTypedefs::heapSnapshot):
>+        (WebCore::JSTestTypedefsOwner::isReachableFromOpaqueRoots):
>+        * bindings/scripts/test/JS/JSTestTypedefs.h:
>+        * dom/Document.idl:
>+        * page/DOMWindow.idl:
>+
> 2018-08-23  Eric Carlson  <eric.carlson@apple.com>
> 
>         [MediaStream] Store video preset sizes in a map
>diff --git a/Source/WebInspectorUI/ChangeLog b/Source/WebInspectorUI/ChangeLog
>index 646875d092b90aa0e42766367cb2cc6e38535713..f971fadfe2e6e98a42eff70590839426e7fb75e6 100644
>--- a/Source/WebInspectorUI/ChangeLog
>+++ b/Source/WebInspectorUI/ChangeLog
>@@ -1,3 +1,29 @@
>+2018-08-23  Simon Fraser  <simon.fraser@apple.com>
>+
>+        Add support for dumping GC heap snapshots, and a viewer
>+        https://bugs.webkit.org/show_bug.cgi?id=186416
>+
>+        Reviewed by Joseph Pecoraro.
>+
>+        Make a way to dump information about the GC heap that is useful for looking for leaked
>+        or abandoned objects. This dump is obtained (on Apple platforms) via:
>+            notifyutil -p com.apple.WebKit.dumpGCHeap
>+        which writes a JSON file to /tmp which can then be loaded into the viewer in Tools/GCHeapInspector.
>+
>+        This leverages the heap snapshot used by Web Inspector, adding an alternate format for
>+        the snapshot JSON that adds additional data about objects and why they are GC roots.
>+
>+        The generated bindings code is changed to include the output root reason from isReachableFromOpaqueRoots(),
>+        and to implement heapSnapshot() which provides the address of the wrapped object. A new IDL attribute,
>+        CustomHeapSnapshot, is used to allow custom heapSnapshot() implementations for classes like JSDocument
>+        that need to decorate the heap snapshot cell data with things like the document URL.
>+
>+        GCController registers a notifyutil callback which gathers the debug heap snapshot, and dumps it
>+        to a file in /tmp. The file path is printed out to the system log.
>+
>+        * UserInterface/Workers/HeapSnapshot/HeapSnapshot.js:
>+        (HeapSnapshot):
>+
> 2018-08-23  Devin Rousso  <drousso@apple.com>
> 
>         Web Inspector: `console.inspect(sessionStorage)` first time does not show Session Storage content view if Storage tab was previously unvisited
>diff --git a/Source/JavaScriptCore/API/JSAPIWrapperObject.mm b/Source/JavaScriptCore/API/JSAPIWrapperObject.mm
>index 979ff9b9ed0af2170423ccf9452d1a9a6d0fefbd..8a84bf77b3e2dee1060696ba3717f5d6418cb0ac 100644
>--- a/Source/JavaScriptCore/API/JSAPIWrapperObject.mm
>+++ b/Source/JavaScriptCore/API/JSAPIWrapperObject.mm
>@@ -37,7 +37,7 @@
> class JSAPIWrapperObjectHandleOwner : public JSC::WeakHandleOwner {
> public:
>     void finalize(JSC::Handle<JSC::Unknown>, void*) override;
>-    bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&) override;
>+    bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**) override;
> };
> 
> static JSAPIWrapperObjectHandleOwner* jsAPIWrapperObjectHandleOwner()
>@@ -56,7 +56,7 @@ void JSAPIWrapperObjectHandleOwner::finalize(JSC::Handle<JSC::Unknown> handle, v
>     JSC::WeakSet::deallocate(JSC::WeakImpl::asWeakImpl(handle.slot()));
> }
> 
>-bool JSAPIWrapperObjectHandleOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor)
>+bool JSAPIWrapperObjectHandleOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor, const char**)
> {
>     JSC::JSAPIWrapperObject* wrapperObject = JSC::jsCast<JSC::JSAPIWrapperObject*>(handle.get().asCell());
>     // We use the JSGlobalObject when processing weak handles to prevent the situation where using
>diff --git a/Source/JavaScriptCore/API/JSManagedValue.mm b/Source/JavaScriptCore/API/JSManagedValue.mm
>index 29c717db3b3d1d18571855b8e13e67e58a44cbde..c545fad6d58c5651749f5c91d1c119f933a40295 100644
>--- a/Source/JavaScriptCore/API/JSManagedValue.mm
>+++ b/Source/JavaScriptCore/API/JSManagedValue.mm
>@@ -43,7 +43,7 @@
> class JSManagedValueHandleOwner : public JSC::WeakHandleOwner {
> public:
>     void finalize(JSC::Handle<JSC::Unknown>, void* context) override;
>-    bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&) override;
>+    bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**) override;
> };
> 
> static JSManagedValueHandleOwner& managedValueHandleOwner()
>@@ -182,8 +182,10 @@ @interface JSManagedValue (PrivateMethods)
> - (void)disconnectValue;
> @end
> 
>-bool JSManagedValueHandleOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor& visitor)
>+bool JSManagedValueHandleOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor& visitor, const char** reason)
> {
>+    if (UNLIKELY(reason))
>+        *reason = "JSManagedValue is opaque root";
>     JSManagedValue *managedValue = (__bridge JSManagedValue *)context;
>     return visitor.containsOpaqueRoot((__bridge void*)managedValue);
> }
>diff --git a/Source/JavaScriptCore/API/glib/JSAPIWrapperObjectGLib.cpp b/Source/JavaScriptCore/API/glib/JSAPIWrapperObjectGLib.cpp
>index 18dd13eb70f6d1057eba63f23fceebdc2ff3012c..87d198ccb2f8f80650369632fe5cd0decc4dee1d 100644
>--- a/Source/JavaScriptCore/API/glib/JSAPIWrapperObjectGLib.cpp
>+++ b/Source/JavaScriptCore/API/glib/JSAPIWrapperObjectGLib.cpp
>@@ -36,7 +36,7 @@
> class JSAPIWrapperObjectHandleOwner : public JSC::WeakHandleOwner {
> public:
>     void finalize(JSC::Handle<JSC::Unknown>, void*) override;
>-    bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&) override;
>+    bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**) override;
> };
> 
> static JSAPIWrapperObjectHandleOwner* jsAPIWrapperObjectHandleOwner()
>@@ -55,7 +55,7 @@ void JSAPIWrapperObjectHandleOwner::finalize(JSC::Handle<JSC::Unknown> handle, v
>     JSC::WeakSet::deallocate(JSC::WeakImpl::asWeakImpl(handle.slot()));
> }
> 
>-bool JSAPIWrapperObjectHandleOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor)
>+bool JSAPIWrapperObjectHandleOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor, const char**)
> {
>     JSC::JSAPIWrapperObject* wrapperObject = JSC::jsCast<JSC::JSAPIWrapperObject*>(handle.get().asCell());
>     // We use the JSGlobalObject when processing weak handles to prevent the situation where using
>diff --git a/Source/JavaScriptCore/CMakeLists.txt b/Source/JavaScriptCore/CMakeLists.txt
>index f610d720486be1781fc2ca1aa08783f1b8124201..fb1d0cc6b246a7a6a5d7ec0921927327e195994b 100644
>--- a/Source/JavaScriptCore/CMakeLists.txt
>+++ b/Source/JavaScriptCore/CMakeLists.txt
>@@ -534,6 +534,7 @@ set(JavaScriptCore_PRIVATE_FRAMEWORK_HEADERS
>     heap/HeapFinalizerCallback.h
>     heap/HeapInlines.h
>     heap/HeapObserver.h
>+    heap/HeapSnapshotBuilder.h
>     heap/IncrementalSweeper.h
>     heap/IsoCellSet.h
>     heap/IsoSubspace.h
>diff --git a/Source/JavaScriptCore/heap/ConservativeRoots.h b/Source/JavaScriptCore/heap/ConservativeRoots.h
>index 0f9a42a6fcd694f0a9efeb10f32062246f96410d..83e5ce4694267c3143f08b44b0a861df450ce1a6 100644
>--- a/Source/JavaScriptCore/heap/ConservativeRoots.h
>+++ b/Source/JavaScriptCore/heap/ConservativeRoots.h
>@@ -41,7 +41,7 @@ public:
>     void add(void* begin, void* end);
>     void add(void* begin, void* end, JITStubRoutineSet&, CodeBlockSet&);
>     
>-    size_t size();
>+    size_t size() const;
>     HeapCell** roots();
> 
> private:
>@@ -63,7 +63,7 @@ private:
>     HeapCell* m_inlineRoots[inlineCapacity];
> };
> 
>-inline size_t ConservativeRoots::size()
>+inline size_t ConservativeRoots::size() const
> {
>     return m_size;
> }
>diff --git a/Source/JavaScriptCore/heap/Heap.cpp b/Source/JavaScriptCore/heap/Heap.cpp
>index c782fcbb7051dbd65b13dedfa526626b67c73c5f..e0baee52fd4c92ea723f4751b872e01f5013b1f7 100644
>--- a/Source/JavaScriptCore/heap/Heap.cpp
>+++ b/Source/JavaScriptCore/heap/Heap.cpp
>@@ -2641,11 +2641,15 @@ void Heap::addCoreConstraints()
>             
>             TimingScope preConvergenceTimingScope(*this, "Constraint: conservative scan");
>             m_objectSpace.prepareForConservativeScan();
>+
>             ConservativeRoots conservativeRoots(*this);
>             SuperSamplerScope superSamplerScope(false);
>+
>             gatherStackRoots(conservativeRoots);
>             gatherJSStackRoots(conservativeRoots);
>             gatherScratchBufferRoots(conservativeRoots);
>+
>+            SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::ConservativeScan);
>             slotVisitor.append(conservativeRoots);
>             
>             lastVersion = m_phaseVersion;
>@@ -2655,27 +2659,38 @@ void Heap::addCoreConstraints()
>     m_constraintSet->add(
>         "Msr", "Misc Small Roots",
>         [this] (SlotVisitor& slotVisitor) {
>+
> #if JSC_OBJC_API_ENABLED
>             scanExternalRememberedSet(*m_vm, slotVisitor);
> #endif
>-
>-            if (m_vm->smallStrings.needsToBeVisited(*m_collectionScope))
>+            if (m_vm->smallStrings.needsToBeVisited(*m_collectionScope)) {
>+                SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::StrongReferences);
>                 m_vm->smallStrings.visitStrongReferences(slotVisitor);
>+            }
>             
>-            for (auto& pair : m_protectedValues)
>-                slotVisitor.appendUnbarriered(pair.key);
>+            {
>+                SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::ProtectedValues);
>+                for (auto& pair : m_protectedValues)
>+                    slotVisitor.appendUnbarriered(pair.key);
>+            }
>             
>-            if (m_markListSet && m_markListSet->size())
>+            if (m_markListSet && m_markListSet->size()) {
>+                SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::ConservativeScan);
>                 MarkedArgumentBuffer::markLists(slotVisitor, *m_markListSet);
>-            
>-            slotVisitor.appendUnbarriered(m_vm->exception());
>-            slotVisitor.appendUnbarriered(m_vm->lastException());
>+            }
>+
>+            {
>+                SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::VMExceptions);
>+                slotVisitor.appendUnbarriered(m_vm->exception());
>+                slotVisitor.appendUnbarriered(m_vm->lastException());
>+            }
>         },
>         ConstraintVolatility::GreyedByExecution);
>     
>     m_constraintSet->add(
>         "Sh", "Strong Handles",
>         [this] (SlotVisitor& slotVisitor) {
>+            SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::StrongHandles);
>             m_handleSet.visitStrongHandles(slotVisitor);
>         },
>         ConstraintVolatility::GreyedByExecution);
>@@ -2683,6 +2698,8 @@ void Heap::addCoreConstraints()
>     m_constraintSet->add(
>         "D", "Debugger",
>         [this] (SlotVisitor& slotVisitor) {
>+            SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::Debugger);
>+
> #if ENABLE(SAMPLING_PROFILER)
>             if (SamplingProfiler* samplingProfiler = m_vm->samplingProfiler()) {
>                 LockHolder locker(samplingProfiler->getLock());
>@@ -2692,7 +2709,7 @@ void Heap::addCoreConstraints()
>                     dataLog("Sampling Profiler data:\n", slotVisitor);
>             }
> #endif // ENABLE(SAMPLING_PROFILER)
>-            
>+
>             if (m_vm->typeProfiler())
>                 m_vm->typeProfilerLog()->visit(slotVisitor);
>             
>@@ -2703,6 +2720,7 @@ void Heap::addCoreConstraints()
>     m_constraintSet->add(
>         "Jsr", "JIT Stub Routines",
>         [this] (SlotVisitor& slotVisitor) {
>+            SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::JITStubRoutines);
>             m_jitStubRoutines->traceMarkedStubRoutines(slotVisitor);
>         },
>         ConstraintVolatility::GreyedByExecution);
>@@ -2710,6 +2728,7 @@ void Heap::addCoreConstraints()
>     m_constraintSet->add(
>         "Ws", "Weak Sets",
>         [this] (SlotVisitor& slotVisitor) {
>+            SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::WeakSets);
>             m_objectSpace.visitWeakSets(slotVisitor);
>         },
>         ConstraintVolatility::GreyedByMarking);
>@@ -2718,8 +2737,9 @@ void Heap::addCoreConstraints()
>         "O", "Output",
>         [] (SlotVisitor& slotVisitor) {
>             VM& vm = slotVisitor.vm();
>-            
>+
>             auto callOutputConstraint = [] (SlotVisitor& slotVisitor, HeapCell* heapCell, HeapCell::Kind) {
>+                SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::Output);
>                 VM& vm = slotVisitor.vm();
>                 JSCell* cell = static_cast<JSCell*>(heapCell);
>                 cell->methodTable(vm)->visitOutputConstraints(cell, slotVisitor);
>@@ -2739,6 +2759,8 @@ void Heap::addCoreConstraints()
>     m_constraintSet->add(
>         "Dw", "DFG Worklists",
>         [this] (SlotVisitor& slotVisitor) {
>+            SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::DFGWorkLists);
>+
>             for (unsigned i = DFG::numberOfWorklists(); i--;)
>                 DFG::existingWorklistForIndex(i).visitWeakReferences(slotVisitor);
>             
>@@ -2759,6 +2781,7 @@ void Heap::addCoreConstraints()
>     m_constraintSet->add(
>         "Cb", "CodeBlocks",
>         [this] (SlotVisitor& slotVisitor) {
>+            SetRootMarkReasonScope rootScope(slotVisitor, SlotVisitor::RootMarkReason::CodeBlocks);
>             iterateExecutingAndCompilingCodeBlocksWithoutHoldingLocks(
>                 [&] (CodeBlock* codeBlock) {
>                     // Visit the CodeBlock as a constraint only if it's black.
>diff --git a/Source/JavaScriptCore/heap/HeapSnapshotBuilder.cpp b/Source/JavaScriptCore/heap/HeapSnapshotBuilder.cpp
>index 624935a7b33f2c9f5e638df649e0dbb17695effd..aa7fce2fca476ad2128b23f33740c14c7f50557c 100644
>--- a/Source/JavaScriptCore/heap/HeapSnapshotBuilder.cpp
>+++ b/Source/JavaScriptCore/heap/HeapSnapshotBuilder.cpp
>@@ -37,24 +37,33 @@
> #include <wtf/text/StringBuilder.h>
> 
> namespace JSC {
>-    
>-unsigned HeapSnapshotBuilder::nextAvailableObjectIdentifier = 1;
>-unsigned HeapSnapshotBuilder::getNextObjectIdentifier() { return nextAvailableObjectIdentifier++; }
>+
>+static const char* rootTypeToString(SlotVisitor::RootMarkReason);
>+
>+NodeIdentifier HeapSnapshotBuilder::nextAvailableObjectIdentifier = 1;
>+NodeIdentifier HeapSnapshotBuilder::getNextObjectIdentifier() { return nextAvailableObjectIdentifier++; }
> void HeapSnapshotBuilder::resetNextAvailableObjectIdentifier() { HeapSnapshotBuilder::nextAvailableObjectIdentifier = 1; }
> 
>-HeapSnapshotBuilder::HeapSnapshotBuilder(HeapProfiler& profiler)
>+HeapSnapshotBuilder::HeapSnapshotBuilder(HeapProfiler& profiler, SnapshotType type)
>     : m_profiler(profiler)
>+    , m_snapshotType(type)
> {
> }
> 
> HeapSnapshotBuilder::~HeapSnapshotBuilder()
> {
>+    if (m_snapshotType == SnapshotType::GCDebuggingSnapshot)
>+        m_profiler.clearSnapshots();
> }
> 
> void HeapSnapshotBuilder::buildSnapshot()
> {
>+    // GCDebuggingSnapshot are always full snapshots, so clear any existing snapshots.
>+    if (m_snapshotType == SnapshotType::GCDebuggingSnapshot)
>+        m_profiler.clearSnapshots();
>+
>     PreventCollectionScope preventCollectionScope(m_profiler.vm().heap);
>-    
>+
>     m_snapshot = std::make_unique<HeapSnapshot>(m_profiler.mostRecentSnapshot());
>     {
>         m_profiler.setActiveSnapshotBuilder(this);
>@@ -69,17 +78,18 @@ void HeapSnapshotBuilder::buildSnapshot()
> void HeapSnapshotBuilder::appendNode(JSCell* cell)
> {
>     ASSERT(m_profiler.activeSnapshotBuilder() == this);
>+    
>     ASSERT(Heap::isMarked(cell));
> 
>-    if (hasExistingNodeForCell(cell))
>+    NodeIdentifier identifier;
>+    if (previousSnapshotHasNodeForCell(cell, identifier))
>         return;
> 
>     std::lock_guard<Lock> lock(m_buildingNodeMutex);
>-
>     m_snapshot->appendNode(HeapSnapshotNode(cell, getNextObjectIdentifier()));
> }
> 
>-void HeapSnapshotBuilder::appendEdge(JSCell* from, JSCell* to)
>+void HeapSnapshotBuilder::appendEdge(JSCell* from, JSCell* to, SlotVisitor::RootMarkReason rootMarkReason)
> {
>     ASSERT(m_profiler.activeSnapshotBuilder() == this);
>     ASSERT(to);
>@@ -90,6 +100,15 @@ void HeapSnapshotBuilder::appendEdge(JSCell* from, JSCell* to)
> 
>     std::lock_guard<Lock> lock(m_buildingEdgeMutex);
> 
>+    if (m_snapshotType == SnapshotType::GCDebuggingSnapshot) {
>+        if (rootMarkReason == SlotVisitor::RootMarkReason::None && m_snapshotType == SnapshotType::GCDebuggingSnapshot)
>+            WTFLogAlways("Cell %p is a root but no root marking reason was supplied", to);
>+
>+        m_rootData.ensure(to, [] () -> RootData {
>+            return { };
>+        }).iterator->value.markReason = rootMarkReason;
>+    }
>+
>     m_edges.append(HeapSnapshotEdge(from, to));
> }
> 
>@@ -123,22 +142,46 @@ void HeapSnapshotBuilder::appendIndexEdge(JSCell* from, JSCell* to, uint32_t ind
>     m_edges.append(HeapSnapshotEdge(from, to, index));
> }
> 
>-bool HeapSnapshotBuilder::hasExistingNodeForCell(JSCell* cell)
>+void HeapSnapshotBuilder::setOpaqueRootReachabilityReasonForCell(JSCell* cell, const char* reason)
>+{
>+    if (!reason || !*reason || m_snapshotType != SnapshotType::GCDebuggingSnapshot)
>+        return;
>+
>+    m_rootData.ensure(cell, [] () -> RootData {
>+        return { };
>+    }).iterator->value.reachabilityFromOpaqueRootReasons = reason;
>+}
>+
>+void HeapSnapshotBuilder::setWrappedObjectForCell(JSCell* cell, void* wrappedPtr)
>+{
>+    m_wrappedObjectPointers.set(cell, wrappedPtr);
>+}
>+
>+bool HeapSnapshotBuilder::previousSnapshotHasNodeForCell(JSCell* cell, NodeIdentifier& identifier)
> {
>     if (!m_snapshot->previous())
>         return false;
> 
>-    return !!m_snapshot->previous()->nodeForCell(cell);
>-}
>+    auto existingNode = m_snapshot->previous()->nodeForCell(cell);
>+    if (existingNode) {
>+        identifier = existingNode.value().identifier;
>+        return true;
>+    }
> 
>+    return false;
>+}
> 
> // Heap Snapshot JSON Format:
> //
>+//  Inspector snapshots:
>+//
> //   {
> //      "version": 1.0,
>+//      "type": "Inspector",
>+//      // [<address>, <labelIndex>, <wrappedEddress>] only present in GCDebuggingSnapshot-type snapshots
> //      "nodes": [
>-//          <nodeId>, <sizeInBytes>, <nodeClassNameIndex>, <internal>,
>-//          <nodeId>, <sizeInBytes>, <nodeClassNameIndex>, <internal>,
>+//          <nodeId>, <sizeInBytes>, <nodeClassNameIndex>, <internal>
>+//          <nodeId>, <sizeInBytes>, <nodeClassNameIndex>, <internal>
> //          ...
> //      ],
> //      "nodeClassNames": [
>@@ -157,6 +200,40 @@ bool HeapSnapshotBuilder::hasExistingNodeForCell(JSCell* cell)
> //      ]
> //   }
> //
>+//  GC heap debugger snapshots:
>+//
>+//   {
>+//      "version": 1.0,
>+//      "type": "GCDebugging",
>+//      "nodes": [
>+//          <nodeId>, <sizeInBytes>, <nodeClassNameIndex>, <internal>, <labelIndex>, <cellEddress>, <wrappedEddress>,
>+//          <nodeId>, <sizeInBytes>, <nodeClassNameIndex>, <internal>, <labelIndex>, <cellEddress>, <wrappedEddress>,
>+//          ...
>+//      ],
>+//      "nodeClassNames": [
>+//          "string", "Structure", "Object", ...
>+//      ],
>+//      "edges": [
>+//          <fromNodeId>, <toNodeId>, <edgeTypeIndex>, <edgeExtraData>,
>+//          <fromNodeId>, <toNodeId>, <edgeTypeIndex>, <edgeExtraData>,
>+//          ...
>+//      ],
>+//      "edgeTypes": [
>+//          "Internal", "Property", "Index", "Variable"
>+//      ],
>+//      "edgeNames": [
>+//          "propertyName", "variableName", ...
>+//      ],
>+//      "roots" : [
>+//          <nodeId>, <rootReasonIndex>, <reachabilityReasonIndex>,
>+//          <nodeId>, <rootReasonIndex>, <reachabilityReasonIndex>,
>+//          ... // <nodeId> may be repeated
>+//      ],
>+//      "labels" : [
>+//          "foo", "bar", ...
>+//      ]
>+//   }
>+//
> // Notes:
> //
> //     <nodeClassNameIndex>
>@@ -172,6 +249,9 @@ bool HeapSnapshotBuilder::hasExistingNodeForCell(JSCell* cell)
> //       - for Internal edges this should be ignored (0).
> //       - for Index edges this is the index value.
> //       - for Property or Variable edges this is an index into the "edgeNames" list.
>+//
>+//      <rootReasonIndex>
>+//       - index into the "labels" list.
> 
> static uint8_t edgeTypeToNumber(EdgeType type)
> {
>@@ -194,24 +274,98 @@ static const char* edgeTypeToString(EdgeType type)
>     return "Internal";
> }
> 
>+static const char* snapshotTypeToString(HeapSnapshotBuilder::SnapshotType type)
>+{
>+    switch (type) {
>+    case HeapSnapshotBuilder::SnapshotType::InspectorSnapshot:
>+        return "Inspector";
>+    case HeapSnapshotBuilder::SnapshotType::GCDebuggingSnapshot:
>+        return "GCDebugging";
>+    }
>+    ASSERT_NOT_REACHED();
>+    return "Inspector";
>+}
>+
>+static const char* rootTypeToString(SlotVisitor::RootMarkReason type)
>+{
>+    switch (type) {
>+    case SlotVisitor::RootMarkReason::None:
>+        return "None";
>+    case SlotVisitor::RootMarkReason::ConservativeScan:
>+        return "Conservative scan";
>+    case SlotVisitor::RootMarkReason::StrongReferences:
>+        return "Strong references";
>+    case SlotVisitor::RootMarkReason::ProtectedValues:
>+        return "Protected values";
>+    case SlotVisitor::RootMarkReason::MarkListSet:
>+        return "Mark list set";
>+    case SlotVisitor::RootMarkReason::VMExceptions:
>+        return "VM exceptions";
>+    case SlotVisitor::RootMarkReason::StrongHandles:
>+        return "Strong handles";
>+    case SlotVisitor::RootMarkReason::Debugger:
>+        return "Debugger";
>+    case SlotVisitor::RootMarkReason::JITStubRoutines:
>+        return "JIT stub routines";
>+    case SlotVisitor::RootMarkReason::WeakSets:
>+        return "Weak sets";
>+    case SlotVisitor::RootMarkReason::Output:
>+        return "Output";
>+    case SlotVisitor::RootMarkReason::DFGWorkLists:
>+        return "DFG work lists";
>+    case SlotVisitor::RootMarkReason::CodeBlocks:
>+        return "Code blocks";
>+    case SlotVisitor::RootMarkReason::DOMGCOutput:
>+        return "DOM GC output";
>+    }
>+    ASSERT_NOT_REACHED();
>+    return "None";
>+}
>+
> String HeapSnapshotBuilder::json()
> {
>     return json([] (const HeapSnapshotNode&) { return true; });
> }
> 
>+void HeapSnapshotBuilder::setLabelForCell(JSCell* cell, const String& label)
>+{
>+    m_cellLabels.set(cell, label);
>+}
>+
>+String HeapSnapshotBuilder::descriptionForCell(JSCell *cell) const
>+{
>+    if (cell->isString())
>+        return emptyString(); // FIXME: get part of string.
>+
>+    VM& vm = m_profiler.vm();
>+    Structure* structure = cell->structure(vm);
>+
>+    if (structure->classInfo()->isSubClassOf(Structure::info())) {
>+        Structure* cellAsStructure = jsCast<Structure*>(cell);
>+        return cellAsStructure->classInfo()->className;
>+    }
>+
>+    return emptyString();
>+}
>+
> String HeapSnapshotBuilder::json(Function<bool (const HeapSnapshotNode&)> allowNodeCallback)
> {
>     VM& vm = m_profiler.vm();
>     DeferGCForAWhile deferGC(vm.heap);
> 
>     // Build a node to identifier map of allowed nodes to use when serializing edges.
>-    HashMap<JSCell*, unsigned> allowedNodeIdentifiers;
>+    HashMap<JSCell*, NodeIdentifier> allowedNodeIdentifiers;
> 
>     // Build a list of used class names.
>     HashMap<const char*, unsigned> classNameIndexes;
>     classNameIndexes.set("<root>", 0);
>     unsigned nextClassNameIndex = 1;
> 
>+    // Build a list of labels (this is just a string table).
>+    HashMap<String, unsigned> labelIndexes;
>+    labelIndexes.set(emptyString(), 0);
>+    unsigned nextLabelIndex = 1;
>+
>     // Build a list of used edge names.
>     HashMap<UniquedStringImpl*, unsigned> edgeNameIndexes;
>     unsigned nextEdgeNameIndex = 0;
>@@ -231,12 +385,44 @@ String HeapSnapshotBuilder::json(Function<bool (const HeapSnapshotNode&)> allowN
>         unsigned classNameIndex = result.iterator->value;
> 
>         bool isInternal = false;
>+        void* wrappedAddress = 0;
>+        unsigned labelIndex = 0;
>         if (!node.cell->isString()) {
>             Structure* structure = node.cell->structure(vm);
>             isInternal = !structure || !structure->globalObject();
>+
>+            if (m_snapshotType == SnapshotType::GCDebuggingSnapshot) {
>+                String nodeLabel;
>+                auto it = m_cellLabels.find(node.cell);
>+                if (it != m_cellLabels.end())
>+                    nodeLabel = it->value;
>+
>+                if (nodeLabel.isEmpty()) {
>+                    if (auto* object = jsDynamicCast<JSObject*>(vm, node.cell)) {
>+                        if (auto* function = jsDynamicCast<JSFunction*>(vm, object))
>+                            nodeLabel = function->calculatedDisplayName(vm);
>+                    }
>+                }
>+                
>+                String description = descriptionForCell(node.cell);
>+                if (description.length()) {
>+                    if (nodeLabel.length())
>+                        nodeLabel.append(' ');
>+                    nodeLabel.append(description);
>+                }
>+
>+                if (!nodeLabel.isEmpty() && m_snapshotType == SnapshotType::GCDebuggingSnapshot) {
>+                    auto result = labelIndexes.add(nodeLabel, nextLabelIndex);
>+                    if (result.isNewEntry)
>+                        nextLabelIndex++;
>+                    labelIndex = result.iterator->value;
>+                }
>+                
>+                wrappedAddress = m_wrappedObjectPointers.get(node.cell);
>+            }
>         }
> 
>-        // <nodeId>, <sizeInBytes>, <className>, <optionalInternalBoolean>
>+        // <nodeId>, <sizeInBytes>, <nodeClassNameIndex>, <internal>, [<labelIndex>, <cellEddress>, <wrappedEddress>]
>         json.append(',');
>         json.appendNumber(node.identifier);
>         json.append(',');
>@@ -245,6 +431,14 @@ String HeapSnapshotBuilder::json(Function<bool (const HeapSnapshotNode&)> allowN
>         json.appendNumber(classNameIndex);
>         json.append(',');
>         json.append(isInternal ? '1' : '0');
>+        if (m_snapshotType == SnapshotType::GCDebuggingSnapshot) {
>+            json.append(',');
>+            json.appendNumber(labelIndex);
>+            json.append(',');
>+            json.append(String::format("\"%p\"", node.cell)); // FIXME: Should add StringBuilder::appendAddress(void*).
>+            json.append(',');
>+            json.append(String::format("\"%p\"", wrappedAddress));
>+        }
>     };
> 
>     bool firstEdge = true;
>@@ -285,11 +479,21 @@ String HeapSnapshotBuilder::json(Function<bool (const HeapSnapshotNode&)> allowN
>     // version
>     json.appendLiteral("\"version\":1");
> 
>+    // type
>+    json.append(',');
>+    json.appendLiteral("\"type\":");
>+    json.appendQuotedJSONString(snapshotTypeToString(m_snapshotType));
>+
>     // nodes
>     json.append(',');
>     json.appendLiteral("\"nodes\":");
>     json.append('[');
>-    json.appendLiteral("0,0,0,0"); // <root>
>+    // <root>
>+    if (m_snapshotType == SnapshotType::GCDebuggingSnapshot)
>+        json.appendLiteral("0,0,0,0,0,\"0x0\",\"0x0\"");
>+    else
>+        json.appendLiteral("0,0,0,0");
>+
>     for (HeapSnapshot* snapshot = m_profiler.mostRecentSnapshot(); snapshot; snapshot = snapshot->previous()) {
>         for (auto& node : snapshot->m_nodes)
>             appendNodeJSON(node);
>@@ -323,8 +527,11 @@ String HeapSnapshotBuilder::json(Function<bool (const HeapSnapshotNode&)> allowN
>             edge.from.identifier = 0;
>         else {
>             auto fromLookup = allowedNodeIdentifiers.find(edge.from.cell);
>-            if (fromLookup == allowedNodeIdentifiers.end())
>+            if (fromLookup == allowedNodeIdentifiers.end()) {
>+                if (m_snapshotType == SnapshotType::GCDebuggingSnapshot)
>+                    WTFLogAlways("Failed to find node for from-edge cell %p", edge.from.cell);
>                 return true;
>+            }
>             edge.from.identifier = fromLookup->value;
>         }
> 
>@@ -332,13 +539,17 @@ String HeapSnapshotBuilder::json(Function<bool (const HeapSnapshotNode&)> allowN
>             edge.to.identifier = 0;
>         else {
>             auto toLookup = allowedNodeIdentifiers.find(edge.to.cell);
>-            if (toLookup == allowedNodeIdentifiers.end())
>+            if (toLookup == allowedNodeIdentifiers.end()) {
>+                if (m_snapshotType == SnapshotType::GCDebuggingSnapshot)
>+                    WTFLogAlways("Failed to find node for to-edge cell %p", edge.to.cell);
>                 return true;
>+            }
>             edge.to.identifier = toLookup->value;
>         }
> 
>         return false;
>     });
>+
>     allowedNodeIdentifiers.clear();
>     m_edges.shrinkToFit();
> 
>@@ -386,6 +597,73 @@ String HeapSnapshotBuilder::json(Function<bool (const HeapSnapshotNode&)> allowN
>     orderedEdgeNames.clear();
>     json.append(']');
> 
>+    if (m_snapshotType == SnapshotType::GCDebuggingSnapshot) {
>+        json.append(',');
>+        json.appendLiteral("\"roots\":");
>+        json.append('[');
>+        
>+        HeapSnapshot* snapshot = m_profiler.mostRecentSnapshot();
>+
>+        bool firstNode = true;
>+        for (auto it : m_rootData) {
>+            auto snapshotNode = snapshot->nodeForCell(it.key);
>+            if (!snapshotNode) {
>+                WTFLogAlways("Failed to find snapshot node for cell %p", it.key);
>+                continue;
>+            }
>+
>+            if (!firstNode)
>+                json.append(',');
>+
>+            firstNode = false;
>+            json.appendNumber(snapshotNode.value().identifier);
>+            
>+            // Maybe we should just always encode the root names.
>+            const char* rootName = rootTypeToString(it.value.markReason);
>+            auto result = labelIndexes.add(rootName, nextLabelIndex);
>+            if (result.isNewEntry)
>+                nextLabelIndex++;
>+            unsigned labelIndex = result.iterator->value;
>+            json.append(',');
>+            json.appendNumber(labelIndex);
>+
>+            unsigned reachabilityReasonIndex = 0;
>+            if (it.value.reachabilityFromOpaqueRootReasons) {
>+                auto result = labelIndexes.add(it.value.reachabilityFromOpaqueRootReasons, nextLabelIndex);
>+                if (result.isNewEntry)
>+                    nextLabelIndex++;
>+                reachabilityReasonIndex = result.iterator->value;
>+            }
>+            json.append(',');
>+            json.appendNumber(reachabilityReasonIndex);
>+        }
>+
>+        json.append(']');
>+    }
>+
>+    if (m_snapshotType == SnapshotType::GCDebuggingSnapshot) {
>+        // internal node descriptions
>+        json.append(',');
>+        json.appendLiteral("\"labels\":");
>+        json.append('[');
>+
>+        Vector<String> orderedLabels(labelIndexes.size());
>+        for (auto& entry : labelIndexes)
>+            orderedLabels[entry.value] = entry.key;
>+        labelIndexes.clear();
>+        bool firstLabel = true;
>+        for (auto& label : orderedLabels) {
>+            if (!firstLabel)
>+                json.append(',');
>+
>+            firstLabel = false;
>+            json.appendQuotedJSONString(label);
>+        }
>+        orderedLabels.clear();
>+
>+        json.append(']');
>+    }
>+
>     json.append('}');
>     return json.toString();
> }
>diff --git a/Source/JavaScriptCore/heap/HeapSnapshotBuilder.h b/Source/JavaScriptCore/heap/HeapSnapshotBuilder.h
>index ae7f0bd0c3e423145643d73dd22ff5c8048ec18a..d69869298b36eb0e10e606c0a55421350a03caa7 100644
>--- a/Source/JavaScriptCore/heap/HeapSnapshotBuilder.h
>+++ b/Source/JavaScriptCore/heap/HeapSnapshotBuilder.h
>@@ -25,6 +25,7 @@
> 
> #pragma once
> 
>+#include "SlotVisitor.h"
> #include <functional>
> #include <wtf/Lock.h>
> #include <wtf/Vector.h>
>@@ -33,10 +34,13 @@
> 
> namespace JSC {
> 
>+class ConservativeRoots;
> class HeapProfiler;
> class HeapSnapshot;
> class JSCell;
> 
>+typedef unsigned NodeIdentifier;
>+
> struct HeapSnapshotNode {
>     HeapSnapshotNode(JSCell* cell, unsigned identifier)
>         : cell(cell)
>@@ -44,7 +48,7 @@ struct HeapSnapshotNode {
>     { }
> 
>     JSCell* cell;
>-    unsigned identifier;
>+    NodeIdentifier identifier;
> };
> 
> enum class EdgeType : uint8_t {
>@@ -82,12 +86,12 @@ struct HeapSnapshotEdge {
> 
>     union {
>         JSCell *cell;
>-        unsigned identifier;
>+        NodeIdentifier identifier;
>     } from;
> 
>     union {
>         JSCell *cell;
>-        unsigned identifier;
>+        NodeIdentifier identifier;
>     } to;
> 
>     union {
>@@ -101,33 +105,47 @@ struct HeapSnapshotEdge {
> class JS_EXPORT_PRIVATE HeapSnapshotBuilder {
>     WTF_MAKE_FAST_ALLOCATED;
> public:
>-    HeapSnapshotBuilder(HeapProfiler&);
>+    enum SnapshotType { InspectorSnapshot, GCDebuggingSnapshot };
>+
>+    HeapSnapshotBuilder(HeapProfiler&, SnapshotType = SnapshotType::InspectorSnapshot);
>     ~HeapSnapshotBuilder();
> 
>-    static unsigned nextAvailableObjectIdentifier;
>-    static unsigned getNextObjectIdentifier();
>     static void resetNextAvailableObjectIdentifier();
> 
>     // Performs a garbage collection that builds a snapshot of all live cells.
>     void buildSnapshot();
> 
>-    // A marked cell.
>+    // A root or marked cell.
>     void appendNode(JSCell*);
> 
>     // A reference from one cell to another.
>-    void appendEdge(JSCell* from, JSCell* to);
>+    void appendEdge(JSCell* from, JSCell* to, SlotVisitor::RootMarkReason);
>     void appendPropertyNameEdge(JSCell* from, JSCell* to, UniquedStringImpl* propertyName);
>     void appendVariableNameEdge(JSCell* from, JSCell* to, UniquedStringImpl* variableName);
>     void appendIndexEdge(JSCell* from, JSCell* to, uint32_t index);
> 
>+    void setOpaqueRootReachabilityReasonForCell(JSCell*, const char*);
>+    void setWrappedObjectForCell(JSCell*, void*);
>+    void setLabelForCell(JSCell*, const String&);
>+
>     String json();
>     String json(Function<bool (const HeapSnapshotNode&)> allowNodeCallback);
> 
> private:
>+    static NodeIdentifier nextAvailableObjectIdentifier;
>+    static NodeIdentifier getNextObjectIdentifier();
>+
>     // Finalized snapshots are not modified during building. So searching them
>     // for an existing node can be done concurrently without a lock.
>-    bool hasExistingNodeForCell(JSCell*);
>-
>+    bool previousSnapshotHasNodeForCell(JSCell*, NodeIdentifier&);
>+    
>+    String descriptionForCell(JSCell*) const;
>+    
>+    struct RootData {
>+        const char* reachabilityFromOpaqueRootReasons { nullptr };
>+        SlotVisitor::RootMarkReason markReason { SlotVisitor::RootMarkReason::None };
>+    };
>+    
>     HeapProfiler& m_profiler;
> 
>     // SlotVisitors run in parallel.
>@@ -135,6 +153,10 @@ private:
>     std::unique_ptr<HeapSnapshot> m_snapshot;
>     Lock m_buildingEdgeMutex;
>     Vector<HeapSnapshotEdge> m_edges;
>+    HashMap<JSCell*, RootData> m_rootData;
>+    HashMap<JSCell*, void*> m_wrappedObjectPointers;
>+    HashMap<JSCell*, String> m_cellLabels;
>+    SnapshotType m_snapshotType;
> };
> 
> } // namespace JSC
>diff --git a/Source/JavaScriptCore/heap/SlotVisitor.cpp b/Source/JavaScriptCore/heap/SlotVisitor.cpp
>index e737f39450e90c5425eb3a146c39245d8e96554e..33c37fc6e89b28b6347869dbe0dac77db1eab357 100644
>--- a/Source/JavaScriptCore/heap/SlotVisitor.cpp
>+++ b/Source/JavaScriptCore/heap/SlotVisitor.cpp
>@@ -225,7 +225,7 @@ void SlotVisitor::appendJSCellOrAuxiliary(HeapCell* heapCell)
> void SlotVisitor::appendSlow(JSCell* cell, Dependency dependency)
> {
>     if (UNLIKELY(m_heapSnapshotBuilder))
>-        m_heapSnapshotBuilder->appendEdge(m_currentCell, cell);
>+        m_heapSnapshotBuilder->appendEdge(m_currentCell, cell, m_rootMarkReason);
>     
>     appendHiddenSlowImpl(cell, dependency);
> }
>diff --git a/Source/JavaScriptCore/heap/SlotVisitor.h b/Source/JavaScriptCore/heap/SlotVisitor.h
>index e8a27743a6c63b12ad99773abd260d839ad31731..ee590efa057957a5998be1dc69aa3d051b73137c 100644
>--- a/Source/JavaScriptCore/heap/SlotVisitor.h
>+++ b/Source/JavaScriptCore/heap/SlotVisitor.h
>@@ -57,6 +57,23 @@ class SlotVisitor {
>     friend class Heap;
> 
> public:
>+    enum RootMarkReason {
>+        None,
>+        ConservativeScan,
>+        StrongReferences,
>+        ProtectedValues,
>+        MarkListSet,
>+        VMExceptions,
>+        StrongHandles,
>+        Debugger,
>+        JITStubRoutines,
>+        WeakSets,
>+        Output,
>+        DFGWorkLists,
>+        CodeBlocks,
>+        DOMGCOutput,
>+    };
>+
>     SlotVisitor(Heap&, CString codeName);
>     ~SlotVisitor();
> 
>@@ -142,7 +159,11 @@ public:
>     void dump(PrintStream&) const;
> 
>     bool isBuildingHeapSnapshot() const { return !!m_heapSnapshotBuilder; }
>+    HeapSnapshotBuilder* heapSnapshotBuilder() const { return m_heapSnapshotBuilder; }
>     
>+    RootMarkReason rootMarkReason() const { return m_rootMarkReason; }
>+    void setRootMarkReason(RootMarkReason reason) { m_rootMarkReason = reason; }
>+
>     HeapVersion markingVersion() const { return m_markingVersion; }
> 
>     bool mutatorIsStopped() const { return m_mutatorIsStopped; }
>@@ -224,6 +245,7 @@ private:
> 
>     HeapSnapshotBuilder* m_heapSnapshotBuilder { nullptr };
>     JSCell* m_currentCell { nullptr };
>+    RootMarkReason m_rootMarkReason { RootMarkReason::None };
>     bool m_isFirstVisit { false };
>     bool m_mutatorIsStopped { false };
>     bool m_canOptimizeForStoppedMutator { false };
>@@ -262,4 +284,23 @@ private:
>     SlotVisitor& m_stack;
> };
> 
>+class SetRootMarkReasonScope {
>+public:
>+    SetRootMarkReasonScope(SlotVisitor& visitor, SlotVisitor::RootMarkReason reason)
>+        : m_visitor(visitor)
>+        , m_previousReason(visitor.rootMarkReason())
>+    {
>+        m_visitor.setRootMarkReason(reason);
>+    }
>+
>+    ~SetRootMarkReasonScope()
>+    {
>+        m_visitor.setRootMarkReason(m_previousReason);
>+    }
>+
>+private:
>+    SlotVisitor& m_visitor;
>+    SlotVisitor::RootMarkReason m_previousReason;
>+};
>+
> } // namespace JSC
>diff --git a/Source/JavaScriptCore/heap/WeakBlock.cpp b/Source/JavaScriptCore/heap/WeakBlock.cpp
>index f328f96c451a5153b89d5b3c4ae424880675f0c3..323ce6ffac688ae14b10dc84c85d122df021fba8 100644
>--- a/Source/JavaScriptCore/heap/WeakBlock.cpp
>+++ b/Source/JavaScriptCore/heap/WeakBlock.cpp
>@@ -28,6 +28,7 @@
> 
> #include "CellContainerInlines.h"
> #include "Heap.h"
>+#include "HeapSnapshotBuilder.h"
> #include "JSCInlines.h"
> #include "JSObject.h"
> #include "WeakHandleOwner.h"
>@@ -114,10 +115,20 @@ void WeakBlock::specializedVisit(ContainerType& container, SlotVisitor& visitor)
>         if (container.isMarked(markingVersion, jsValue.asCell()))
>             continue;
>         
>-        if (!weakHandleOwner->isReachableFromOpaqueRoots(Handle<Unknown>::wrapSlot(&const_cast<JSValue&>(jsValue)), weakImpl->context(), visitor))
>+        const char* reason = "";
>+        const char** reasonPtr = nullptr;
>+        if (UNLIKELY(visitor.isBuildingHeapSnapshot()))
>+            reasonPtr = &reason;
>+
>+        if (!weakHandleOwner->isReachableFromOpaqueRoots(Handle<Unknown>::wrapSlot(&const_cast<JSValue&>(jsValue)), weakImpl->context(), visitor, reasonPtr))
>             continue;
> 
>         visitor.appendUnbarriered(jsValue);
>+
>+        if (UNLIKELY(visitor.isBuildingHeapSnapshot())) {
>+            if (jsValue.isCell())
>+                visitor.heapSnapshotBuilder()->setOpaqueRootReachabilityReasonForCell(jsValue.asCell(), *reasonPtr);
>+        }
>     }
> }
> 
>diff --git a/Source/JavaScriptCore/heap/WeakHandleOwner.cpp b/Source/JavaScriptCore/heap/WeakHandleOwner.cpp
>index 044518f7a31307cf366c18fa3b2e67fde805fe1c..4572405977f5971a17775de4b9e41ad6c4e27ad1 100644
>--- a/Source/JavaScriptCore/heap/WeakHandleOwner.cpp
>+++ b/Source/JavaScriptCore/heap/WeakHandleOwner.cpp
>@@ -37,7 +37,7 @@ WeakHandleOwner::~WeakHandleOwner()
> {
> }
> 
>-bool WeakHandleOwner::isReachableFromOpaqueRoots(Handle<Unknown>, void*, SlotVisitor&)
>+bool WeakHandleOwner::isReachableFromOpaqueRoots(Handle<Unknown>, void*, SlotVisitor&, const char**)
> {
>     return false;
> }
>diff --git a/Source/JavaScriptCore/heap/WeakHandleOwner.h b/Source/JavaScriptCore/heap/WeakHandleOwner.h
>index 219a9c59189dd1413e4ea6e724d7da1a36505db1..450456f40169a7a45640b5e6554768edd415e92f 100644
>--- a/Source/JavaScriptCore/heap/WeakHandleOwner.h
>+++ b/Source/JavaScriptCore/heap/WeakHandleOwner.h
>@@ -34,7 +34,8 @@ class SlotVisitor;
> class JS_EXPORT_PRIVATE WeakHandleOwner {
> public:
>     virtual ~WeakHandleOwner();
>-    virtual bool isReachableFromOpaqueRoots(Handle<Unknown>, void* context, SlotVisitor&);
>+    // reason will only be non-null when generating a debug GC heap snapshot.
>+    virtual bool isReachableFromOpaqueRoots(Handle<Unknown>, void* context, SlotVisitor&, char const** reason = nullptr);
>     virtual void finalize(Handle<Unknown>, void* context);
> };
> 
>diff --git a/Source/JavaScriptCore/runtime/SimpleTypedArrayController.cpp b/Source/JavaScriptCore/runtime/SimpleTypedArrayController.cpp
>index a25028cd752f83f7ad666783fe62bc9a510c7a36..6afe2309f14a21f38d33e31219bb5709322b9c0e 100644
>--- a/Source/JavaScriptCore/runtime/SimpleTypedArrayController.cpp
>+++ b/Source/JavaScriptCore/runtime/SimpleTypedArrayController.cpp
>@@ -58,8 +58,10 @@ bool SimpleTypedArrayController::isAtomicsWaitAllowedOnCurrentThread()
>     return true;
> }
> 
>-bool SimpleTypedArrayController::JSArrayBufferOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor)
>+bool SimpleTypedArrayController::JSArrayBufferOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor, const char** reason)
> {
>+    if (UNLIKELY(reason))
>+        *reason = "JSArrayBuffer is opaque root";
>     auto& wrapper = *JSC::jsCast<JSC::JSArrayBuffer*>(handle.slot()->asCell());
>     return visitor.containsOpaqueRoot(wrapper.impl());
> }
>diff --git a/Source/JavaScriptCore/runtime/SimpleTypedArrayController.h b/Source/JavaScriptCore/runtime/SimpleTypedArrayController.h
>index d2b03c2639590d37ecd12c6a998d3569bae279a6..33d089f617bd88cbe4df81c411bdf4e40c77d510 100644
>--- a/Source/JavaScriptCore/runtime/SimpleTypedArrayController.h
>+++ b/Source/JavaScriptCore/runtime/SimpleTypedArrayController.h
>@@ -58,7 +58,7 @@ public:
> private:
>     class JSArrayBufferOwner : public WeakHandleOwner {
>     public:
>-        bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, SlotVisitor&) override;
>+        bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, SlotVisitor&, const char** reason) override;
>         void finalize(JSC::Handle<JSC::Unknown>, void* context) override;
>     };
> 
>diff --git a/Source/JavaScriptCore/tools/JSDollarVM.cpp b/Source/JavaScriptCore/tools/JSDollarVM.cpp
>index dde310911c0dc43c1b67c35292d108677fbd2295..eb57c784c98180462d4a8e43c86b50baaae8493b 100644
>--- a/Source/JavaScriptCore/tools/JSDollarVM.cpp
>+++ b/Source/JavaScriptCore/tools/JSDollarVM.cpp
>@@ -172,8 +172,10 @@ private:
> 
> class ElementHandleOwner : public WeakHandleOwner {
> public:
>-    bool isReachableFromOpaqueRoots(Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor) override
>+    bool isReachableFromOpaqueRoots(Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason) override
>     {
>+        if (UNLIKELY(reason))
>+            *reason = "JSC::Element is opaque root";
>         Element* element = jsCast<Element*>(handle.slot()->asCell());
>         return visitor.containsOpaqueRoot(element->root());
>     }
>diff --git a/Source/WebCore/bindings/js/DOMGCOutputConstraint.cpp b/Source/WebCore/bindings/js/DOMGCOutputConstraint.cpp
>index 6886343a77534848d4b59c34eff65bfed7c7cf25..7d27da466b85c5d5c9a15b7391c13f9652c7c8d4 100644
>--- a/Source/WebCore/bindings/js/DOMGCOutputConstraint.cpp
>+++ b/Source/WebCore/bindings/js/DOMGCOutputConstraint.cpp
>@@ -29,6 +29,7 @@
> #include "WebCoreJSClientData.h"
> #include <JavaScriptCore/BlockDirectoryInlines.h>
> #include <JavaScriptCore/HeapInlines.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/MarkedBlockInlines.h>
> #include <JavaScriptCore/SubspaceInlines.h>
> #include <JavaScriptCore/VM.h>
>@@ -61,6 +62,7 @@ void DOMGCOutputConstraint::executeImpl(SlotVisitor& visitor)
>     m_clientData.forEachOutputConstraintSpace(
>         [&] (Subspace& subspace) {
>             auto func = [] (SlotVisitor& visitor, HeapCell* heapCell, HeapCell::Kind) {
>+                SetRootMarkReasonScope rootScope(visitor, SlotVisitor::RootMarkReason::DOMGCOutput);
>                 JSCell* cell = static_cast<JSCell*>(heapCell);
>                 cell->methodTable(visitor.vm())->visitOutputConstraints(cell, visitor);
>             };
>diff --git a/Source/WebCore/bindings/js/GCController.cpp b/Source/WebCore/bindings/js/GCController.cpp
>index 43856f5ee5cafc23f563d5fc765499a6f6ac21e5..d251d2ece1f18b240a88bbfb3a22c2c24f80f6cd 100644
>--- a/Source/WebCore/bindings/js/GCController.cpp
>+++ b/Source/WebCore/bindings/js/GCController.cpp
>@@ -27,9 +27,14 @@
> #include "GCController.h"
> 
> #include "CommonVM.h"
>+#include "FileSystem.h"
>+#include "JSHTMLDocument.h"
>+#include "Location.h"
> #include <JavaScriptCore/Heap.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSLock.h>
> #include <JavaScriptCore/VM.h>
>+#include <pal/Logging.h>
> #include <wtf/FastMalloc.h>
> #include <wtf/NeverDestroyed.h>
> #include <wtf/StdLibExtras.h>
>@@ -52,6 +57,12 @@ GCController& GCController::singleton()
> GCController::GCController()
>     : m_GCTimer(*this, &GCController::gcTimerFired)
> {
>+    static std::once_flag onceFlag;
>+    std::call_once(onceFlag, [] {
>+        PAL::registerNotifyCallback("com.apple.WebKit.dumpGCHeap", [] {
>+            GCController::singleton().dumpHeap();
>+        });
>+    });
> }
> 
> void GCController::garbageCollectSoon()
>@@ -127,4 +138,36 @@ void GCController::deleteAllLinkedCode(DeleteAllCodeEffort effort)
>     commonVM().deleteAllLinkedCode(effort);
> }
> 
>+void GCController::dumpHeap()
>+{
>+    FileSystem::PlatformFileHandle fileHandle;
>+    String tempFilePath = FileSystem::openTemporaryFile("GCHeap"_s, fileHandle);
>+    if (!FileSystem::isHandleValid(fileHandle)) {
>+        WTFLogAlways("Dumping GC heap failed to open temporary file");
>+        return;
>+    }
>+
>+    VM& vm = commonVM();
>+    JSLockHolder lock(vm);
>+
>+    sanitizeStackForVM(&vm);
>+
>+    String jsonData;
>+    {
>+        DeferGCForAWhile deferGC(vm.heap); // Prevent concurrent GC from interfering with the full GC that the snapshot does.
>+
>+        HeapSnapshotBuilder snapshotBuilder(vm.ensureHeapProfiler(), HeapSnapshotBuilder::SnapshotType::GCDebuggingSnapshot);
>+        snapshotBuilder.buildSnapshot();
>+
>+        jsonData = snapshotBuilder.json();
>+    }
>+
>+    CString utf8String = jsonData.utf8();
>+
>+    FileSystem::writeToFile(fileHandle, utf8String.data(), utf8String.length());
>+    FileSystem::closeFile(fileHandle);
>+    
>+    WTFLogAlways("Dumped GC heap to %s", tempFilePath.utf8().data());
>+}
>+
> } // namespace WebCore
>diff --git a/Source/WebCore/bindings/js/GCController.h b/Source/WebCore/bindings/js/GCController.h
>index ef6d1386f72d50819031a6c1d32438afde90f8f1..0ce45950c5391f60696fd40d89d927da386a87d3 100644
>--- a/Source/WebCore/bindings/js/GCController.h
>+++ b/Source/WebCore/bindings/js/GCController.h
>@@ -51,6 +51,8 @@ public:
> private:
>     GCController(); // Use singleton() instead.
> 
>+    void dumpHeap();
>+
>     void gcTimerFired();
>     Timer m_GCTimer;
> };
>diff --git a/Source/WebCore/bindings/js/JSCSSRuleListCustom.cpp b/Source/WebCore/bindings/js/JSCSSRuleListCustom.cpp
>index f0550aa6443ee9d44655b95c09720e462dcd7bfe..020e458c05fb350d0351d04dc175e11935067cba 100644
>--- a/Source/WebCore/bindings/js/JSCSSRuleListCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSCSSRuleListCustom.cpp
>@@ -36,15 +36,25 @@
> namespace WebCore {
> using namespace JSC;
> 
>-bool JSCSSRuleListOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+bool JSCSSRuleListOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     JSCSSRuleList* jsCSSRuleList = jsCast<JSCSSRuleList*>(handle.slot()->asCell());
>     if (!jsCSSRuleList->hasCustomProperties(*jsCSSRuleList->vm()))
>         return false;
>-    if (CSSStyleSheet* styleSheet = jsCSSRuleList->wrapped().styleSheet())
>+
>+    if (CSSStyleSheet* styleSheet = jsCSSRuleList->wrapped().styleSheet()) {
>+        if (UNLIKELY(reason))
>+            *reason = "CSSStyleSheet is opaque root";
>+
>         return visitor.containsOpaqueRoot(root(styleSheet));
>-    if (CSSRule* cssRule = jsCSSRuleList->wrapped().item(0))
>+    }
>+    
>+    if (CSSRule* cssRule = jsCSSRuleList->wrapped().item(0)) {
>+        if (UNLIKELY(reason))
>+            *reason = "CSSRule is opaque root";
>+
>         return visitor.containsOpaqueRoot(root(cssRule));
>+    }
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/js/JSCallbackData.cpp b/Source/WebCore/bindings/js/JSCallbackData.cpp
>index 9ba0eaf19faddc1c4be1a530aeb447dcb39eef1f..186762a90d2853547462318c25caea46856fd9ca 100644
>--- a/Source/WebCore/bindings/js/JSCallbackData.cpp
>+++ b/Source/WebCore/bindings/js/JSCallbackData.cpp
>@@ -90,8 +90,10 @@ void JSCallbackDataWeak::visitJSFunction(JSC::SlotVisitor& vistor)
>     vistor.append(m_callback);
> }
> 
>-bool JSCallbackDataWeak::WeakOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, SlotVisitor& visitor)
>+bool JSCallbackDataWeak::WeakOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, SlotVisitor& visitor, const char** reason)
> {
>+    if (UNLIKELY(reason))
>+        *reason = "Context is opaque root"; // FIXME: what is the context.
>     return visitor.containsOpaqueRoot(context);
> }
> 
>diff --git a/Source/WebCore/bindings/js/JSCallbackData.h b/Source/WebCore/bindings/js/JSCallbackData.h
>index 60192e61422d123a2297927b5bdaff03d447d765..a5bc9542e54752334af0d790305263e94fe5bdda 100644
>--- a/Source/WebCore/bindings/js/JSCallbackData.h
>+++ b/Source/WebCore/bindings/js/JSCallbackData.h
>@@ -116,7 +116,7 @@ public:
> 
> private:
>     class WeakOwner : public JSC::WeakHandleOwner {
>-        bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&) override;
>+        bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**) override;
>     };
>     WeakOwner m_weakOwner;
>     JSC::Weak<JSC::JSObject> m_callback;
>diff --git a/Source/WebCore/bindings/js/JSCanvasRenderingContext2DCustom.cpp b/Source/WebCore/bindings/js/JSCanvasRenderingContext2DCustom.cpp
>index f1018f4d57dfb3b4256d171edb68fda183c480a8..4d500429807753580491facec170e6ba7770250f 100644
>--- a/Source/WebCore/bindings/js/JSCanvasRenderingContext2DCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSCanvasRenderingContext2DCustom.cpp
>@@ -22,8 +22,11 @@
> 
> namespace WebCore {
> 
>-bool JSCanvasRenderingContext2DOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor)
>+bool JSCanvasRenderingContext2DOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor, const char** reason)
> {
>+    if (UNLIKELY(reason))
>+        *reason = "Canvas is opaque root";
>+
>     JSCanvasRenderingContext2D* jsCanvasRenderingContext = jsCast<JSCanvasRenderingContext2D*>(handle.slot()->asCell());
>     void* root = WebCore::root(jsCanvasRenderingContext->wrapped().canvas());
>     return visitor.containsOpaqueRoot(root);
>diff --git a/Source/WebCore/bindings/js/JSDOMWindowCustom.cpp b/Source/WebCore/bindings/js/JSDOMWindowCustom.cpp
>index a761db123e62c8bd97f0e78172c16e06a227b427..496d2e7d6f2468feb6d81a2d84ed26d837a36eca 100644
>--- a/Source/WebCore/bindings/js/JSDOMWindowCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSDOMWindowCustom.cpp
>@@ -45,6 +45,7 @@
> #include "ScheduledAction.h"
> #include "Settings.h"
> #include "WebCoreJSClientData.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/JSMicrotask.h>
> #include <JavaScriptCore/Lookup.h>
>@@ -202,7 +203,7 @@ bool JSDOMWindow::getOwnPropertySlot(JSObject* object, ExecState* state, Propert
>     if (!BindingSecurity::shouldAllowAccessToDOMWindow(*state, thisObject->wrapped(), errorMessage))
>         return jsDOMWindowGetOwnPropertySlotRestrictedAccess<DOMWindowType::Local>(thisObject, thisObject->wrapped(), *state, propertyName, slot, errorMessage);
> 
>-    // FIXME: this need more explanation.
>+    // FIXME: this needs more explanation.
>     // (Particularly, is it correct that this exists here but not in getOwnPropertySlotByIndex?)
>     slot.setWatchpointSet(thisObject->m_windowCloseWatchpoints);
> 
>@@ -328,6 +329,15 @@ bool JSDOMWindow::deletePropertyByIndex(JSCell* cell, ExecState* exec, unsigned
>     return Base::deletePropertyByIndex(thisObject, exec, propertyName);
> }
> 
>+void JSDOMWindow::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    JSDOMWindow* thisObject = jsCast<JSDOMWindow*>(cell);
>+    if (auto* location = thisObject->wrapped().location())
>+        builder.setLabelForCell(cell, location->href());
>+
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> // https://html.spec.whatwg.org/#crossoriginproperties-(-o-)
> static void addCrossOriginWindowPropertyNames(ExecState& state, AbstractDOMWindow& window, PropertyNameArray& propertyNames)
> {
>diff --git a/Source/WebCore/bindings/js/JSDeprecatedCSSOMValueCustom.cpp b/Source/WebCore/bindings/js/JSDeprecatedCSSOMValueCustom.cpp
>index 35c75fe347126421775609c4428cacf0a3cdf103..f86fee63c76b956b3f3c2bcc1d265fa87769559c 100644
>--- a/Source/WebCore/bindings/js/JSDeprecatedCSSOMValueCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSDeprecatedCSSOMValueCustom.cpp
>@@ -35,11 +35,15 @@
> namespace WebCore {
> using namespace JSC;
> 
>-bool JSDeprecatedCSSOMValueOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+bool JSDeprecatedCSSOMValueOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     JSDeprecatedCSSOMValue* jsCSSValue = jsCast<JSDeprecatedCSSOMValue*>(handle.slot()->asCell());
>     if (!jsCSSValue->hasCustomProperties(*jsCSSValue->vm()))
>         return false;
>+
>+    if (UNLIKELY(reason))
>+        *reason = "CSSStyleDeclaration is opaque root";
>+
>     return visitor.containsOpaqueRoot(root(&jsCSSValue->wrapped().owner()));
> }
> 
>diff --git a/Source/WebCore/bindings/js/JSDocumentCustom.cpp b/Source/WebCore/bindings/js/JSDocumentCustom.cpp
>index 6605f1e7ff2f9576ff89fe70f1a348ee4c8bdf3c..5687e4d78c62d08f194b93824646979699c17f8d 100644
>--- a/Source/WebCore/bindings/js/JSDocumentCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSDocumentCustom.cpp
>@@ -26,6 +26,7 @@
> #include "JSXMLDocument.h"
> #include "NodeTraversal.h"
> #include "SVGDocument.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> 
> 
> namespace WebCore {
>@@ -93,4 +94,11 @@ void JSDocument::visitAdditionalChildren(SlotVisitor& visitor)
>     visitor.addOpaqueRoot(static_cast<ScriptExecutionContext*>(&wrapped()));
> }
> 
>+void JSDocument::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    Base::heapSnapshot(cell, builder);
>+    auto* thisObject = jsCast<JSDocument*>(cell);
>+    builder.setLabelForCell(cell, thisObject->wrapped().url().string());
>+}
>+
> } // namespace WebCore
>diff --git a/Source/WebCore/bindings/js/JSMicrotaskCallback.h b/Source/WebCore/bindings/js/JSMicrotaskCallback.h
>index c9b8e64b854b443ffd3f97bc23e25c8d1664c26a..1a0b53fc343d5c5b14f737e258b37a64d8244e4c 100644
>--- a/Source/WebCore/bindings/js/JSMicrotaskCallback.h
>+++ b/Source/WebCore/bindings/js/JSMicrotaskCallback.h
>@@ -40,8 +40,8 @@ public:
>     void call()
>     {
>         auto protectedThis { makeRef(*this) };
>-        VM& vm = m_globalObject->vm();
>-        JSLockHolder lock(vm);
>+        JSC::VM& vm = m_globalObject->vm();
>+        JSC::JSLockHolder lock(vm);
>         auto scope = DECLARE_THROW_SCOPE(vm);
>         JSExecState::runTask(m_globalObject->globalExec(), m_task);
>         scope.assertNoException();
>@@ -54,7 +54,7 @@ private:
>     {
>     }
> 
>-    Strong<JSDOMGlobalObject> m_globalObject;
>+    JSC::Strong<JSDOMGlobalObject> m_globalObject;
>     Ref<JSC::Microtask> m_task;
> };
> 
>diff --git a/Source/WebCore/bindings/js/JSMutationObserverCustom.cpp b/Source/WebCore/bindings/js/JSMutationObserverCustom.cpp
>index 4ddef8afc87b01dc6a551d1b92dd2ff19430dbe3..195ad015cd599b4561ae5ea10fc09346674bf557 100644
>--- a/Source/WebCore/bindings/js/JSMutationObserverCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSMutationObserverCustom.cpp
>@@ -44,11 +44,14 @@ void JSMutationObserver::visitAdditionalChildren(JSC::SlotVisitor& visitor)
>     wrapped().callback().visitJSFunction(visitor);
> }
> 
>-bool JSMutationObserverOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+bool JSMutationObserverOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char**reason)
> {
>     for (auto* node : jsCast<JSMutationObserver*>(handle.slot()->asCell())->wrapped().observedNodes()) {
>-        if (visitor.containsOpaqueRoot(root(node)))
>+        if (visitor.containsOpaqueRoot(root(node))) {
>+            if (UNLIKELY(reason))
>+                *reason = "Reachable from observed nodes";
>             return true;
>+        }
>     }
>     return false;
> }
>diff --git a/Source/WebCore/bindings/js/JSNavigatorCustom.cpp b/Source/WebCore/bindings/js/JSNavigatorCustom.cpp
>index 49cda064dc66f2bb38901a2e3e6e3195739c0f81..21699004aa751edbb46f190d57f4308c04087fda 100644
>--- a/Source/WebCore/bindings/js/JSNavigatorCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSNavigatorCustom.cpp
>@@ -28,7 +28,7 @@
> 
> namespace WebCore {
> 
>-void JSNavigator::visitAdditionalChildren(SlotVisitor& visitor)
>+void JSNavigator::visitAdditionalChildren(JSC::SlotVisitor& visitor)
> {
> #if ENABLE(SERVICE_WORKER)
>     visitor.addOpaqueRoot(&wrapped().serviceWorker());
>diff --git a/Source/WebCore/bindings/js/JSNodeCustom.cpp b/Source/WebCore/bindings/js/JSNodeCustom.cpp
>index 09a70f22e88cf1cb987ae7d7181cfe2595359422..c485e303d5a36e31105f83fd574d5c3117ab2490 100644
>--- a/Source/WebCore/bindings/js/JSNodeCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSNodeCustom.cpp
>@@ -71,7 +71,7 @@ using namespace JSC;
> 
> using namespace HTMLNames;
> 
>-static inline bool isReachableFromDOM(Node* node, SlotVisitor& visitor)
>+static inline bool isReachableFromDOM(Node* node, SlotVisitor& visitor, const char** reason)
> {
>     if (!node->isConnected()) {
>         if (is<Element>(*node)) {
>@@ -83,30 +83,42 @@ static inline bool isReachableFromDOM(Node* node, SlotVisitor& visitor)
>             // the element is destroyed, its load event will not fire.
>             // FIXME: The DOM should manage this issue without the help of JavaScript wrappers.
>             if (is<HTMLImageElement>(element)) {
>-                if (downcast<HTMLImageElement>(element).hasPendingActivity())
>+                if (downcast<HTMLImageElement>(element).hasPendingActivity()) {
>+                    if (UNLIKELY(reason))
>+                        *reason = "Image element with pending activity";
>                     return true;
>+                }
>             }
> #if ENABLE(VIDEO)
>             else if (is<HTMLAudioElement>(element)) {
>-                if (!downcast<HTMLAudioElement>(element).paused())
>+                if (!downcast<HTMLAudioElement>(element).paused()) {
>+                    if (UNLIKELY(reason))
>+                        *reason = "Audio element which is not paused";
>                     return true;
>+                }
>             }
> #endif
>         }
> 
>         // If a node is firing event listeners, its wrapper is observable because
>         // its wrapper is responsible for marking those event listeners.
>-        if (node->isFiringEventListeners())
>+        if (node->isFiringEventListeners()) {
>+            if (UNLIKELY(reason))
>+                *reason = "Node which is firing event listeners";
>             return true;
>+        }
>     }
> 
>+    if (UNLIKELY(reason))
>+        *reason = "Connected node";
>+
>     return visitor.containsOpaqueRoot(root(node));
> }
> 
>-bool JSNodeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+bool JSNodeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     JSNode* jsNode = jsCast<JSNode*>(handle.slot()->asCell());
>-    return isReachableFromDOM(&jsNode->wrapped(), visitor);
>+    return isReachableFromDOM(&jsNode->wrapped(), visitor, reason);
> }
> 
> JSScope* JSNode::pushEventHandlerScope(ExecState* exec, JSScope* node) const
>diff --git a/Source/WebCore/bindings/js/JSNodeListCustom.cpp b/Source/WebCore/bindings/js/JSNodeListCustom.cpp
>index 9a1c873446953e321fb8e5569c94f3481ccef8f8..7f6d9e72da50795fef35ffd62bd2688c5a10eaf2 100644
>--- a/Source/WebCore/bindings/js/JSNodeListCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSNodeListCustom.cpp
>@@ -37,17 +37,32 @@
> namespace WebCore {
> using namespace JSC;
> 
>-bool JSNodeListOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+bool JSNodeListOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     JSNodeList* jsNodeList = jsCast<JSNodeList*>(handle.slot()->asCell());
>     if (!jsNodeList->hasCustomProperties(*jsNodeList->vm()))
>         return false;
>-    if (jsNodeList->wrapped().isLiveNodeList())
>+
>+    if (jsNodeList->wrapped().isLiveNodeList()) {
>+        if (UNLIKELY(reason))
>+            *reason = "LiveNodeList owner is opaque root";
>+
>         return visitor.containsOpaqueRoot(root(static_cast<LiveNodeList&>(jsNodeList->wrapped()).ownerNode()));
>-    if (jsNodeList->wrapped().isChildNodeList())
>+    }
>+
>+    if (jsNodeList->wrapped().isChildNodeList()) {
>+        if (UNLIKELY(reason))
>+            *reason = "ChildNodeList owner is opaque root";
>+
>         return visitor.containsOpaqueRoot(root(static_cast<ChildNodeList&>(jsNodeList->wrapped()).ownerNode()));
>-    if (jsNodeList->wrapped().isEmptyNodeList())
>+    }
>+
>+    if (jsNodeList->wrapped().isEmptyNodeList()) {
>+        if (UNLIKELY(reason))
>+            *reason = "EmptyNodeList owner is opaque root";
>+
>         return visitor.containsOpaqueRoot(root(static_cast<EmptyNodeList&>(jsNodeList->wrapped()).ownerNode()));
>+    }
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/js/JSOffscreenCanvasRenderingContext2DCustom.cpp b/Source/WebCore/bindings/js/JSOffscreenCanvasRenderingContext2DCustom.cpp
>index 33b08e78a2c2f76574ad8c52d230f6ca0774a529..34d7a099b36678753f54f5ddfc648974b27d4a7f 100644
>--- a/Source/WebCore/bindings/js/JSOffscreenCanvasRenderingContext2DCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSOffscreenCanvasRenderingContext2DCustom.cpp
>@@ -28,8 +28,11 @@ inline void* root(OffscreenCanvas* canvas)
>     return canvas;
> }
> 
>-bool JSOffscreenCanvasRenderingContext2DOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+bool JSOffscreenCanvasRenderingContext2DOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>+    if (UNLIKELY(reason))
>+        *reason = "Canvas is opaque root";
>+
>     JSOffscreenCanvasRenderingContext2D* jsOffscreenCanvasRenderingContext = jsCast<JSOffscreenCanvasRenderingContext2D*>(handle.slot()->asCell());
>     void* root = WebCore::root(&jsOffscreenCanvasRenderingContext->wrapped().canvas());
>     return visitor.containsOpaqueRoot(root);
>diff --git a/Source/WebCore/bindings/js/JSPerformanceObserverCustom.cpp b/Source/WebCore/bindings/js/JSPerformanceObserverCustom.cpp
>index 37e95171122ddda05dd376b61eea1f8e1d3c729f..6efa3dcd2e7ebd35ac1b8627bc2bdb8080a1ca35 100644
>--- a/Source/WebCore/bindings/js/JSPerformanceObserverCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSPerformanceObserverCustom.cpp
>@@ -35,8 +35,11 @@ void JSPerformanceObserver::visitAdditionalChildren(JSC::SlotVisitor& visitor)
>     wrapped().callback().visitJSFunction(visitor);
> }
> 
>-bool JSPerformanceObserverOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor&)
>+bool JSPerformanceObserverOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor&, const char** reason)
> {
>+    if (UNLIKELY(reason))
>+        *reason = "Registered PerformanceObserver callback";
>+
>     return jsCast<JSPerformanceObserver*>(handle.slot()->asCell())->wrapped().isRegistered();
> }
> 
>diff --git a/Source/WebCore/bindings/js/JSPopStateEventCustom.cpp b/Source/WebCore/bindings/js/JSPopStateEventCustom.cpp
>index 722dd9e43403f9dc3432960cf737f4c8f1ad601e..d31bdc779b917ec5e5821f6f89f40bbfe02ada88 100644
>--- a/Source/WebCore/bindings/js/JSPopStateEventCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSPopStateEventCustom.cpp
>@@ -37,6 +37,7 @@
> #include <JavaScriptCore/JSCJSValueInlines.h>
> 
> namespace WebCore {
>+using namespace JSC;
> 
> JSValue JSPopStateEvent::state(ExecState& state) const
> {
>diff --git a/Source/WebCore/bindings/js/JSTextTrackCueCustom.cpp b/Source/WebCore/bindings/js/JSTextTrackCueCustom.cpp
>index c2944513f95e93d0b1aae2a7b50673d813e3a4e3..3c487952c3865a0770a47dabfcd5ce8fab5fd4dd 100644
>--- a/Source/WebCore/bindings/js/JSTextTrackCueCustom.cpp
>+++ b/Source/WebCore/bindings/js/JSTextTrackCueCustom.cpp
>@@ -38,20 +38,26 @@
> namespace WebCore {
> using namespace JSC;
> 
>-bool JSTextTrackCueOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+bool JSTextTrackCueOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     JSTextTrackCue* jsTextTrackCue = jsCast<JSTextTrackCue*>(handle.slot()->asCell());
>     TextTrackCue& textTrackCue = jsTextTrackCue->wrapped();
> 
>     // If the cue is firing event listeners, its wrapper is reachable because
>     // the wrapper is responsible for marking those event listeners.
>-    if (textTrackCue.isFiringEventListeners())
>+    if (textTrackCue.isFiringEventListeners()) {
>+        if (UNLIKELY(reason))
>+            *reason = "TextTrackCue is firing event listeners";
>         return true;
>+    }
> 
>     // If the cue is not associated with a track, it is not reachable.
>     if (!textTrackCue.track())
>         return false;
> 
>+    if (UNLIKELY(reason))
>+        *reason = "TextTrack is an opaque root";
>+
>     return visitor.containsOpaqueRoot(root(textTrackCue.track()));
> }
> 
>diff --git a/Source/WebCore/bindings/js/WebCoreTypedArrayController.cpp b/Source/WebCore/bindings/js/WebCoreTypedArrayController.cpp
>index 84fb3c4f15a8c8380be35b15e61f8e76f5a972a2..20b5f98168dbde3a7f19069319335fa9a54adf5b 100644
>--- a/Source/WebCore/bindings/js/WebCoreTypedArrayController.cpp
>+++ b/Source/WebCore/bindings/js/WebCoreTypedArrayController.cpp
>@@ -53,8 +53,10 @@ bool WebCoreTypedArrayController::isAtomicsWaitAllowedOnCurrentThread()
>     return !isMainThread();
> }
> 
>-bool WebCoreTypedArrayController::JSArrayBufferOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor)
>+bool WebCoreTypedArrayController::JSArrayBufferOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, JSC::SlotVisitor& visitor, const char** reason)
> {
>+    if (UNLIKELY(reason))
>+        *reason = "ArrayBuffer is opaque root";
>     auto& wrapper = *JSC::jsCast<JSC::JSArrayBuffer*>(handle.slot()->asCell());
>     return visitor.containsOpaqueRoot(wrapper.impl());
> }
>diff --git a/Source/WebCore/bindings/js/WebCoreTypedArrayController.h b/Source/WebCore/bindings/js/WebCoreTypedArrayController.h
>index 25260122ebf1197edcf1274971961ef229344736..e0bb956b9944ba22bce97a29676cf81dd0a2b841 100644
>--- a/Source/WebCore/bindings/js/WebCoreTypedArrayController.h
>+++ b/Source/WebCore/bindings/js/WebCoreTypedArrayController.h
>@@ -48,7 +48,7 @@ public:
> private:
>     class JSArrayBufferOwner : public JSC::WeakHandleOwner {
>     public:
>-        bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&) override;
>+        bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**) override;
>         void finalize(JSC::Handle<JSC::Unknown>, void* context) override;
>     };
> 
>diff --git a/Source/WebCore/bindings/scripts/CodeGeneratorJS.pm b/Source/WebCore/bindings/scripts/CodeGeneratorJS.pm
>index 873ed6b02de17f182bcd6ec3022d0d5a7310c784..7f4ac10c9e8af8bdc6dbc865cc7bbfe4a642f3dc 100644
>--- a/Source/WebCore/bindings/scripts/CodeGeneratorJS.pm
>+++ b/Source/WebCore/bindings/scripts/CodeGeneratorJS.pm
>@@ -2657,7 +2657,7 @@ sub GenerateHeader
>     if ($interface->extendedAttributes->{CustomPreventExtensions}) {
>         push(@headerContent, "    static bool preventExtensions(JSC::JSObject*, JSC::ExecState*);\n");
>     }
>-    
>+
>     if (InstanceNeedsEstimatedSize($interface)) {
>         push(@headerContent, "    static size_t estimatedSize(JSCell*, JSC::VM&);\n");
>     }
>@@ -2758,6 +2758,10 @@ sub GenerateHeader
>             push(@headerContent, "    template<typename> static JSC::CompleteSubspace* subspaceFor(JSC::VM& vm) { return $subspaceFunc(vm); }\n");
>         }
>     }
>+
>+    if (NeedsImplementationClass($interface)) {
>+        push(@headerContent, "    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);\n");
>+    }
>     
>     if ($numCustomAttributes > 0) {
>         push(@headerContent, "\n    // Custom attributes\n");
>@@ -2867,7 +2871,7 @@ sub GenerateHeader
>         }
>         $headerIncludes{"<wtf/NeverDestroyed.h>"} = 1;
>         push(@headerContent, "public:\n");
>-        push(@headerContent, "    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);\n");
>+        push(@headerContent, "    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);\n");
>         push(@headerContent, "    virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);\n");
>         push(@headerContent, "};\n");
>         push(@headerContent, "\n");
>@@ -4521,6 +4525,20 @@ sub GenerateImplementation
>         push(@implContent, "}\n\n");
>     }
> 
>+    if (NeedsImplementationClass($interface) && !$interface->extendedAttributes->{CustomHeapSnapshot}) {
>+        AddToImplIncludes("<JavaScriptCore/HeapSnapshotBuilder.h>");
>+        AddToImplIncludes("ScriptExecutionContext.h");
>+        AddToImplIncludes("URL.h");
>+        push(@implContent, "void ${className}::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)\n");
>+        push(@implContent, "{\n");
>+        push(@implContent, "    auto* thisObject = jsCast<${className}*>(cell);\n");
>+        push(@implContent, "    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());\n");
>+        push(@implContent, "    if (thisObject->scriptExecutionContext())\n");
>+        push(@implContent, "        builder.setLabelForCell(cell, String::format(\"url %s\", thisObject->scriptExecutionContext()->url().string().utf8().data()));\n");
>+        push(@implContent, "    Base::heapSnapshot(cell, builder);\n");
>+        push(@implContent, "}\n\n");
>+    }
>+
>     if ($indexedGetterOperation) {
>         $implIncludes{"URL.h"} = 1 if $indexedGetterOperation->type->name eq "DOMString";
>         if ($interfaceName =~ /^HTML\w*Collection$/ or $interfaceName eq "RadioNodeList") {
>@@ -4530,7 +4548,7 @@ sub GenerateImplementation
>     }
> 
>     if (ShouldGenerateWrapperOwnerCode($hasParent, $interface) && !GetCustomIsReachable($interface)) {
>-        push(@implContent, "bool JS${interfaceName}Owner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)\n");
>+        push(@implContent, "bool JS${interfaceName}Owner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)\n");
>         push(@implContent, "{\n");
>         # All ActiveDOMObjects implement hasPendingActivity(), but not all of them
>         # increment their C++ reference counts when hasPendingActivity() becomes
>@@ -4543,23 +4561,29 @@ sub GenerateImplementation
>         if ($codeGenerator->InheritsExtendedAttribute($interface, "ActiveDOMObject")) {
>             push(@implContent, "    auto* js${interfaceName} = jsCast<JS${interfaceName}*>(handle.slot()->asCell());\n");
>             $emittedJSCast = 1;
>-            push(@implContent, "    if (js${interfaceName}->wrapped().hasPendingActivity())\n");
>+            push(@implContent, "    if (js${interfaceName}->wrapped().hasPendingActivity()) {\n");
>+            push(@implContent, "        if (UNLIKELY(reason))\n");
>+            push(@implContent, "            *reason = \"ActiveDOMObject with pending activity\";\n");
>             push(@implContent, "        return true;\n");
>+            push(@implContent, "     }\n");
>         }
>         if ($codeGenerator->InheritsInterface($interface, "EventTarget")) {
>             if (!$emittedJSCast) {
>                 push(@implContent, "    auto* js${interfaceName} = jsCast<JS${interfaceName}*>(handle.slot()->asCell());\n");
>                 $emittedJSCast = 1;
>             }
>-            push(@implContent, "    if (js${interfaceName}->wrapped().isFiringEventListeners())\n");
>+            push(@implContent, "    if (js${interfaceName}->wrapped().isFiringEventListeners()) {\n");
>+            push(@implContent, "        if (UNLIKELY(reason))\n");
>+            push(@implContent, "            *reason = \"EventTarget firing event listeners\";\n");
>             push(@implContent, "        return true;\n");
>+            push(@implContent, "    }\n");
>         }
>         if ($codeGenerator->InheritsInterface($interface, "Node")) {
>             if (!$emittedJSCast) {
>                 push(@implContent, "    auto* js${interfaceName} = jsCast<JS${interfaceName}*>(handle.slot()->asCell());\n");
>                 $emittedJSCast = 1;
>             }
>-            push(@implContent, "    if (JSNodeOwner::isReachableFromOpaqueRoots(handle, 0, visitor))\n");
>+            push(@implContent, "    if (JSNodeOwner::isReachableFromOpaqueRoots(handle, 0, visitor, reason))\n");
>             push(@implContent, "        return true;\n");
>         }
>         if (GetGenerateIsReachable($interface)) {
>@@ -4571,33 +4595,49 @@ sub GenerateImplementation
>             my $rootString;
>             if (GetGenerateIsReachable($interface) eq "Impl") {
>                 $rootString  = "    ${implType}* root = &js${interfaceName}->wrapped();\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from ${interfaceName}\";\n";
>             } elsif (GetGenerateIsReachable($interface) eq "ImplWebGLRenderingContext") {
>                 $rootString  = "    WebGLRenderingContextBase* root = WTF::getPtr(js${interfaceName}->wrapped().context());\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from ${interfaceName}\";\n";
>             } elsif (GetGenerateIsReachable($interface) eq "ImplFrame") {
>                 $rootString  = "    Frame* root = WTF::getPtr(js${interfaceName}->wrapped().frame());\n";
>                 $rootString .= "    if (!root)\n";
>                 $rootString .= "        return false;\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from Frame\";\n";
>             } elsif (GetGenerateIsReachable($interface) eq "ImplDocument") {
>                 $rootString  = "    Document* root = WTF::getPtr(js${interfaceName}->wrapped().document());\n";
>                 $rootString .= "    if (!root)\n";
>                 $rootString .= "        return false;\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from Document\";\n";
>             } elsif (GetGenerateIsReachable($interface) eq "ImplElementRoot") {
>                 $implIncludes{"Element.h"} = 1;
>                 $implIncludes{"JSNodeCustom.h"} = 1;
>                 $rootString  = "    Element* element = WTF::getPtr(js${interfaceName}->wrapped().element());\n";
>                 $rootString .= "    if (!element)\n";
>                 $rootString .= "        return false;\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from ${interfaceName}Owner\";\n";
>                 $rootString .= "    void* root = WebCore::root(element);\n";
>             } elsif (GetGenerateIsReachable($interface) eq "ImplOwnerNodeRoot") {
>                 $implIncludes{"Element.h"} = 1;
>                 $implIncludes{"JSNodeCustom.h"} = 1;
>                 $rootString  = "    void* root = WebCore::root(js${interfaceName}->wrapped().ownerNode());\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from ${interfaceName} ownerNode\";\n";
>             } elsif (GetGenerateIsReachable($interface) eq "ImplScriptExecutionContext") {
>                 $rootString  = "    ScriptExecutionContext* root = WTF::getPtr(js${interfaceName}->wrapped().scriptExecutionContext());\n";
>                 $rootString .= "    if (!root)\n";
>                 $rootString .= "        return false;\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from ScriptExecutionContext\";\n";
>             } else {
>                 $rootString  = "    void* root = WebCore::root(&js${interfaceName}->wrapped());\n";
>+                $rootString .= "    if (UNLIKELY(reason))\n";
>+                $rootString .= "        *reason = \"Reachable from js${interfaceName}\";\n";
>             }
> 
>             push(@implContent, $rootString);
>@@ -4607,6 +4647,7 @@ sub GenerateImplementation
>                 push(@implContent, "    UNUSED_PARAM(handle);\n");
>             }
>             push(@implContent, "    UNUSED_PARAM(visitor);\n");
>+            push(@implContent, "    UNUSED_PARAM(reason);\n");
>             push(@implContent, "    return false;\n");
>         }
>         push(@implContent, "}\n\n");
>diff --git a/Source/WebCore/bindings/scripts/IDLAttributes.json b/Source/WebCore/bindings/scripts/IDLAttributes.json
>index a9991c284bfdfe1809e70bdd3781aa070fd1c42b..73fe7a25f9cd19dcac7e35743fe6ff5fffae21c8 100644
>--- a/Source/WebCore/bindings/scripts/IDLAttributes.json
>+++ b/Source/WebCore/bindings/scripts/IDLAttributes.json
>@@ -118,6 +118,9 @@
>         "CustomGetter": {
>             "contextsAllowed": ["attribute"]
>         },
>+        "CustomHeapSnapshot": {
>+            "contextsAllowed": ["interface"]
>+        },
>         "CustomIndexedSetter": {
>             "contextsAllowed": ["interface"]
>         },
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.cpp b/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.cpp
>index d8c10a2a9b89af090f8fa2edfd134e41f169d060..11795875351bf2e363c914edd741351b742a29ac 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.cpp
>@@ -25,7 +25,10 @@
> #include "JSDOMConstructorNotConstructable.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -168,10 +171,20 @@ size_t JSInterfaceName::estimatedSize(JSCell* cell, VM& vm)
>     return Base::estimatedSize(thisObject, vm) + thisObject->wrapped().memoryCost();
> }
> 
>-bool JSInterfaceNameOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSInterfaceName::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSInterfaceName*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSInterfaceNameOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.h b/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.h
>index 9ddbbc3e8dd8a6903bc6fd23fe94d93a8c436bf4..c60a9177586c63b802b283e58a3c0fa0784a6094 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSInterfaceName.h
>@@ -52,6 +52,7 @@ public:
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>     static void visitChildren(JSCell*, JSC::SlotVisitor&);
> 
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSInterfaceName(JSC::Structure*, JSDOMGlobalObject&, Ref<InterfaceName>&&);
> 
>@@ -60,7 +61,7 @@ protected:
> 
> class JSInterfaceNameOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSMapLike.cpp b/Source/WebCore/bindings/scripts/test/JS/JSMapLike.cpp
>index 761bbf46d9a6e4160b567214fa956295857b649e..6d7faa2b77dad6a473d09ca6ff29b3896c5c4a2b 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSMapLike.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSMapLike.cpp
>@@ -30,8 +30,11 @@
> #include "JSDOMMapLike.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/BuiltinNames.h>
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -337,10 +340,20 @@ EncodedJSValue JSC_HOST_CALL jsMapLikePrototypeFunctionDelete(ExecState* state)
>     return IDLOperation<JSMapLike>::call<jsMapLikePrototypeFunctionDeleteBody>(*state, "delete");
> }
> 
>-bool JSMapLikeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSMapLike::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSMapLike*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSMapLikeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSMapLike.h b/Source/WebCore/bindings/scripts/test/JS/JSMapLike.h
>index d80402d84f6ccc98e4ee2d0f1e2c39842ee878f9..206cd2ef23f51ff266dd948f86b0104b8709c213 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSMapLike.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSMapLike.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSMapLike(JSC::Structure*, JSDOMGlobalObject&, Ref<MapLike>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSMapLikeOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.cpp b/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.cpp
>index c52d101f4b0100e773fe67b52d9611ac83e06579..008dceba42c6d4c6fb2c9a913ffb6d33fd0f1c83 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.cpp
>@@ -30,8 +30,11 @@
> #include "JSDOMMapLike.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/BuiltinNames.h>
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -286,10 +289,20 @@ EncodedJSValue JSC_HOST_CALL jsReadOnlyMapLikePrototypeFunctionForEach(ExecState
>     return IDLOperation<JSReadOnlyMapLike>::call<jsReadOnlyMapLikePrototypeFunctionForEachBody>(*state, "forEach");
> }
> 
>-bool JSReadOnlyMapLikeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSReadOnlyMapLike::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSReadOnlyMapLike*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSReadOnlyMapLikeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.h b/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.h
>index e2dc79f3e43558e93e56a3503509321ffd25bb1d..b6fbb20bc7e71cfcd8518f4e5509678795a5e4d7 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSReadOnlyMapLike.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSReadOnlyMapLike(JSC::Structure*, JSDOMGlobalObject&, Ref<ReadOnlyMapLike>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSReadOnlyMapLikeOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.cpp
>index 7d4907d15fd2ddb6c7bc3887b0df5cfd4d84bf8b..62916d36bb55a6932262ffa55dc24ce6ee3be863 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.cpp
>@@ -32,7 +32,10 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSNode.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -247,10 +250,20 @@ EncodedJSValue JSC_HOST_CALL jsTestActiveDOMObjectPrototypeFunctionPostMessage(E
>     return IDLOperation<JSTestActiveDOMObject>::call<jsTestActiveDOMObjectPrototypeFunctionPostMessageBody>(*state, "postMessage");
> }
> 
>-bool JSTestActiveDOMObjectOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestActiveDOMObject::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestActiveDOMObject*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestActiveDOMObjectOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.h b/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.h
>index 0dd833f749e050ac3ed85affb5baeba6dbd1ebe8..f224633ead1801b470da1bf05b29f02431f2abd2 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestActiveDOMObject.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::HasStaticPropertyTable | Base::StructureFlags;
> protected:
>@@ -59,7 +60,7 @@ protected:
> 
> class JSTestActiveDOMObjectOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.cpp
>index 83740bf795107af42c036168d61075cd504302fb..6406be99903d14c1b17108b8fa3d1292e1fc9070 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.cpp
>@@ -33,7 +33,10 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSTestCEReactionsStringifier.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -427,10 +430,20 @@ EncodedJSValue JSC_HOST_CALL jsTestCEReactionsPrototypeFunctionMethodWithCEReact
>     return IDLOperation<JSTestCEReactions>::call<jsTestCEReactionsPrototypeFunctionMethodWithCEReactionsNotNeededBody>(*state, "methodWithCEReactionsNotNeeded");
> }
> 
>-bool JSTestCEReactionsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestCEReactions::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestCEReactions*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestCEReactionsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.h b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.h
>index e7f801fcdc482ebc2ffebba8fd33c4a6f8f7b4bb..8ac99c9e8a54284e54fd285aec90cd385d9060c3 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactions.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestCEReactions(JSC::Structure*, JSDOMGlobalObject&, Ref<TestCEReactions>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestCEReactionsOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.cpp
>index 8ef20f352e70cd6bec275379e8bc5c6fdd707dcf..adad049b14c4aa5f25aead9460b1c2bfa619224e 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.cpp
>@@ -29,7 +29,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -256,10 +259,20 @@ EncodedJSValue JSC_HOST_CALL jsTestCEReactionsStringifierPrototypeFunctionToStri
>     return IDLOperation<JSTestCEReactionsStringifier>::call<jsTestCEReactionsStringifierPrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestCEReactionsStringifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestCEReactionsStringifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestCEReactionsStringifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestCEReactionsStringifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.h
>index 63b3e163139a6d8c6d6787ca5c85e1b0302daa4c..05c1edb3eb8873491d68fca571e2b8e4d3dd25d1 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCEReactionsStringifier.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestCEReactionsStringifier(JSC::Structure*, JSDOMGlobalObject&, Ref<TestCEReactionsStringifier>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestCEReactionsStringifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.cpp
>index 330a2cec7e031ecabf35e692f4337f2b1551a85a..461c4d96a5f390dc8f190095cb74564a7107a722 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.cpp
>@@ -36,7 +36,10 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSNode.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -520,10 +523,20 @@ EncodedJSValue JSC_HOST_CALL jsTestCallTracerPrototypeFunctionTestOperationWithD
>     return IDLOperation<JSTestCallTracer>::call<jsTestCallTracerPrototypeFunctionTestOperationWithDefaultVariantArgumentBody>(*state, "testOperationWithDefaultVariantArgument");
> }
> 
>-bool JSTestCallTracerOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestCallTracer::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestCallTracer*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestCallTracerOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.h b/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.h
>index dd75a17258aa3b8b339f6849e5fbc17c34195607..c7bca1f19f13dbdb9ee6c92d3971cb559f1ba41c 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCallTracer.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestCallTracer(JSC::Structure*, JSDOMGlobalObject&, Ref<TestCallTracer>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestCallTracerOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.cpp
>index b8b3101c99b173dcd35746724754a160f43a6e4f..1db0a32d55e5cb7c0f94c85f2afc9c982b2eb00d 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.cpp
>@@ -25,8 +25,11 @@
> #include "JSDOMBuiltinConstructor.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "TestClassWithJSBuiltinConstructorBuiltins.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -160,10 +163,20 @@ bool setJSTestClassWithJSBuiltinConstructorConstructor(ExecState* state, Encoded
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestClassWithJSBuiltinConstructorOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestClassWithJSBuiltinConstructor::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestClassWithJSBuiltinConstructor*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestClassWithJSBuiltinConstructorOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.h b/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.h
>index 061b23af23dac061bde16f509824061336512a79..166ee73b0fc8f83ca65f8dd34f23c0cdb27bb73f 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestClassWithJSBuiltinConstructor.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestClassWithJSBuiltinConstructor(JSC::Structure*, JSDOMGlobalObject&, Ref<TestClassWithJSBuiltinConstructor>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestClassWithJSBuiltinConstructorOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.cpp
>index 84f8df209e115ce83d88453239f3217866f8b958..e1f9f170a6cd723dbe95584dcf5fad04242cee88 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.cpp
>@@ -25,7 +25,10 @@
> #include "JSDOMConstructor.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -158,10 +161,20 @@ bool setJSTestCustomConstructorWithNoInterfaceObjectConstructor(ExecState* state
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestCustomConstructorWithNoInterfaceObjectOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestCustomConstructorWithNoInterfaceObject::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestCustomConstructorWithNoInterfaceObject*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestCustomConstructorWithNoInterfaceObjectOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.h b/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.h
>index d3b2b05211ca482136839f6f36f5068687446e1d..4cde2e9bd02f385b6562a64574105376f6503451 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestCustomConstructorWithNoInterfaceObject.h
>@@ -48,6 +48,7 @@ public:
>         return JSC::Structure::create(vm, globalObject, prototype, JSC::TypeInfo(JSC::ObjectType, StructureFlags), info());
>     }
> 
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestCustomConstructorWithNoInterfaceObject(JSC::Structure*, JSDOMGlobalObject&, Ref<TestCustomConstructorWithNoInterfaceObject>&&);
> 
>@@ -56,7 +57,7 @@ protected:
> 
> class JSTestCustomConstructorWithNoInterfaceObjectOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.cpp
>index 93872a6a01d7deebf8db37549cbbd0f088349e9c..76c40fb04579503ea5b52c47e04960db21999f3a 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.cpp
>@@ -40,7 +40,10 @@
> #include "JSDOMWrapperCache.h"
> #include "JSElement.h"
> #include "JSNodeList.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FrameTracers.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -1258,5 +1261,14 @@ JSC::EncodedJSValue JIT_OPERATION unsafeJsTestDOMJITPrototypeFunctionGetElements
>     return JSValue::encode(toJS<IDLInterface<NodeList>>(*state, *castedThis->globalObject(), impl.getElementsByName(WTFMove(elementName))));
> }
> 
>+void JSTestDOMJIT::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestDOMJIT*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> 
> }
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.h b/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.h
>index 0b781e62e3be2f21b389170dadf2ecd31ef01e20..35fb126ec4c6e9a518f8c8f41972016d484130ee 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestDOMJIT.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestDOMJIT& wrapped() const
>     {
>         return static_cast<TestDOMJIT&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.cpp
>index d999032ef2e7a2c31148c187a9c95a7de9a8b826..87b353c5b598589d5902b70e64f390721fa92c93 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.cpp
>@@ -29,9 +29,12 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSTestSubObj.h"
>+#include "ScriptExecutionContext.h"
> #include "Settings.h"
>+#include "URL.h"
> #include "WebCoreJSClientData.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -294,10 +297,20 @@ EncodedJSValue JSC_HOST_CALL jsTestEnabledBySettingPrototypeFunctionEnabledBySet
> 
> #endif
> 
>-bool JSTestEnabledBySettingOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestEnabledBySetting::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestEnabledBySetting*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestEnabledBySettingOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.h b/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.h
>index 9ff37051e0a115f6b816dd6227a1e3409fe83ace..ca1ebb63eea784882233ae5a65e470260f9cf2a3 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestEnabledBySetting.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::HasStaticPropertyTable | Base::StructureFlags;
> protected:
>@@ -59,7 +60,7 @@ protected:
> 
> class JSTestEnabledBySettingOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.cpp
>index 8f1f9b4fc9454248d791ff9d07fa3595a052d7cd..55dea8eeab07f7505f1a6bdfcc241b671cbdea56 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.cpp
>@@ -29,6 +29,9 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -303,6 +306,15 @@ EncodedJSValue jsTestEventConstructorAttr3(ExecState* state, EncodedJSValue this
> 
> #endif
> 
>+void JSTestEventConstructor::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestEventConstructor*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> #if ENABLE(BINDING_INTEGRITY)
> #if PLATFORM(WIN)
> #pragma warning(disable: 4483)
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.h b/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.h
>index e104cedd7f9818acee1990f0a70b4a7817d6fb43..9755360e26a8c1748ef8de0a95ca1f0dc7858832 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestEventConstructor.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestEventConstructor& wrapped() const
>     {
>         return static_cast<TestEventConstructor&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.cpp
>index 17f3d03f0bd4a770f1e9e86299a051efc08d41ec..308e34f001ac2d8b353e8051d8ec51bbadd6ee0a 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.cpp
>@@ -31,6 +31,9 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSNode.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -246,6 +249,15 @@ EncodedJSValue JSC_HOST_CALL jsTestEventTargetPrototypeFunctionItem(ExecState* s
>     return IDLOperation<JSTestEventTarget>::call<jsTestEventTargetPrototypeFunctionItemBody>(*state, "item");
> }
> 
>+void JSTestEventTarget::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestEventTarget*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> #if ENABLE(BINDING_INTEGRITY)
> #if PLATFORM(WIN)
> #pragma warning(disable: 4483)
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.h b/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.h
>index 9ac22200d88b39e04256f24479c5f777f18eebda..eb863955e5c49ff99a0451050f74d46f1124992f 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestEventTarget.h
>@@ -53,6 +53,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestEventTarget& wrapped() const
>     {
>         return static_cast<TestEventTarget&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestException.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestException.cpp
>index 348190e93b1fd58cac0dbb5e50a6061ab6351c16..238a48f7c65f0d3cb6f1e03f7a29e9e829ebcbcf 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestException.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestException.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -177,10 +180,20 @@ EncodedJSValue jsTestExceptionName(ExecState* state, EncodedJSValue thisValue, P
>     return IDLAttribute<JSTestException>::get<jsTestExceptionNameGetter, CastedThisErrorBehavior::Assert>(*state, thisValue, "name");
> }
> 
>-bool JSTestExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestException::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestException*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestException.h b/Source/WebCore/bindings/scripts/test/JS/JSTestException.h
>index 59c8d4c2af7d19a61972bb7b1c59175bcb8c9715..ab25de750382978a00c5e094b8463b5d731f7da1 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestException.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestException.h
>@@ -50,6 +50,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestException(JSC::Structure*, JSDOMGlobalObject&, Ref<TestException>&&);
> 
>@@ -58,7 +59,7 @@ protected:
> 
> class JSTestExceptionOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.cpp
>index 708ef66d69120b9978dcc998454d0c432dcc2525..34238bc5d213ec499fbe5a255c5618f30685b2dc 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.cpp
>@@ -28,7 +28,9 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
> #include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -183,10 +185,21 @@ EncodedJSValue jsTestGenerateIsReachableASecretAttribute(ExecState* state, Encod
>     return IDLAttribute<JSTestGenerateIsReachable>::get<jsTestGenerateIsReachableASecretAttributeGetter, CastedThisErrorBehavior::Assert>(*state, thisValue, "aSecretAttribute");
> }
> 
>-bool JSTestGenerateIsReachableOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestGenerateIsReachable::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestGenerateIsReachable*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestGenerateIsReachableOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     auto* jsTestGenerateIsReachable = jsCast<JSTestGenerateIsReachable*>(handle.slot()->asCell());
>     TestGenerateIsReachable* root = &jsTestGenerateIsReachable->wrapped();
>+    if (UNLIKELY(reason))
>+        *reason = "Reachable from TestGenerateIsReachable";
>     return visitor.containsOpaqueRoot(root);
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.h b/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.h
>index c788ce1ccb7a63aca8adf4b304a6c4b6dc86a937..0065023501d3e8368482cb8ea6a8b7b2dd882e99 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestGenerateIsReachable.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestGenerateIsReachable(JSC::Structure*, JSDOMGlobalObject&, Ref<TestGenerateIsReachable>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestGenerateIsReachableOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.cpp
>index 561d51471bdef373f2ef1a6f83f618320607b476..bda32c37b044bfbd7346b7270a6cab4488c84ea5 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.cpp
>@@ -34,8 +34,10 @@
> #include "JSDOMWrapperCache.h"
> #include "RuntimeEnabledFeatures.h"
> #include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include "WebCoreJSClientData.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -568,10 +570,20 @@ EncodedJSValue JSC_HOST_CALL jsTestGlobalObjectInstanceFunctionTestFeatureGetSec
> 
> #endif
> 
>-bool JSTestGlobalObjectOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestGlobalObject::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestGlobalObject*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestGlobalObjectOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.h b/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.h
>index fe261f5ae10a6d05d85835c93603f1d4fde32281..7fd48750c1df4cc275aa56dee7a423906148545d 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestGlobalObject.h
>@@ -51,6 +51,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::HasStaticPropertyTable | Base::StructureFlags;
> protected:
>@@ -61,7 +62,7 @@ protected:
> 
> class JSTestGlobalObjectOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.cpp
>index efd18c9b6098cbd825f4208dd2064efe07cadef5..66078bd6cb511e1d2f468882582a7f91fbc06bc9 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.cpp
>@@ -26,8 +26,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -246,10 +248,20 @@ bool setJSTestIndexedSetterNoIdentifierConstructor(ExecState* state, EncodedJSVa
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestIndexedSetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestIndexedSetterNoIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestIndexedSetterNoIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestIndexedSetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.h
>index 866c415953a0e8a3cde28c9466ecb2bfa0e8bea0..b7aa998c868bb6dda46ae300acfdfa534fb19c4f 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterNoIdentifier.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestIndexedSetterNoIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.cpp
>index 0119e6bea7a937d5a2615a92f95c71ee848e7f82..6203cc0b987392477a1d9bf7b570cbe37c10e34f 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.cpp
>@@ -26,8 +26,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -246,10 +248,20 @@ bool setJSTestIndexedSetterThrowingExceptionConstructor(ExecState* state, Encode
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestIndexedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestIndexedSetterThrowingException::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestIndexedSetterThrowingException*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestIndexedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.h b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.h
>index 088c4bbf03d88f62be60ff5691c2dd49ccee3c3c..29ad2be67c157c9bbc5391054dc5d465588f76e1 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterThrowingException.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestIndexedSetterThrowingExceptionOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.cpp
>index 8c2974e8edb448eac6f97892b8e75f47ba4434fe..3fdc56040350e1321db24697b7272557dd1a908d 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.cpp
>@@ -28,8 +28,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -278,10 +280,20 @@ EncodedJSValue JSC_HOST_CALL jsTestIndexedSetterWithIdentifierPrototypeFunctionI
>     return IDLOperation<JSTestIndexedSetterWithIdentifier>::call<jsTestIndexedSetterWithIdentifierPrototypeFunctionIndexedSetterBody>(*state, "indexedSetter");
> }
> 
>-bool JSTestIndexedSetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestIndexedSetterWithIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestIndexedSetterWithIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestIndexedSetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.h
>index 83ebf5299e98ef94f7f9fd8c4cf7191481cc28d0..040dfaaae0c54555c25a004fe7f91f28b490a124 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIndexedSetterWithIdentifier.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestIndexedSetterWithIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.cpp
>index b7dee1a8eafa4d94c9a18898fe12947024c5e202..4c64c19eb85826029f60b925b319e11a703d94f7 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.cpp
>@@ -32,8 +32,11 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "TestSupplemental.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -984,12 +987,25 @@ EncodedJSValue JSC_HOST_CALL jsTestInterfaceConstructorFunctionSupplementalMetho
> 
> #endif
> 
>-bool JSTestInterfaceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestInterface::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestInterface*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestInterfaceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     auto* jsTestInterface = jsCast<JSTestInterface*>(handle.slot()->asCell());
>-    if (jsTestInterface->wrapped().hasPendingActivity())
>+    if (jsTestInterface->wrapped().hasPendingActivity()) {
>+        if (UNLIKELY(reason))
>+            *reason = "ActiveDOMObject with pending activity";
>         return true;
>+     }
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.h b/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.h
>index be94dc5c3b6c1899b854e6797f838e31b221c19a..a55a4e1aea426de16a7b8acb2276169c8d4b5cc0 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestInterface.h
>@@ -53,6 +53,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> 
>     // Custom attributes
> #if ENABLE(Condition22) || ENABLE(Condition23)
>@@ -83,7 +84,7 @@ protected:
> 
> class JSTestInterfaceOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.cpp
>index 11a78d7943ace14c58c98c7f05a2ab120056fbb7..55ffd67a150fe32c5eda35e32d246f6932e8ebaa 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -177,10 +180,20 @@ EncodedJSValue jsTestInterfaceLeadingUnderscoreReadonly(ExecState* state, Encode
>     return IDLAttribute<JSTestInterfaceLeadingUnderscore>::get<jsTestInterfaceLeadingUnderscoreReadonlyGetter, CastedThisErrorBehavior::Assert>(*state, thisValue, "readonly");
> }
> 
>-bool JSTestInterfaceLeadingUnderscoreOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestInterfaceLeadingUnderscore::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestInterfaceLeadingUnderscore*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestInterfaceLeadingUnderscoreOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.h b/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.h
>index cd206ca31b003000156aece6d8913958c3325cb3..4cc8d8c728cdd898934c2e0a5ccf8c211ba176f4 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestInterfaceLeadingUnderscore.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestInterfaceLeadingUnderscore(JSC::Structure*, JSDOMGlobalObject&, Ref<TestInterfaceLeadingUnderscore>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestInterfaceLeadingUnderscoreOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.cpp
>index 78c3ff921791d15bf00ad1ffa4f0ac8101beb008..979c0f8cc5bad846f0961cbda264e0cb76c1830c 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.cpp
>@@ -27,8 +27,11 @@
> #include "JSDOMIterator.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/BuiltinNames.h>
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -230,10 +233,20 @@ JSC::EncodedJSValue JSC_HOST_CALL jsTestIterablePrototypeFunctionForEach(JSC::Ex
>     return IDLOperation<JSTestIterable>::call<jsTestIterablePrototypeFunctionForEachCaller>(*state, "forEach");
> }
> 
>-bool JSTestIterableOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestIterable::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestIterable*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestIterableOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.h b/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.h
>index 4630d7e286a0781e56e1216816c7ae011436cec1..6489f26100c7cf67ed58c8a36e29a540de9a006b 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestIterable.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestIterable(JSC::Structure*, JSDOMGlobalObject&, Ref<TestIterable>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestIterableOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.cpp
>index e0d83898cc9925d439a6624a9347d50d95a78735..50ada8caf0ee6d02bada801b0b698aee959d9360 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.cpp
>@@ -29,7 +29,10 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSMediaQueryListListener.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -186,10 +189,20 @@ EncodedJSValue JSC_HOST_CALL jsTestMediaQueryListListenerPrototypeFunctionMethod
>     return IDLOperation<JSTestMediaQueryListListener>::call<jsTestMediaQueryListListenerPrototypeFunctionMethodBody>(*state, "method");
> }
> 
>-bool JSTestMediaQueryListListenerOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestMediaQueryListListener::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestMediaQueryListListener*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestMediaQueryListListenerOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.h b/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.h
>index d34bbde576732e43f4b6633fc7711b9105adf361..d0375e3af77feb133d74d48e21d84da29f15e9ab 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestMediaQueryListListener.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestMediaQueryListListener(JSC::Structure*, JSDOMGlobalObject&, Ref<TestMediaQueryListListener>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestMediaQueryListListenerOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.cpp
>index 17cf7e5034d2262b580f3025083bd2e2120aab81..3187d2daf3d6e721e7e0085f27ccb22f2b41daf9 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.cpp
>@@ -27,8 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -312,10 +314,20 @@ bool setJSTestNamedAndIndexedSetterNoIdentifierConstructor(ExecState* state, Enc
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedAndIndexedSetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedAndIndexedSetterNoIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedAndIndexedSetterNoIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedAndIndexedSetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.h
>index 2b82b0927eec1194f6115136af0e49af9e7547a8..10390d972626a944b3910070bd4a16bcad213c1e 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterNoIdentifier.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedAndIndexedSetterNoIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.cpp
>index d3c324a2d0b90f6aa7978c39275955a2a2aac3ed..d27194e83e6ac776531a9db7730f0574c2dfae1d 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.cpp
>@@ -27,8 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -312,10 +314,20 @@ bool setJSTestNamedAndIndexedSetterThrowingExceptionConstructor(ExecState* state
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedAndIndexedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedAndIndexedSetterThrowingException::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedAndIndexedSetterThrowingException*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedAndIndexedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.h
>index 3cdb6dc7cf0af377deec11f0ddce8e2202d48042..1bfd74c486cfe217490e31f1e736ff4ba12a4437 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterThrowingException.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedAndIndexedSetterThrowingExceptionOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.cpp
>index 99574b20547a301f3b274249bb312b9bf5bbe9b7..b1f36d73e035afe78efe932b14df93ceffba4df2 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.cpp
>@@ -29,8 +29,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -366,10 +368,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedAndIndexedSetterWithIdentifierPrototypeF
>     return IDLOperation<JSTestNamedAndIndexedSetterWithIdentifier>::call<jsTestNamedAndIndexedSetterWithIdentifierPrototypeFunctionIndexedSetterBody>(*state, "indexedSetter");
> }
> 
>-bool JSTestNamedAndIndexedSetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedAndIndexedSetterWithIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedAndIndexedSetterWithIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedAndIndexedSetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.h
>index ddcb655da9b90b2350156ea0292f0398a9f2293c..7e88cf4d41d725d40b9ff9d1951fef51b1887269 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedAndIndexedSetterWithIdentifier.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedAndIndexedSetterWithIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.cpp
>index e3ce42191733972e02bcf6404cce6f39fcd5663d..9298a1760c0835266c42eb664864bf64482c7b30 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.cpp
>@@ -28,7 +28,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMNamedConstructor.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -197,12 +200,25 @@ bool setJSTestNamedConstructorConstructor(ExecState* state, EncodedJSValue thisV
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedConstructorOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedConstructor::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedConstructor*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedConstructorOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     auto* jsTestNamedConstructor = jsCast<JSTestNamedConstructor*>(handle.slot()->asCell());
>-    if (jsTestNamedConstructor->wrapped().hasPendingActivity())
>+    if (jsTestNamedConstructor->wrapped().hasPendingActivity()) {
>+        if (UNLIKELY(reason))
>+            *reason = "ActiveDOMObject with pending activity";
>         return true;
>+     }
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.h
>index 5732bc0e447de180bf0800ba2f23329df97e26be..25c51bd7af3fbf8aa6434bed6c533ecdfd30b2ad 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedConstructor.h
>@@ -50,6 +50,7 @@ public:
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>     static JSC::JSValue getNamedConstructor(JSC::VM&, JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestNamedConstructor(JSC::Structure*, JSDOMGlobalObject&, Ref<TestNamedConstructor>&&);
> 
>@@ -58,7 +59,7 @@ protected:
> 
> class JSTestNamedConstructorOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.cpp
>index 524a60348e2998a4cae7a7048a60a758e16f23aa..57d67c234e26b4ddd0947e402ef89b4502b6dd43 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -225,10 +228,20 @@ bool setJSTestNamedDeleterNoIdentifierConstructor(ExecState* state, EncodedJSVal
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedDeleterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedDeleterNoIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedDeleterNoIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedDeleterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.h
>index fc93e802a011ed0492eefffc23abd1b4f0c34c2d..2479f4f8c8b1dc5479db1093c7297105c5a5b493 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterNoIdentifier.h
>@@ -54,6 +54,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -64,7 +65,7 @@ protected:
> 
> class JSTestNamedDeleterNoIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.cpp
>index e07c9a1aac87397a7221fe0d696e7359aae0b600..d0fcd2f2b9032495b4b658be9b31b7fe55a0886a 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -239,10 +242,20 @@ bool setJSTestNamedDeleterThrowingExceptionConstructor(ExecState* state, Encoded
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedDeleterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedDeleterThrowingException::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedDeleterThrowingException*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedDeleterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.h
>index cde11c68195087221eee36dd51e6535dad286184..7bde9b18964ac2a72fd0e39819432f7df69a7e23 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterThrowingException.h
>@@ -54,6 +54,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -64,7 +65,7 @@ protected:
> 
> class JSTestNamedDeleterThrowingExceptionOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.cpp
>index 6a597416ee3403066dcdc98cceddf30de8975453..251410b5faef7004c0116666fce8e1e90a3379a9 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.cpp
>@@ -28,7 +28,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -256,10 +259,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedDeleterWithIdentifierPrototypeFunctionNa
>     return IDLOperation<JSTestNamedDeleterWithIdentifier>::call<jsTestNamedDeleterWithIdentifierPrototypeFunctionNamedDeleterBody>(*state, "namedDeleter");
> }
> 
>-bool JSTestNamedDeleterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedDeleterWithIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedDeleterWithIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedDeleterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.h
>index 0141708395045b6f6bcd51a331f0915b2f83fd82..4ef3ec18788c40afc7ac26c9497b37d067bab1a3 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIdentifier.h
>@@ -54,6 +54,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -64,7 +65,7 @@ protected:
> 
> class JSTestNamedDeleterWithIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.cpp
>index 1f83a9a67ba88561e3ffd212211f7027b4fdc564..2758ca10a1412c4139b32584f3cf92b403abb014 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.cpp
>@@ -27,8 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -243,10 +245,20 @@ bool setJSTestNamedDeleterWithIndexedGetterConstructor(ExecState* state, Encoded
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedDeleterWithIndexedGetterOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedDeleterWithIndexedGetter::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedDeleterWithIndexedGetter*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedDeleterWithIndexedGetterOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.h
>index 153db7eca306c286430c08e6c7434bbee7f10e17..b3d501787346353e479d7f626179b3a1717731e6 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedDeleterWithIndexedGetter.h
>@@ -54,6 +54,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -64,7 +65,7 @@ protected:
> 
> class JSTestNamedDeleterWithIndexedGetterOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.cpp
>index 5d34b7b891b4edb5687928ee19313e816bb92e04..ca2bf612acbc2fd4e044a159a76467e5aacab9b2 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -204,10 +207,20 @@ bool setJSTestNamedGetterCallWithConstructor(ExecState* state, EncodedJSValue th
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedGetterCallWithOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedGetterCallWith::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedGetterCallWith*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedGetterCallWithOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.h
>index 6bd2e8df879970b2d5bfd8f686fbecf8469c7ef3..36db01798c601c9cee889842ddbcec8b3393e7fd 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterCallWith.h
>@@ -52,6 +52,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -62,7 +63,7 @@ protected:
> 
> class JSTestNamedGetterCallWithOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.cpp
>index 9b0c4b71cfa7f55e2814d06fd6c0983f580cfdfa..9120f4be7f21b78d2c9bff2db1725a94974bfcd1 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -204,10 +207,20 @@ bool setJSTestNamedGetterNoIdentifierConstructor(ExecState* state, EncodedJSValu
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedGetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedGetterNoIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedGetterNoIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedGetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.h
>index 73180d5e2bb04132d08714f254eae69eb7557cae..d37c83f80b7c79ee99eb3226b591d5406ff88ea0 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterNoIdentifier.h
>@@ -52,6 +52,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -62,7 +63,7 @@ protected:
> 
> class JSTestNamedGetterNoIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.cpp
>index c752fd00c9a09eda75bbf498cc63d6c780f54637..3bb7d3b7e12f829c88c344babdb486822f57cba2 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.cpp
>@@ -28,7 +28,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -232,10 +235,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedGetterWithIdentifierPrototypeFunctionGet
>     return IDLOperation<JSTestNamedGetterWithIdentifier>::call<jsTestNamedGetterWithIdentifierPrototypeFunctionGetterNameBody>(*state, "getterName");
> }
> 
>-bool JSTestNamedGetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedGetterWithIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedGetterWithIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedGetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.h
>index f57b9d5fc6e39754d71058a67a18a3d3049cd371..0d55fef234485013f8d761c3c19b3f270b49edc0 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedGetterWithIdentifier.h
>@@ -52,6 +52,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -62,7 +63,7 @@ protected:
> 
> class JSTestNamedGetterWithIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.cpp
>index 48c07113a4c45d41ea350c4bb2bba67ff8f1a8c5..cbd5d31a6267ea1ee25de0914d61468bfcd925b5 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -266,10 +269,20 @@ bool setJSTestNamedSetterNoIdentifierConstructor(ExecState* state, EncodedJSValu
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedSetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterNoIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterNoIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterNoIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.h
>index 4544c51559cf7953304c46c6466abc7c393780f0..637570f216ee383255bc89b4e28ee9ff260aed50 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterNoIdentifier.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterNoIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.cpp
>index 2b7dc95a89d134c154bb35165c566546e1289088..9fcbf697b639ae03ddc0ebb2083ce799f7388353 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -266,10 +269,20 @@ bool setJSTestNamedSetterThrowingExceptionConstructor(ExecState* state, EncodedJ
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterThrowingException::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterThrowingException*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterThrowingExceptionOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.h
>index ef4a26d9001f60daa0592032c61ab6563200a901..5b1713c0cf2bfe1a1214b76fb527168eb3ea3491 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterThrowingException.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterThrowingExceptionOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.cpp
>index e659eb347538fd7c75c4959731129d539ebefd15..b8a9fade85b4e987c7052a029a9dd8fcda6d3250 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.cpp
>@@ -28,7 +28,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -297,10 +300,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedSetterWithIdentifierPrototypeFunctionNam
>     return IDLOperation<JSTestNamedSetterWithIdentifier>::call<jsTestNamedSetterWithIdentifierPrototypeFunctionNamedSetterBody>(*state, "namedSetter");
> }
> 
>-bool JSTestNamedSetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterWithIdentifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterWithIdentifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterWithIdentifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.h
>index bc885b3ca95728467b1a5b73abe56223a0cc8656..0ec45f40f44e0e272fe2c3673666e37be6e1d96a 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIdentifier.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterWithIdentifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.cpp
>index 64a5ca632598657941d75652e3392b6cc6d716d9..171477550b0c38459562f6a75c2e83d8315db924 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.cpp
>@@ -29,8 +29,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -340,10 +342,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedSetterWithIndexedGetterPrototypeFunction
>     return IDLOperation<JSTestNamedSetterWithIndexedGetter>::call<jsTestNamedSetterWithIndexedGetterPrototypeFunctionIndexedSetterBody>(*state, "indexedSetter");
> }
> 
>-bool JSTestNamedSetterWithIndexedGetterOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterWithIndexedGetter::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterWithIndexedGetter*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterWithIndexedGetterOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.h
>index 2f24520fbf19fea040e112fe79e03144d23a802a..dbebe4c48628457f194f10c4807a351c1d6a727e 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetter.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterWithIndexedGetterOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.cpp
>index cccdeb12760eb394f059be87e48073d4d488cde8..103f91c1baa2aa39f1e399aafece581191116dee 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.cpp
>@@ -29,8 +29,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
> #include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/PropertyNameArray.h>
> #include <wtf/GetPtr.h>
>@@ -390,10 +392,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedSetterWithIndexedGetterAndSetterPrototyp
>     return IDLOperation<JSTestNamedSetterWithIndexedGetterAndSetter>::call<jsTestNamedSetterWithIndexedGetterAndSetterPrototypeFunctionIndexedSetterOverloadDispatcher>(*state, "indexedSetter");
> }
> 
>-bool JSTestNamedSetterWithIndexedGetterAndSetterOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterWithIndexedGetterAndSetter::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterWithIndexedGetterAndSetter*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterWithIndexedGetterAndSetterOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.h
>index 45060c755f6575208140dc41eafb66874853a105..c160d68f251a8f6d8523ce3464ebc124e24ed7db 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithIndexedGetterAndSetter.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterWithIndexedGetterAndSetterOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.cpp
>index fb2514cbdd035475592fc002d5a2bc734882663b..fc122a2e2bbf87ba14b16f70799e6c280f1e4c4d 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -253,10 +256,20 @@ bool setJSTestNamedSetterWithOverrideBuiltinsConstructor(ExecState* state, Encod
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestNamedSetterWithOverrideBuiltinsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterWithOverrideBuiltins::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterWithOverrideBuiltins*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterWithOverrideBuiltinsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.h
>index d084ca5f42145c05ea18d0ca3c15f2fd21c35f61..fe037c31325ec1f4496fce1fd7e9574af69f8f9d 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithOverrideBuiltins.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpure | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterWithOverrideBuiltinsOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.cpp
>index c54943a3938d0450be87b24244a53e93064b3dc7..4807b36553318e042a9b67b44408119d684ed1de 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.cpp
>@@ -29,7 +29,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -335,10 +338,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedSetterWithUnforgablePropertiesInstanceFu
>     return IDLOperation<JSTestNamedSetterWithUnforgableProperties>::call<jsTestNamedSetterWithUnforgablePropertiesInstanceFunctionUnforgeableOperationBody>(*state, "unforgeableOperation");
> }
> 
>-bool JSTestNamedSetterWithUnforgablePropertiesOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterWithUnforgableProperties::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterWithUnforgableProperties*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterWithUnforgablePropertiesOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.h
>index 964a17df1268d8464aad9b59c16e1364dc8d4ed4..a16868f2f2c31156e386da03211b1e2f97fc5ef3 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgableProperties.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpureForPropertyAbsence | JSC::HasStaticPropertyTable | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterWithUnforgablePropertiesOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.cpp
>index 4982e17064ba1ea40c086c484207b1415bae0e1e..dd51930733b6cdaa49e1f6856b6b71f1e117cade 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.cpp
>@@ -29,7 +29,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -322,10 +325,20 @@ EncodedJSValue JSC_HOST_CALL jsTestNamedSetterWithUnforgablePropertiesAndOverrid
>     return IDLOperation<JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins>::call<jsTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltinsInstanceFunctionUnforgeableOperationBody>(*state, "unforgeableOperation");
> }
> 
>-bool JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltinsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltinsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.h
>index 385b0a5b60229ba23342200a9aa13169fc2e6c45..9e4267cb60760532e08d19a1d1eb5f69643e9647 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltins.h
>@@ -55,6 +55,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpure | JSC::HasStaticPropertyTable | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -65,7 +66,7 @@ protected:
> 
> class JSTestNamedSetterWithUnforgablePropertiesAndOverrideBuiltinsOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNode.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestNode.cpp
>index 246b17a2449061820180e9fe9dbf16349f7f4cf3..de96d3b8cbf20128b39635db36e0684e9f8667c6 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNode.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNode.cpp
>@@ -34,7 +34,9 @@
> #include "JSDOMWrapperCache.h"
> #include "RuntimeEnabledFeatures.h"
> #include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/BuiltinNames.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/ObjectConstructor.h>
> #include <wtf/GetPtr.h>
>@@ -412,6 +414,15 @@ EncodedJSValue JSC_HOST_CALL jsTestNodePrototypeFunctionToJSON(ExecState* state)
>     return IDLOperation<JSTestNode>::call<jsTestNodePrototypeFunctionToJSONBody>(*state, "toJSON");
> }
> 
>+void JSTestNode::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestNode*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> #if ENABLE(BINDING_INTEGRITY)
> #if PLATFORM(WIN)
> #pragma warning(disable: 4483)
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestNode.h b/Source/WebCore/bindings/scripts/test/JS/JSTestNode.h
>index 8f8d2c388bf7bb97227a4e96866c92f3d11dfde6..e7c25d554ca284fcc251736aeb4d4736a4a474b5 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestNode.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestNode.h
>@@ -49,6 +49,7 @@ public:
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>     static JSC::JSObject* serialize(JSC::ExecState&, JSTestNode& thisObject, JSDOMGlobalObject&, JSC::ThrowScope&);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestNode& wrapped() const
>     {
>         return static_cast<TestNode&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestObj.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestObj.cpp
>index 5deebbb11a1138996f802d1f4a5691741b905557..e4d988597acf92d4fd555ad2fbdf87e04a66d58f 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestObj.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestObj.cpp
>@@ -82,6 +82,7 @@
> #include <JavaScriptCore/ArrayPrototype.h>
> #include <JavaScriptCore/BuiltinNames.h>
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/IteratorOperations.h>
> #include <JavaScriptCore/JSArray.h>
> #include <JavaScriptCore/JSCInlines.h>
>@@ -8389,10 +8390,20 @@ void JSTestObj::visitChildren(JSCell* cell, SlotVisitor& visitor)
>     visitor.append(thisObject->m_cachedAttribute2);
> }
> 
>-bool JSTestObjOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestObj::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestObj*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestObjOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestObj.h b/Source/WebCore/bindings/scripts/test/JS/JSTestObj.h
>index 85424433f82e4febd75f635507b0e7af9d58f954..1e1b467ffe17da906e8faf1d3942f88e1a15c1c8 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestObj.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestObj.h
>@@ -62,6 +62,7 @@ public:
>     mutable JSC::WriteBarrier<JSC::Unknown> m_cachedAttribute2;
>     static void visitChildren(JSCell*, JSC::SlotVisitor&);
> 
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> 
>     // Custom attributes
>     JSC::JSValue customAttr(JSC::ExecState&) const;
>@@ -84,7 +85,7 @@ protected:
> 
> class JSTestObjOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.cpp
>index 38aa4ab268ba82ddb4f0e4bc1937e5038d2f0b71..eaf092af3f3a71e9d3c4cf6011bfd9192ab21b5a 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.cpp
>@@ -31,7 +31,10 @@
> #include "JSDOMConvertVariadic.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -249,10 +252,20 @@ bool setJSTestOverloadedConstructorsConstructor(ExecState* state, EncodedJSValue
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestOverloadedConstructorsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestOverloadedConstructors::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestOverloadedConstructors*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestOverloadedConstructorsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.h b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.h
>index b78d83a78a88a95e0a9faa20c5131e3cc7f55967..289a635abf5196673aa11bd46ab0fadb835ae913 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructors.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestOverloadedConstructors(JSC::Structure*, JSDOMGlobalObject&, Ref<TestOverloadedConstructors>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestOverloadedConstructorsOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.cpp
>index 1dc8bc98288f6cfe06a6b6f8f9ccebf81641f01f..058143ba14bb4eb22409c8d28473753d84f03e7f 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.cpp
>@@ -28,7 +28,10 @@
> #include "JSDOMConvertStrings.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/IteratorOperations.h>
> #include <JavaScriptCore/JSArray.h>
> #include <JavaScriptCore/JSCInlines.h>
>@@ -205,10 +208,20 @@ bool setJSTestOverloadedConstructorsWithSequenceConstructor(ExecState* state, En
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestOverloadedConstructorsWithSequenceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestOverloadedConstructorsWithSequence::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestOverloadedConstructorsWithSequence*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestOverloadedConstructorsWithSequenceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.h b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.h
>index e358daa4d24fc3d23e09cde4ea18e907774c471d..e0f54ebedffe2f2dd1b59ebb0a8739b6b6df1682 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestOverloadedConstructorsWithSequence.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestOverloadedConstructorsWithSequence(JSC::Structure*, JSDOMGlobalObject&, Ref<TestOverloadedConstructorsWithSequence>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestOverloadedConstructorsWithSequenceOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.cpp
>index d890d3e7197e2a056bc77934e61228dd3010a43e..0db774af125f5d1850f79a6171475e137ab7cd61 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.cpp
>@@ -31,7 +31,10 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSNode.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -235,10 +238,20 @@ EncodedJSValue JSC_HOST_CALL jsTestOverrideBuiltinsPrototypeFunctionNamedItem(Ex
>     return IDLOperation<JSTestOverrideBuiltins>::call<jsTestOverrideBuiltinsPrototypeFunctionNamedItemBody>(*state, "namedItem");
> }
> 
>-bool JSTestOverrideBuiltinsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestOverrideBuiltins::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestOverrideBuiltins*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestOverrideBuiltinsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.h b/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.h
>index f5be072cc8d6e8a1370546955d9817db0f569082..f2a26af39a46c1879f56cb3cd81c67970cf2dbfa 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestOverrideBuiltins.h
>@@ -52,6 +52,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::GetOwnPropertySlotIsImpure | JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetOwnPropertySlot | JSC::OverridesGetPropertyNames | Base::StructureFlags;
> protected:
>@@ -62,7 +63,7 @@ protected:
> 
> class JSTestOverrideBuiltinsOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.cpp
>index de490c5f09a9aaa8cd1fdba42d1764a4c09af804..28362473a709f59646bbeb604be3da111a0a8ced 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.cpp
>@@ -26,7 +26,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
> #include "JSPluginElementFunctions.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -208,10 +211,20 @@ bool setJSTestPluginInterfaceConstructor(ExecState* state, EncodedJSValue thisVa
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>-bool JSTestPluginInterfaceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestPluginInterface::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestPluginInterface*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestPluginInterfaceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.h b/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.h
>index 37a30ec9bf25254d26d23bcb328b681d24d46f10..6ddb79c5c5cf01ad04febfd7133577c60516f7e8 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestPluginInterface.h
>@@ -56,6 +56,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | JSC::OverridesGetCallData | JSC::OverridesGetOwnPropertySlot | Base::StructureFlags;
> protected:
>@@ -66,7 +67,7 @@ protected:
> 
> class JSTestPluginInterfaceOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.cpp
>index 07a93b39e11ed663fa2c3e8d6af6077b67db1808..8b0f51ed31941c84aeb24a373be9b17c07f763d7 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.cpp
>@@ -33,6 +33,9 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMGlobalObject.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -285,6 +288,15 @@ EncodedJSValue jsTestPromiseRejectionEventReason(ExecState* state, EncodedJSValu
>     return IDLAttribute<JSTestPromiseRejectionEvent>::get<jsTestPromiseRejectionEventReasonGetter, CastedThisErrorBehavior::Assert>(*state, thisValue, "reason");
> }
> 
>+void JSTestPromiseRejectionEvent::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestPromiseRejectionEvent*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> #if ENABLE(BINDING_INTEGRITY)
> #if PLATFORM(WIN)
> #pragma warning(disable: 4483)
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.h b/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.h
>index 442682434b02c1495ed55067fb94f3a23ffb477f..f4695fa4c5ad6665805026918ee9fd6ddc9646e9 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestPromiseRejectionEvent.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestPromiseRejectionEvent& wrapped() const
>     {
>         return static_cast<TestPromiseRejectionEvent&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.cpp
>index 2b2cc626b4908131f99bf2a14bae216c6c4511d8..e3c1cc259a1d27879a267d9f4458d5d489a68792 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.cpp
>@@ -35,7 +35,10 @@
> #include "JSTestException.h"
> #include "JSTestSerializationIndirectInheritance.h"
> #include "JSTestSerializationInheritFinal.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/ObjectConstructor.h>
> #include <wtf/GetPtr.h>
>@@ -542,10 +545,20 @@ EncodedJSValue JSC_HOST_CALL jsTestSerializationPrototypeFunctionToJSON(ExecStat
>     return IDLOperation<JSTestSerialization>::call<jsTestSerializationPrototypeFunctionToJSONBody>(*state, "toJSON");
> }
> 
>-bool JSTestSerializationOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestSerialization::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestSerialization*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestSerializationOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.h b/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.h
>index 009d66f4bcdd4e147bceb9805976b8780a00681e..d272847f7b43d9ed5381367b3541bbbf31347ef8 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerialization.h
>@@ -50,6 +50,7 @@ public:
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>     static JSC::JSObject* serialize(JSC::ExecState&, JSTestSerialization& thisObject, JSDOMGlobalObject&, JSC::ThrowScope&);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestSerialization(JSC::Structure*, JSDOMGlobalObject&, Ref<TestSerialization>&&);
> 
>@@ -58,7 +59,7 @@ protected:
> 
> class JSTestSerializationOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.cpp
>index 86e1cd99f6636b5ba90e4bc70e75f669eec4104c..96368f3e6f7bffa8e1a7efbfe7f5063f646dbda7 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.cpp
>@@ -25,6 +25,9 @@
> #include "JSDOMConstructorNotConstructable.h"
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -146,5 +149,14 @@ bool setJSTestSerializationIndirectInheritanceConstructor(ExecState* state, Enco
>     return prototype->putDirect(vm, vm.propertyNames->constructor, JSValue::decode(encodedValue));
> }
> 
>+void JSTestSerializationIndirectInheritance::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestSerializationIndirectInheritance*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> 
> }
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.h b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.h
>index a1335fadc915d454cc5a4b88775ed48da5323b80..0dd045ae685eddacf95ecdbab3eddae15c8a5bee 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationIndirectInheritance.h
>@@ -48,6 +48,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestSerializationIndirectInheritance& wrapped() const
>     {
>         return static_cast<TestSerializationIndirectInheritance&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.cpp
>index a77a5888f3072d185f95a123269b37127e8d1976..37f654ddfcd7c15b78eb671c4aac6785d5400bb7 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.cpp
>@@ -28,6 +28,9 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/ObjectConstructor.h>
> #include <wtf/GetPtr.h>
>@@ -221,5 +224,14 @@ EncodedJSValue JSC_HOST_CALL jsTestSerializationInheritPrototypeFunctionToJSON(E
>     return IDLOperation<JSTestSerializationInherit>::call<jsTestSerializationInheritPrototypeFunctionToJSONBody>(*state, "toJSON");
> }
> 
>+void JSTestSerializationInherit::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestSerializationInherit*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> 
> }
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.h b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.h
>index 0673854c2f01cce43c8ef8ad1fea4164bb81edd1..02873a26fcc041bc98a64a557b3ec978e3ebca50 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInherit.h
>@@ -49,6 +49,7 @@ public:
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>     static JSC::JSObject* serialize(JSC::ExecState&, JSTestSerializationInherit& thisObject, JSDOMGlobalObject&, JSC::ThrowScope&);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestSerializationInherit& wrapped() const
>     {
>         return static_cast<TestSerializationInherit&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.cpp
>index 52504f18136472a1261b34747905ccb742295be6..5e86910ab52088e317793649d4c6072ac4e72b94 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.cpp
>@@ -28,6 +28,9 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <JavaScriptCore/ObjectConstructor.h>
> #include <wtf/GetPtr.h>
>@@ -255,5 +258,14 @@ EncodedJSValue JSC_HOST_CALL jsTestSerializationInheritFinalPrototypeFunctionToJ
>     return IDLOperation<JSTestSerializationInheritFinal>::call<jsTestSerializationInheritFinalPrototypeFunctionToJSONBody>(*state, "toJSON");
> }
> 
>+void JSTestSerializationInheritFinal::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestSerializationInheritFinal*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
> 
> }
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.h b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.h
>index d16f85fc53fb0de09f91bd69df5e681c9163cd48..7c475752d40c301b244933ce0103968943101682 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializationInheritFinal.h
>@@ -49,6 +49,7 @@ public:
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>     static JSC::JSObject* serialize(JSC::ExecState&, JSTestSerializationInheritFinal& thisObject, JSDOMGlobalObject&, JSC::ThrowScope&);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
>     TestSerializationInheritFinal& wrapped() const
>     {
>         return static_cast<TestSerializationInheritFinal&>(Base::wrapped());
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.cpp
>index 32af6939f07a5c899f9bcb3ddc5470f6eb752453..342ed1aa50b46868bfb39166ae0f83d08c6802a8 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.cpp
>@@ -35,8 +35,11 @@
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
> #include "JSMessagePort.h"
>+#include "ScriptExecutionContext.h"
> #include "SerializedScriptValue.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSArray.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
>@@ -345,10 +348,20 @@ void JSTestSerializedScriptValueInterface::visitChildren(JSCell* cell, SlotVisit
>     visitor.append(thisObject->m_cachedReadonlyValue);
> }
> 
>-bool JSTestSerializedScriptValueInterfaceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestSerializedScriptValueInterface::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestSerializedScriptValueInterface*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestSerializedScriptValueInterfaceOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.h b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.h
>index 069a1d5de134cf050ffb9a1a87da4f4f14aa1bf8..feaec894c070202752c76f5da4b3eea80fc59a00 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestSerializedScriptValueInterface.h
>@@ -55,6 +55,7 @@ public:
>     mutable JSC::WriteBarrier<JSC::Unknown> m_cachedReadonlyValue;
>     static void visitChildren(JSCell*, JSC::SlotVisitor&);
> 
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestSerializedScriptValueInterface(JSC::Structure*, JSDOMGlobalObject&, Ref<TestSerializedScriptValueInterface>&&);
> 
>@@ -63,7 +64,7 @@ protected:
> 
> class JSTestSerializedScriptValueInterfaceOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.cpp
>index ac39bc84e91dea974c7594d3c31c9707753eea6a..ff2cad2caffc9adf1affa53cdf0977cfc83e8715 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -179,10 +182,20 @@ EncodedJSValue JSC_HOST_CALL jsTestStringifierPrototypeFunctionToString(ExecStat
>     return IDLOperation<JSTestStringifier>::call<jsTestStringifierPrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestStringifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestStringifier::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestStringifier*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestStringifierOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.h b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.h
>index f432bee618a68fbfb024efbf87c76dda6d1a62b7..55381f70a52d8d53b33bfe059c78ad5031815284 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifier.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestStringifier(JSC::Structure*, JSDOMGlobalObject&, Ref<TestStringifier>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestStringifierOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.cpp
>index 1e4730eb20e9495b2da8040d74ea713e36d2eaf9..89edfb4d54ffce29312e9dc370a2904e8638948f 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -179,10 +182,20 @@ EncodedJSValue JSC_HOST_CALL jsTestStringifierAnonymousOperationPrototypeFunctio
>     return IDLOperation<JSTestStringifierAnonymousOperation>::call<jsTestStringifierAnonymousOperationPrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestStringifierAnonymousOperationOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestStringifierAnonymousOperation::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestStringifierAnonymousOperation*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestStringifierAnonymousOperationOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.h b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.h
>index f29d3e0fde5aae9e97d3d46936e743f96f573ecf..14aa0ed3ee9aced16a57bcb45932282d0d49aa82 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierAnonymousOperation.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestStringifierAnonymousOperation(JSC::Structure*, JSDOMGlobalObject&, Ref<TestStringifierAnonymousOperation>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestStringifierAnonymousOperationOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.cpp
>index d16ed4bb9ecc0e657d0884c317317fe38b852df4..b16b6fb4bd14e24d2dae0f44babea64d84700f44 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -194,10 +197,20 @@ EncodedJSValue JSC_HOST_CALL jsTestStringifierNamedOperationPrototypeFunctionToS
>     return IDLOperation<JSTestStringifierNamedOperation>::call<jsTestStringifierNamedOperationPrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestStringifierNamedOperationOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestStringifierNamedOperation::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestStringifierNamedOperation*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestStringifierNamedOperationOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.h b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.h
>index 860de6cbb5aa6358347bc93e0853c3cd452793f9..5bc10ef65d2194401a53e3e5cc2318fd45fbbe54 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierNamedOperation.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestStringifierNamedOperation(JSC::Structure*, JSDOMGlobalObject&, Ref<TestStringifierNamedOperation>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestStringifierNamedOperationOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.cpp
>index b8da5d6f5dcc3f1e5c22522aef406073ebf510d1..5a54ac143cb279364e646c3334b0cbc17ae83435 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -194,10 +197,20 @@ EncodedJSValue JSC_HOST_CALL jsTestStringifierOperationImplementedAsPrototypeFun
>     return IDLOperation<JSTestStringifierOperationImplementedAs>::call<jsTestStringifierOperationImplementedAsPrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestStringifierOperationImplementedAsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestStringifierOperationImplementedAs::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestStringifierOperationImplementedAs*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestStringifierOperationImplementedAsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.h b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.h
>index 9d407009141bdacb6decc6e3cb3831de80d2c63a..f93ff116869596bf7efc28c9ee6275adffb586ec 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationImplementedAs.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestStringifierOperationImplementedAs(JSC::Structure*, JSDOMGlobalObject&, Ref<TestStringifierOperationImplementedAs>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestStringifierOperationImplementedAsOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.cpp
>index c2edea8d3bc91c926a3c77be9c4646629db81123..f2518a71ed6861c5a8caf6beb34c5fcf6bd30def 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.cpp
>@@ -27,7 +27,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -179,10 +182,20 @@ EncodedJSValue JSC_HOST_CALL jsTestStringifierOperationNamedToStringPrototypeFun
>     return IDLOperation<JSTestStringifierOperationNamedToString>::call<jsTestStringifierOperationNamedToStringPrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestStringifierOperationNamedToStringOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestStringifierOperationNamedToString::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestStringifierOperationNamedToString*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestStringifierOperationNamedToStringOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.h b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.h
>index ff84566bc8f27480a9706c081fbcb15a8fa2d2ab..12de238e0f628d609fb18c9c05c5f800c6b24ac8 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierOperationNamedToString.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestStringifierOperationNamedToString(JSC::Structure*, JSDOMGlobalObject&, Ref<TestStringifierOperationNamedToString>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestStringifierOperationNamedToStringOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.cpp
>index d1e37d163d76109c63af969514940771df5e7e53..812372b4d4ee70c423a284334651d1158eaace59 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.cpp
>@@ -28,7 +28,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -201,10 +204,20 @@ EncodedJSValue JSC_HOST_CALL jsTestStringifierReadOnlyAttributePrototypeFunction
>     return IDLOperation<JSTestStringifierReadOnlyAttribute>::call<jsTestStringifierReadOnlyAttributePrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestStringifierReadOnlyAttributeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestStringifierReadOnlyAttribute::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestStringifierReadOnlyAttribute*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestStringifierReadOnlyAttributeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.h b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.h
>index 342ca59fa1dd818984f8340c9fd1ec44badf0cb5..6dcbdc8d955e5b2deb5eb0125053c22528b5d491 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadOnlyAttribute.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestStringifierReadOnlyAttribute(JSC::Structure*, JSDOMGlobalObject&, Ref<TestStringifierReadOnlyAttribute>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestStringifierReadOnlyAttributeOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.cpp
>index aa54121f8c270b317e4bcecb27824eb3bb0301cb..3917b4d9296ba523dc13070619cf4ced38a83fb1 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.cpp
>@@ -28,7 +28,10 @@
> #include "JSDOMExceptionHandling.h"
> #include "JSDOMOperation.h"
> #include "JSDOMWrapperCache.h"
>+#include "ScriptExecutionContext.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
> #include <wtf/PointerPreparations.h>
>@@ -219,10 +222,20 @@ EncodedJSValue JSC_HOST_CALL jsTestStringifierReadWriteAttributePrototypeFunctio
>     return IDLOperation<JSTestStringifierReadWriteAttribute>::call<jsTestStringifierReadWriteAttributePrototypeFunctionToStringBody>(*state, "toString");
> }
> 
>-bool JSTestStringifierReadWriteAttributeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestStringifierReadWriteAttribute::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestStringifierReadWriteAttribute*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestStringifierReadWriteAttributeOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.h b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.h
>index dda4d9d1041ee057077a6263d407623f99d189a7..cb21c3ed45501284a35d2f645cfa9b1035bf9398 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestStringifierReadWriteAttribute.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> protected:
>     JSTestStringifierReadWriteAttribute(JSC::Structure*, JSDOMGlobalObject&, Ref<TestStringifierReadWriteAttribute>&&);
> 
>@@ -57,7 +58,7 @@ protected:
> 
> class JSTestStringifierReadWriteAttributeOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.cpp b/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.cpp
>index ad7fa7fe057229b2bc99d3dfec6b1595598c6e80..1e613aab06524af5560633d8822497fd1c01e0a2 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.cpp
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.cpp
>@@ -43,8 +43,11 @@
> #include "JSTestCallbackInterface.h"
> #include "JSTestEventTarget.h"
> #include "JSTestSubObj.h"
>+#include "ScriptExecutionContext.h"
> #include "SerializedScriptValue.h"
>+#include "URL.h"
> #include <JavaScriptCore/FunctionPrototype.h>
>+#include <JavaScriptCore/HeapSnapshotBuilder.h>
> #include <JavaScriptCore/JSArray.h>
> #include <JavaScriptCore/JSCInlines.h>
> #include <wtf/GetPtr.h>
>@@ -741,10 +744,20 @@ EncodedJSValue JSC_HOST_CALL jsTestTypedefsPrototypeFunctionMethodWithException(
>     return IDLOperation<JSTestTypedefs>::call<jsTestTypedefsPrototypeFunctionMethodWithExceptionBody>(*state, "methodWithException");
> }
> 
>-bool JSTestTypedefsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
>+void JSTestTypedefs::heapSnapshot(JSCell* cell, HeapSnapshotBuilder& builder)
>+{
>+    auto* thisObject = jsCast<JSTestTypedefs*>(cell);
>+    builder.setWrappedObjectForCell(cell, &thisObject->wrapped());
>+    if (thisObject->scriptExecutionContext())
>+        builder.setLabelForCell(cell, String::format("url %s", thisObject->scriptExecutionContext()->url().string().utf8().data()));
>+    Base::heapSnapshot(cell, builder);
>+}
>+
>+bool JSTestTypedefsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor, const char** reason)
> {
>     UNUSED_PARAM(handle);
>     UNUSED_PARAM(visitor);
>+    UNUSED_PARAM(reason);
>     return false;
> }
> 
>diff --git a/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.h b/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.h
>index d8fb0f5c5566717dc7b19c949b9fddeb7eca1e5e..36f2aed83a4a1a617ccfd38285745b0eb895f6ee 100644
>--- a/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.h
>+++ b/Source/WebCore/bindings/scripts/test/JS/JSTestTypedefs.h
>@@ -49,6 +49,7 @@ public:
>     }
> 
>     static JSC::JSValue getConstructor(JSC::VM&, const JSC::JSGlobalObject*);
>+    static void heapSnapshot(JSCell*, JSC::HeapSnapshotBuilder&);
> public:
>     static const unsigned StructureFlags = JSC::HasStaticPropertyTable | Base::StructureFlags;
> protected:
>@@ -59,7 +60,7 @@ protected:
> 
> class JSTestTypedefsOwner : public JSC::WeakHandleOwner {
> public:
>-    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&);
>+    virtual bool isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void* context, JSC::SlotVisitor&, const char**);
>     virtual void finalize(JSC::Handle<JSC::Unknown>, void* context);
> };
> 
>diff --git a/Source/WebCore/dom/Document.idl b/Source/WebCore/dom/Document.idl
>index edf34a9e0a71b2bb7b04317e47f047ad93942897..35ff9c057c3e473ff0fe97485ec0dc48cc848b28 100644
>--- a/Source/WebCore/dom/Document.idl
>+++ b/Source/WebCore/dom/Document.idl
>@@ -35,6 +35,7 @@ typedef (
>     Constructor,
>     ConstructorCallWith=Document,
>     CustomToJSObject,
>+    CustomHeapSnapshot,
>     DOMJIT,
>     ExportMacro=WEBCORE_EXPORT,
>     JSCustomHeader,
>diff --git a/Source/WebCore/page/DOMWindow.idl b/Source/WebCore/page/DOMWindow.idl
>index a32d0f60839510c1353e802c6dad31c825ff223d..2b4150f6387d057c107eedeff56149d1ac730fe0 100644
>--- a/Source/WebCore/page/DOMWindow.idl
>+++ b/Source/WebCore/page/DOMWindow.idl
>@@ -33,6 +33,7 @@ typedef USVString CSSOMString;
>     CustomGetOwnPropertyNames,
>     CustomGetOwnPropertySlot,
>     CustomGetPrototype,
>+    CustomHeapSnapshot,
>     CustomPreventExtensions,
>     CustomProxyToJSObject,
>     CustomPut,
>diff --git a/Source/WebInspectorUI/UserInterface/Workers/HeapSnapshot/HeapSnapshot.js b/Source/WebInspectorUI/UserInterface/Workers/HeapSnapshot/HeapSnapshot.js
>index a8f2c3303177a65f869ef0aca50ceb1bbc2f83b0..745774bb39abb60ecc230ce34b1d4f3fdc402d44 100644
>--- a/Source/WebInspectorUI/UserInterface/Workers/HeapSnapshot/HeapSnapshot.js
>+++ b/Source/WebInspectorUI/UserInterface/Workers/HeapSnapshot/HeapSnapshot.js
>@@ -84,6 +84,7 @@ HeapSnapshot = class HeapSnapshot
> 
>         let {version, nodes, nodeClassNames, edges, edgeTypes, edgeNames} = json;
>         console.assert(version === 1, "Expect JavaScriptCore Heap Snapshot version 1");
>+        console.assert(!type || type === "Inspector", "Expect an Inspector Heap Snapshot");
> 
>         this._nodes = nodes;
>         this._nodeCount = nodes.length / nodeFieldCount;
>diff --git a/Tools/ChangeLog b/Tools/ChangeLog
>index f038e3809fb958d6ddb93f464f5fe26c2009b7d0..afe75f9a89f251b9190686671b70569385adcbdb 100644
>--- a/Tools/ChangeLog
>+++ b/Tools/ChangeLog
>@@ -1,3 +1,20 @@
>+2018-08-23  Simon Fraser  <simon.fraser@apple.com>
>+
>+        Add support for dumping GC heap snapshots, and a viewer
>+        https://bugs.webkit.org/show_bug.cgi?id=186416
>+
>+        Reviewed by Joseph Pecoraro.
>+
>+        Add a viewer for GC heap snapshots. A snapshot JSON file can be dragged into this
>+        page for inspection (or set via the 'filename' URL parameter).
>+
>+        For now, this page shows all objects, all roots, and the shortest path from a root
>+        to all HTMLDocuments and Windows.
>+
>+        * GCHeapInspector/gc-heap-inspector.html: Added.
>+        * GCHeapInspector/heap-analysis/HeapSnapshot.js: Copied from Source/WebInspectorUI/UserInterface/Workers/HeapSnapshot/HeapSnapshot.js.
>+        * GCHeapInspector/script/interface.js: Added.
>+
> 2018-08-23  Myles C. Maxfield  <mmaxfield@apple.com>
> 
>         [WHLSL] Allow native types to have type arguments (like "vector<float, 4>")
>diff --git a/Tools/GCHeapInspector/gc-heap-inspector.html b/Tools/GCHeapInspector/gc-heap-inspector.html
>new file mode 100644
>index 0000000000000000000000000000000000000000..2739a93ae96b1bd123d897a8ea32187263f2c99a
>--- /dev/null
>+++ b/Tools/GCHeapInspector/gc-heap-inspector.html
>@@ -0,0 +1,178 @@
>+<!DOCTYPE html>
>+<head>
>+    <title>GC Heap Inspector</title>
>+    <meta charset="utf8">
>+    <style type="text/css" media="screen">
>+
>+        body {
>+            font-family: "Helvetica Neue";
>+            line-height: 1.3;
>+        }
>+
>+        h1 {
>+            margin-bottom: 0.2em;
>+            font-size: 32px;
>+        }
>+
>+        label {
>+            font-size: 13px;
>+        }
>+
>+        #dropTarget {
>+            font-size: 12pt;
>+            font-weight: bold;
>+            color: #888;
>+            position: absolute;
>+            top: 20px;
>+            right: 20px;
>+            border: 2px solid rgba(0, 0, 0, 0.3);
>+            background-color: rgba(0, 0, 0, 0.1);
>+            padding: 10px;
>+            border-radius: 10px;
>+        }
>+
>+        #dropTarget.dragOver {
>+            border: 2px solid rgba(0, 0, 0, 0.1);
>+            background-color: rgba(0, 0, 0, 0.5);
>+            color: #ddd;
>+        }
>+        
>+        details {
>+            margin-left: 20px;
>+        }
>+        
>+        ul.path {
>+            margin-top: 5px;
>+        }
>+        
>+        ul.instance-list {
>+            margin-top: 0.3em;
>+            padding-left: 1.2em;
>+            list-style-type: none;
>+        }
>+
>+        .node-id::before {
>+            content: '@';
>+        }
>+
>+        .node-id {
>+            font-size: smaller;
>+            color: silver;
>+            display: none; /* Remove if you want to see cell identifiers. */
>+        }
>+
>+        .node-address, .node-size, .node-label {
>+            font-family: monospace;
>+            color: gray;
>+        }
>+        
>+        .node-size {
>+            display: none;
>+        }
>+        
>+        .node-gc-root {
>+            font-size: smaller;
>+            color: maroon;
>+        }
>+        
>+        .node-gc-root img {
>+            margin: 0 3px;
>+            height: 1.4em;
>+            width: 1.4em;
>+            vertical-align: middle;
>+            cursor: pointer;
>+            display: none;
>+        }
>+        
>+        .edge::before {
>+            content: ' â¹ ';
>+            color: gray;
>+        }
>+
>+        .edge::after {
>+            content: ' â¹';
>+            color: gray;
>+        }
>+        
>+        .edge-data {
>+            font-family: monospace;
>+            color: gray;
>+        }
>+        
>+        ul.path {
>+            list-style-type: none;
>+        }
>+        
>+        #description {
>+            position: fixed;
>+            margin: 300px calc((100vw - 500px) / 2);
>+            border: 2px solid rgba(0, 0, 0, 0.3);
>+            background-color: rgba(0, 0, 0, 0.1);
>+            padding: 20px 50px;
>+            border-radius: 10px;
>+            width: 500px;
>+        }
>+
>+        #description > h1 {
>+            margin-top: 10px;
>+            margin-bottom: 15px;
>+        }
>+        
>+        #description code {
>+            display: block;
>+            margin: 15px 0;
>+        }
>+        
>+        #description.hidden {
>+            display: none;
>+        }
>+        
>+        #uiContainer {
>+            display: none;
>+        }
>+        
>+    </style>
>+
>+    <script src="script/interface.js"></script>
>+    <script src="heap-analysis/HeapSnapshot.js"></script>
>+</head>
>+<body>
>+    <header>
>+    <div id="dropTarget">Drop GC heap JSON file here to load.</div>
>+    </header>
>+
>+    <section id="description">
>+        <h1>GC heap debugger</h2>
>+        <p>This page is for analyzing JSON dumps of the GC heap. To debug an issue that you think might be a leaked or abandoned object (often a Document or JSGlobalObject), load the test page, then navigate to about:blank or other simple page. Now simulate a memory warning (to clear the page cache) by issuing the following command in a Terminal window:
>+        <code>notifyutil -p org.WebKit.lowMemory</code>
>+        
>+        You can now inspect the list of live documents via:
>+        <code>notifyutil -p com.apple.WebKit.showAllDocuments</code>
>+        
>+        which prints its output to the system log. If this lists documents other than the current page, you may have a leaked or abandoned Document. To see if the GC heap is referencing that Document, obtain a GC heap dump:
>+        <code>notifyutil -p com.apple.WebKit.dumpGCHeap</code>
>+        
>+        That command will generate a JSON file in /tmp (or equivalent); the file path is dumped to the system log. Drag that file onto the drop target in the top right (or use the ?filename= URL parameter).
>+        </p>
>+    </section>
>+
>+    <section id="uiContainer">
>+
>+        <div id="rooted">
>+        </div>
>+
>+        <h1>All paths toâ¦</h1>
>+        <div id="all-paths">
>+        </div>
>+
>+        <h1>Objects</h1>
>+        <section id="categories">
>+        </section>
>+
>+        <h1>Roots</h1>
>+        <section id="roots">
>+        </section>
>+    </section>
>+    
>+</body>
>+</html>
>diff --git a/Tools/GCHeapInspector/heap-analysis/HeapSnapshot.js b/Tools/GCHeapInspector/heap-analysis/HeapSnapshot.js
>new file mode 100644
>index 0000000000000000000000000000000000000000..727598845a65170ed7e2b8a3bbe920e34c4a3363
>--- /dev/null
>+++ b/Tools/GCHeapInspector/heap-analysis/HeapSnapshot.js
>@@ -0,0 +1,849 @@
>+/*
>+ * Copyright (C) 2011 Google Inc. All rights reserved.
>+ * Copyright (C) 2016 Apple Inc. All rights reserved.
>+ *
>+ * Redistribution and use in source and binary forms, with or without
>+ * modification, are permitted provided that the following conditions are
>+ * met:
>+ *
>+ *     * Redistributions of source code must retain the above copyright
>+ * notice, this list of conditions and the following disclaimer.
>+ *     * Redistributions in binary form must reproduce the above
>+ * copyright notice, this list of conditions and the following disclaimer
>+ * in the documentation and/or other materials provided with the
>+ * distribution.
>+ *     * Neither the name of Google Inc. nor the names of its
>+ * contributors may be used to endorse or promote products derived from
>+ * this software without specific prior written permission.
>+ *
>+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
>+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
>+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
>+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
>+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
>+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
>+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
>+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
>+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
>+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
>+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
>+ */
>+
>+// nodes
>+// [<0:id>, <1:size>, <2:classNameTableIndex>, <3:internal>, <4:labelIndex>, <5:address>, <6:wrapped address>]
>+let nodeFieldCount = 7;
>+const nodeIdOffset = 0;
>+const nodeSizeOffset = 1;
>+const nodeClassNameOffset = 2;
>+const nodeInternalOffset = 3;
>+const nodeLabelOffset = 4;
>+const nodeAddressOffset = 5;
>+const nodeWrappedAddressOffset = 6;
>+
>+// edges
>+// [<0:fromId>, <1:toId>, <2:typeTableIndex>, <3:edgeDataIndexOrEdgeNameIndex>]
>+const edgeFieldCount = 4;
>+const edgeFromIdOffset = 0;
>+const edgeToIdOffset = 1;
>+const edgeTypeOffset = 2;
>+const edgeDataOffset = 3;
>+
>+// roots
>+// [<0:id>, <1:labelIndex>]
>+let rootFieldCount = 3;
>+const rootIdOffset = 0;
>+const rootLabelOffset = 1;
>+const reachabilityReasonOffset = 2;
>+
>+// Other constants.
>+const rootNodeIndex = 0;
>+const rootNodeOrdinal = 0;
>+const rootNodeIdentifier = 0;
>+
>+// Terminology:
>+//   - `nodeIndex` is an index into the `nodes` list.
>+//   - `nodeOrdinal` is the order of the node in the `nodes` list. (nodeIndex / nodeFieldCount).
>+//   - `nodeIdentifier` is the node's id value. (nodes[nodeIndex + nodeIdOffset]).
>+//   - `edgeIndex` is an index into the `edges` list.
>+//
>+// Lists:
>+//   - _nodeOrdinalToFirstOutgoingEdge - `nodeOrdinal` to `edgeIndex` in `edges`.
>+//     Iterate edges by walking `edges` (edgeFieldCount) and checking if fromIdentifier is current.
>+//   - _nodeOrdinalToFirstIncomingEdge - `nodeOrdinal` to `incomingEdgeIndex` in `incomingEdges`.
>+//     Iterate edges by walking `incomingEdges` until `nodeOrdinal+1`'s first incoming edge index.
>+//   - _nodeOrdinalToDominatorNodeOrdinal - `nodeOrdinal` to `nodeOrdinal` of dominator.
>+//   - _nodeOrdinalToRetainedSizes - `nodeOrdinal` to retain size value.
>+//   - _nodeOrdinalIsDead - `nodeOrdinal` is dead or alive.
>+//
>+// Temporary Lists:
>+//   - nodeOrdinalToPostOrderIndex - `nodeOrdinal` to a `postOrderIndex`.
>+//   - postOrderIndexToNodeOrdinal - `postOrderIndex` to a `nodeOrdinal`.
>+
>+let nextSnapshotIdentifier = 1;
>+
>+HeapSnapshot = class HeapSnapshot
>+{
>+    constructor(objectId, snapshotDataString, title = null)
>+    {
>+        this._identifier = nextSnapshotIdentifier++;
>+        this._objectId = objectId;
>+        this._title = title;
>+
>+        let json = JSON.parse(snapshotDataString);
>+        snapshotDataString = null;
>+
>+        let {version, type, nodes, nodeClassNames, edges, edgeTypes, edgeNames, roots, labels} = json;
>+        console.assert(version === 1, "Expect JavaScriptCore Heap Snapshot version 1");
>+        console.assert(type === "GCDebugging", "Expect a GCDebugging-type snapshot");
>+
>+        this._nodes = nodes;
>+        this._nodeCount = nodes.length / nodeFieldCount;
>+
>+        this._roots = roots;
>+        this._rootCount = roots.length / rootFieldCount;
>+
>+        this._edges = edges;
>+        this._edgeCount = edges.length / edgeFieldCount;
>+
>+        this._edgeTypesTable = edgeTypes;
>+        this._edgeNamesTable = edgeNames;
>+        this._nodeClassNamesTable = nodeClassNames;
>+        this._labelsTable = labels;
>+
>+        this._totalSize = 0;
>+        this._nodeIdentifierToOrdinal = new Map; // <node identifier> => nodeOrdinal
>+        this._lastNodeIdentifier = 0;
>+        for (let nodeIndex = 0; nodeIndex < nodes.length; nodeIndex += nodeFieldCount) {
>+            let nodeOrdinal = nodeIndex / nodeFieldCount;
>+            let nodeIdentifier = nodes[nodeIndex + nodeIdOffset];
>+            this._nodeIdentifierToOrdinal.set(nodeIdentifier, nodeOrdinal);
>+            this._totalSize += nodes[nodeIndex + nodeSizeOffset];
>+            if (nodeIdentifier > this._lastNodeIdentifier)
>+                this._lastNodeIdentifier = nodeIdentifier;
>+        }
>+
>+        this._rootIdentifierToReasons = new Map; // <node identifier> => Set of root reasons
>+        for (let rootIndex = 0; rootIndex < roots.length; rootIndex += rootFieldCount) {
>+            let rootOrdinal = rootIndex / rootFieldCount;
>+            let rootIdentifier = roots[rootIndex + rootIdOffset];
>+            let rootReasonIndex = roots[rootIndex + rootLabelOffset];
>+            let rootReachabilityIndex = roots[rootIndex + reachabilityReasonOffset];
>+
>+            let existingReasons = this._rootIdentifierToReasons.get(rootIdentifier);
>+            if (existingReasons) {
>+                existingReasons.add(rootReasonIndex);
>+                existingReasons.add(rootReachabilityIndex);
>+            } else
>+                this._rootIdentifierToReasons.set(rootIdentifier, new Set([rootReasonIndex, rootReachabilityIndex]));
>+        }
>+
>+        // FIXME: Replace toIdentifier and fromIdentifier in edges with nodeIndex to reduce hash lookups?
>+
>+        this._nodeOrdinalToFirstOutgoingEdge = new Uint32Array(this._nodeCount); // nodeOrdinal => edgeIndex
>+        this._buildOutgoingEdges();
>+
>+        this._nodeOrdinalToFirstIncomingEdge = new Uint32Array(this._nodeCount + 1); // nodeOrdinal => incomingNodes/incomingEdges index
>+        this._incomingNodes = new Uint32Array(this._edgeCount); // from nodeOrdinals.
>+        this._incomingEdges = new Uint32Array(this._edgeCount); // edgeIndex.
>+        this._buildIncomingEdges();
>+
>+        let {nodeOrdinalToPostOrderIndex, postOrderIndexToNodeOrdinal} = this._buildPostOrderIndexes();
>+
>+        this._nodeOrdinalToDominatorNodeOrdinal = new Uint32Array(this._nodeCount);
>+        this._nodeOrdinalIsGCRoot = new Uint8Array(this._nodeCount);
>+        this._buildDominatorIndexes(nodeOrdinalToPostOrderIndex, postOrderIndexToNodeOrdinal);
>+
>+        nodeOrdinalToPostOrderIndex = null;
>+
>+        this._nodeOrdinalToRetainedSizes = new Uint32Array(this._nodeCount);
>+        this._buildRetainedSizes(postOrderIndexToNodeOrdinal);
>+
>+        postOrderIndexToNodeOrdinal = null;
>+
>+        this._nodeOrdinalIsDead = new Uint8Array(this._nodeCount);
>+
>+        let {liveSize, categories} = HeapSnapshot.updateCategoriesAndMetadata(this);
>+        this._liveSize = liveSize;
>+        this._categories = categories;
>+    }
>+
>+    get proxyObjectId() { return this._proxyObjectId; }
>+    get identifier() { return this._identifier; }
>+    get title() { return this._title; }
>+    get totalSize() { return this._totalSize; }
>+    get totalObjectCount() { return this._totalObjectCount; }
>+    get liveSize() { return this._liveSize; }
>+    get categories() { return this._categories; }
>+    get invalid() { return this._proxyObjectId === 0; }
>+
>+    // Static
>+
>+    static updateCategoriesAndMetadata(snapshot, allowNodeIdentifierCallback)
>+    {
>+        let liveSize = 0;
>+        let categories = {};
>+
>+        let nodes = snapshot._nodes;
>+        let nodeClassNamesTable = snapshot._nodeClassNamesTable;
>+        let nodeOrdinalToRetainedSizes = snapshot._nodeOrdinalToRetainedSizes;
>+        let nodeOrdinalIsDead = snapshot._nodeOrdinalIsDead;
>+
>+        // Skip the <root> node.
>+        let firstNodeIndex = nodeFieldCount;
>+        let firstNodeOrdinal = 1;
>+        for (let nodeIndex = firstNodeIndex, nodeOrdinal = firstNodeOrdinal; nodeIndex < nodes.length; nodeIndex += nodeFieldCount, nodeOrdinal++) {
>+            if (allowNodeIdentifierCallback && !allowNodeIdentifierCallback(nodes[nodeIndex + nodeIdOffset]))
>+                continue;
>+
>+            let classNameTableIndex = nodes[nodeIndex + nodeClassNameOffset];
>+            let className = nodeClassNamesTable[classNameTableIndex];
>+            let size = nodes[nodeIndex + nodeSizeOffset];
>+            let retainedSize = nodeOrdinalToRetainedSizes[nodeOrdinal];
>+            let internal = nodes[nodeIndex + nodeInternalOffset] ? true : false;
>+            let dead = nodeOrdinalIsDead[nodeOrdinal] ? true : false;
>+
>+            let category = categories[className];
>+            if (!category)
>+                category = categories[className] = {className, size: 0, retainedSize: 0, count: 0, internalCount: 0, deadCount: 0};
>+
>+            category.size += size;
>+            category.retainedSize += retainedSize;
>+            category.count += 1;
>+            if (internal)
>+                category.internalCount += 1;
>+            if (dead)
>+                category.deadCount += 1;
>+            else
>+                liveSize += size;
>+        }
>+
>+        return {liveSize, categories};
>+    }
>+
>+    static allocationBucketCounts(snapshot, bucketSizes, allowNodeIdentifierCallback)
>+    {
>+        let counts = new Array(bucketSizes.length + 1);
>+        let remainderBucket = counts.length - 1;
>+        counts.fill(0);
>+
>+        let nodes = snapshot._nodes;
>+
>+        // Skip the <root> node.
>+        let firstNodeIndex = nodeFieldCount;
>+
>+    outer:
>+        for (let nodeIndex = firstNodeIndex; nodeIndex < nodes.length; nodeIndex += nodeFieldCount) {
>+            if (allowNodeIdentifierCallback && !allowNodeIdentifierCallback(nodes[nodeIndex + nodeIdOffset]))
>+                continue;
>+
>+            let size = nodes[nodeIndex + nodeSizeOffset];
>+            for (let i = 0; i < bucketSizes.length; ++i) {
>+                if (size < bucketSizes[i]) {
>+                    counts[i]++;
>+                    continue outer;
>+                }
>+            }
>+            counts[remainderBucket]++;
>+        }
>+
>+        return counts;
>+    }
>+
>+    static instancesWithClassName(snapshot, className, allowNodeIdentifierCallback)
>+    {
>+        let instances = [];
>+
>+        let nodes = snapshot._nodes;
>+        let nodeClassNamesTable = snapshot._nodeClassNamesTable;
>+
>+        // Skip the <root> node.
>+        let firstNodeIndex = nodeFieldCount;
>+        let firstNodeOrdinal = 1;
>+        for (let nodeIndex = firstNodeIndex, nodeOrdinal = firstNodeOrdinal; nodeIndex < nodes.length; nodeIndex += nodeFieldCount, nodeOrdinal++) {
>+            if (allowNodeIdentifierCallback && !allowNodeIdentifierCallback(nodes[nodeIndex + nodeIdOffset]))
>+                continue;
>+
>+            let classNameTableIndex = nodes[nodeIndex + nodeClassNameOffset];
>+            if (nodeClassNamesTable[classNameTableIndex] === className)
>+                instances.push(nodeIndex);
>+        }
>+
>+        return instances.map(snapshot.serializeNode, snapshot);
>+    }
>+
>+    // Worker Methods
>+
>+    allocationBucketCounts(bucketSizes)
>+    {
>+        return HeapSnapshot.allocationBucketCounts(this, bucketSizes);
>+    }
>+
>+    instancesWithClassName(className)
>+    {
>+        return HeapSnapshot.instancesWithClassName(this, className);
>+    }
>+
>+    update()
>+    {
>+        return HeapSnapshot.updateCategoriesAndMetadata(this);
>+    }
>+
>+    nodeWithIdentifier(nodeIdentifier)
>+    {
>+        let nodeOrdinal = this._nodeIdentifierToOrdinal.get(nodeIdentifier);
>+        let nodeIndex = nodeOrdinal * nodeFieldCount;
>+        return this.serializeNode(nodeIndex);
>+    }
>+
>+    shortestGCRootPath(nodeIdentifier)
>+    {
>+        // Returns an array from this node to a gcRoot node.
>+        // E.g. [Node (target), Edge, Node, Edge, Node (root)].
>+        // Internal nodes are avoided, so if the path is empty this
>+        // node is either a gcRoot or only reachable via Internal nodes.
>+
>+        let paths = this._gcRootPaths(nodeIdentifier);
>+        if (!paths.length)
>+            return [];
>+
>+        paths.sort((a, b) => a.length - b.length);
>+
>+        let shortestPathWithGlobalObject = null;
>+        for (let path of paths) {
>+            let lastNodeIndex = path[path.length - 1].node;
>+            if (this._isNodeGlobalObject(lastNodeIndex)) {
>+                shortestPathWithGlobalObject = path;
>+                break;
>+            }
>+        }
>+
>+        let shortestPath = shortestPathWithGlobalObject || paths[0];
>+        console.assert("node" in shortestPath[0], "Path should start with a node");
>+        console.assert("node" in shortestPath[shortestPath.length - 1], "Path should end with a node");
>+
>+        return shortestPath.map((component) => {
>+            if (component.node)
>+                return this.serializeNode(component.node);
>+            return this.serializeEdge(component.edge);
>+        });
>+    }
>+    
>+    rootNodes()
>+    {
>+        let rootNodeIndexSet = new Set;
>+
>+        // Identifiers can occur multiple times in the roots list.
>+        for (let rootIndex = 0; rootIndex < this._roots.length; rootIndex += rootFieldCount) {
>+            let rootOrdinal = rootIndex / rootFieldCount;
>+            let rootIdentifier = this._roots[rootIndex + rootIdOffset];
>+
>+            let toNodeOrdinal = this._nodeIdentifierToOrdinal.get(rootIdentifier);
>+            let toNodeIndex = toNodeOrdinal * nodeFieldCount;
>+
>+            rootNodeIndexSet.add(toNodeIndex);
>+        }
>+
>+        return Array.from(rootNodeIndexSet.values()).map(this.serializeNode, this);
>+    }
>+    
>+    reasonNamesForRoot(rootIdentifier)
>+    {
>+        let reasonIndexes = this._rootIdentifierToReasons.get(rootIdentifier);
>+        if (!reasonIndexes)
>+            return [];
>+        return Array.from(reasonIndexes).map(this.labelForIndex, this).filter(a => a != 0)
>+    }
>+
>+    dominatedNodes(nodeIdentifier)
>+    {
>+        let dominatedNodes = [];
>+
>+        let targetNodeOrdinal = this._nodeIdentifierToOrdinal.get(nodeIdentifier);
>+        for (let nodeOrdinal = 0; nodeOrdinal < this._nodeCount; ++nodeOrdinal) {
>+            if (this._nodeOrdinalToDominatorNodeOrdinal[nodeOrdinal] === targetNodeOrdinal)
>+                dominatedNodes.push(nodeOrdinal * nodeFieldCount);
>+        }
>+
>+        return dominatedNodes.map(this.serializeNode, this);
>+    }
>+
>+    retainedNodes(nodeIdentifier)
>+    {
>+        let retainedNodes = [];
>+        let edges = [];
>+
>+        let nodeOrdinal = this._nodeIdentifierToOrdinal.get(nodeIdentifier);
>+        let edgeIndex = this._nodeOrdinalToFirstOutgoingEdge[nodeOrdinal];
>+        for (; this._edges[edgeIndex + edgeFromIdOffset] === nodeIdentifier; edgeIndex += edgeFieldCount) {
>+            let toNodeIdentifier = this._edges[edgeIndex + edgeToIdOffset];
>+            let toNodeOrdinal = this._nodeIdentifierToOrdinal.get(toNodeIdentifier);
>+            let toNodeIndex = toNodeOrdinal * nodeFieldCount;
>+            retainedNodes.push(toNodeIndex);
>+            edges.push(edgeIndex);
>+        }
>+
>+        return {
>+            retainedNodes: retainedNodes.map(this.serializeNode, this),
>+            edges: edges.map(this.serializeEdge, this),
>+        };
>+    }
>+
>+    retainers(nodeIdentifier)
>+    {
>+        let retainers = [];
>+        let edges = [];
>+
>+        let nodeOrdinal = this._nodeIdentifierToOrdinal.get(nodeIdentifier);
>+        let incomingEdgeIndex = this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal];
>+        let incomingEdgeIndexEnd = this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal + 1];
>+        for (let edgeIndex = incomingEdgeIndex; edgeIndex < incomingEdgeIndexEnd; ++edgeIndex) {
>+            let fromNodeOrdinal = this._incomingNodes[edgeIndex];
>+            let fromNodeIndex = fromNodeOrdinal * nodeFieldCount;
>+            retainers.push(fromNodeIndex);
>+            edges.push(this._incomingEdges[edgeIndex]);
>+        }
>+
>+        return {
>+            retainers: retainers.map(this.serializeNode, this),
>+            edges: edges.map(this.serializeEdge, this),
>+        };
>+    }
>+
>+    updateDeadNodesAndGatherCollectionData(snapshots)
>+    {
>+        let previousSnapshotIndex = snapshots.indexOf(this) - 1;
>+        let previousSnapshot = snapshots[previousSnapshotIndex];
>+        if (!previousSnapshot)
>+            return null;
>+
>+        let lastNodeIdentifier = previousSnapshot._lastNodeIdentifier;
>+
>+        // All of the node identifiers that could have existed prior to this snapshot.
>+        let known = new Map;
>+        for (let nodeIndex = 0; nodeIndex < this._nodes.length; nodeIndex += nodeFieldCount) {
>+            let nodeIdentifier = this._nodes[nodeIndex + nodeIdOffset];
>+            if (nodeIdentifier > lastNodeIdentifier)
>+                continue;
>+            known.set(nodeIdentifier, nodeIndex);
>+        }
>+
>+        // Determine which node identifiers have since been deleted.
>+        let collectedNodesList = [];
>+        for (let nodeIndex = 0; nodeIndex < previousSnapshot._nodes.length; nodeIndex += nodeFieldCount) {
>+            let nodeIdentifier = previousSnapshot._nodes[nodeIndex + nodeIdOffset];
>+            let wasDeleted = !known.has(nodeIdentifier);
>+            if (wasDeleted)
>+                collectedNodesList.push(nodeIdentifier);
>+        }
>+
>+        // Update dead nodes in previous snapshots.
>+        let affectedSnapshots = [];
>+        for (let snapshot of snapshots) {
>+            if (snapshot === this)
>+                break;
>+            if (snapshot._markDeadNodes(collectedNodesList))
>+                affectedSnapshots.push(snapshot._identifier);
>+        }
>+
>+        // Convert list to a map.
>+        let collectedNodes = {};
>+        for (let i = 0; i < collectedNodesList.length; ++i)
>+            collectedNodes[collectedNodesList[i]] = true;
>+
>+        return {
>+            collectedNodes,
>+            affectedSnapshots,
>+        };
>+    }
>+
>+    // Public
>+
>+    serialize()
>+    {
>+        return {
>+            identifier: this._identifier,
>+            title: this._title,
>+            totalSize: this._totalSize,
>+            totalObjectCount: this._nodeCount - 1, // <root>.
>+            liveSize: this._liveSize,
>+            categories: this._categories,
>+        };
>+    }
>+
>+    serializeNode(nodeIndex)
>+    {
>+        console.assert((nodeIndex % nodeFieldCount) === 0, "Invalid nodeIndex to serialize: " + nodeIndex);
>+
>+        let nodeIdentifier = this._nodes[nodeIndex + nodeIdOffset];
>+        let nodeOrdinal = nodeIndex / nodeFieldCount;
>+        let edgeIndex = this._nodeOrdinalToFirstOutgoingEdge[nodeOrdinal];
>+        let hasChildren = this._edges[edgeIndex + edgeFromIdOffset] === nodeIdentifier;
>+
>+        let dominatorNodeOrdinal = this._nodeOrdinalToDominatorNodeOrdinal[nodeOrdinal];
>+        let dominatorNodeIndex = dominatorNodeOrdinal * nodeFieldCount;
>+        let dominatorNodeIdentifier = this._nodes[dominatorNodeIndex + nodeIdOffset];
>+
>+        let result = {
>+            id: nodeIdentifier,
>+            className: this._nodeClassNamesTable[this._nodes[nodeIndex + nodeClassNameOffset]],
>+            size: this._nodes[nodeIndex + nodeSizeOffset],
>+            retainedSize: this._nodeOrdinalToRetainedSizes[nodeOrdinal],
>+            internal: this._nodes[nodeIndex + nodeInternalOffset] ? true : false,
>+            gcRoot: this._nodeOrdinalIsGCRoot[nodeOrdinal] ? true : false,
>+            markedRoot : this._rootIdentifierToReasons.has(nodeIdentifier),
>+            dead: this._nodeOrdinalIsDead[nodeOrdinal] ? true : false,
>+            address: this._nodes[nodeIndex + nodeAddressOffset],
>+            label: this._labelsTable[this._nodes[nodeIndex + nodeLabelOffset]],
>+            dominatorNodeIdentifier,
>+            hasChildren,
>+        };
>+
>+        let wrappedAddr = this._nodes[nodeIndex + nodeWrappedAddressOffset];
>+        if (wrappedAddr !== "0x0")
>+            result.wrappedAddress = wrappedAddr;
>+        
>+        return result;
>+    }
>+
>+    labelForIndex(index)
>+    {
>+        return this._labelsTable[index];
>+    }
>+
>+    serializeEdge(edgeIndex)
>+    {
>+        console.assert((edgeIndex % edgeFieldCount) === 0, "Invalid edgeIndex to serialize");
>+
>+        let edgeType = this._edgeTypesTable[this._edges[edgeIndex + edgeTypeOffset]];
>+        let edgeData = this._edges[edgeIndex + edgeDataOffset];
>+        switch (edgeType) {
>+        case "Internal":
>+            // edgeData can be ignored.
>+            break;
>+        case "Property":
>+        case "Variable":
>+            // edgeData is a table index.
>+            edgeData = this._edgeNamesTable[edgeData];
>+            break;
>+        case "Index":
>+            // edgeData is the index.
>+            break;
>+        default:
>+            console.error("Unexpected edge type: " + edgeType);
>+            break;
>+        }
>+
>+        return {
>+            from: this._edges[edgeIndex + edgeFromIdOffset],
>+            to: this._edges[edgeIndex + edgeToIdOffset],
>+            type: edgeType,
>+            data: edgeData,
>+        };
>+    }
>+
>+    // Private
>+
>+    _buildOutgoingEdges()
>+    {
>+        let lastFromIdentifier = -1;
>+        for (let edgeIndex = 0; edgeIndex < this._edges.length; edgeIndex += edgeFieldCount) {
>+            let fromIdentifier = this._edges[edgeIndex + edgeFromIdOffset];
>+            console.assert(lastFromIdentifier <= fromIdentifier, "Edge list should be ordered by from node identifier");
>+            if (fromIdentifier !== lastFromIdentifier) {
>+                let nodeOrdinal = this._nodeIdentifierToOrdinal.get(fromIdentifier);
>+                this._nodeOrdinalToFirstOutgoingEdge[nodeOrdinal] = edgeIndex;
>+                lastFromIdentifier = fromIdentifier;
>+            }
>+        }
>+    }
>+
>+    _buildIncomingEdges()
>+    {
>+        // First calculate the count of incoming edges for each node.
>+        for (let edgeIndex = 0; edgeIndex < this._edges.length; edgeIndex += edgeFieldCount) {
>+            let toIdentifier = this._edges[edgeIndex + edgeToIdOffset];
>+            let toNodeOrdinal = this._nodeIdentifierToOrdinal.get(toIdentifier);
>+            this._nodeOrdinalToFirstIncomingEdge[toNodeOrdinal]++;
>+        }
>+
>+        // Replace the counts with what will be the resulting index by running up the counts.
>+        // Store the counts in what will be the edges list to use when placing edges in the list.
>+        let runningFirstIndex = 0;
>+        for (let nodeOrdinal = 0; nodeOrdinal < this._nodeCount; ++nodeOrdinal) {
>+            let count = this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal];
>+            this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal] = runningFirstIndex;
>+            this._incomingNodes[runningFirstIndex] = count;
>+            runningFirstIndex += count;
>+        }
>+
>+        // Fill in the incoming edges list. Use the count as an offset when placing edges in the list.
>+        for (let edgeIndex = 0; edgeIndex < this._edges.length; edgeIndex += edgeFieldCount) {
>+            let fromIdentifier = this._edges[edgeIndex + edgeFromIdOffset];
>+            let fromNodeOrdinal = this._nodeIdentifierToOrdinal.get(fromIdentifier);
>+            let toIdentifier = this._edges[edgeIndex + edgeToIdOffset];
>+            let toNodeOrdinal = this._nodeIdentifierToOrdinal.get(toIdentifier);
>+
>+            let firstIncomingEdgeIndex = this._nodeOrdinalToFirstIncomingEdge[toNodeOrdinal];
>+            console.assert(this._incomingNodes[firstIncomingEdgeIndex] > 0, "Should be expecting edges for this node");
>+            let countAsOffset = this._incomingNodes[firstIncomingEdgeIndex]--;
>+            let index = firstIncomingEdgeIndex + countAsOffset - 1;
>+            this._incomingNodes[index] = fromNodeOrdinal;
>+            this._incomingEdges[index] = edgeIndex;
>+        }
>+
>+        // Duplicate value on the end. Incoming edge iteration walks firstIncomingEdge(ordinal) to firstIncomingEdge(ordinal+1).
>+        this._nodeOrdinalToFirstIncomingEdge[this._nodeCount] = this._nodeOrdinalToFirstIncomingEdge[this._nodeCount - 1];
>+    }
>+
>+    _buildPostOrderIndexes()
>+    {
>+        let postOrderIndex = 0;
>+        let nodeOrdinalToPostOrderIndex = new Uint32Array(this._nodeCount);
>+        let postOrderIndexToNodeOrdinal = new Uint32Array(this._nodeCount);
>+
>+        let stackNodes = new Uint32Array(this._nodeCount); // nodeOrdinal.
>+        let stackEdges = new Uint32Array(this._nodeCount); // edgeIndex.
>+        let visited = new Uint8Array(this._nodeCount);
>+
>+        let stackTop = 0;
>+        stackNodes[stackTop] = rootNodeOrdinal;
>+        stackEdges[stackTop] = this._nodeOrdinalToFirstOutgoingEdge[rootNodeOrdinal];
>+
>+        while (stackTop >= 0) {
>+            let nodeOrdinal = stackNodes[stackTop];
>+            let nodeIdentifier = this._nodes[(nodeOrdinal * nodeFieldCount) + nodeIdOffset];
>+            let edgeIndex = stackEdges[stackTop];
>+
>+            if (this._edges[edgeIndex + edgeFromIdOffset] === nodeIdentifier) {
>+                // Prepare the next child for the current node.
>+                stackEdges[stackTop] += edgeFieldCount;
>+
>+                let toIdentifier = this._edges[edgeIndex + edgeToIdOffset];
>+                let toNodeOrdinal = this._nodeIdentifierToOrdinal.get(toIdentifier);
>+                if (visited[toNodeOrdinal])
>+                    continue;
>+
>+                // Child.
>+                stackTop++;
>+                stackNodes[stackTop] = toNodeOrdinal;
>+                stackEdges[stackTop] = this._nodeOrdinalToFirstOutgoingEdge[toNodeOrdinal];
>+                visited[toNodeOrdinal] = 1;
>+            } else {
>+                // Self.
>+                nodeOrdinalToPostOrderIndex[nodeOrdinal] = postOrderIndex;
>+                postOrderIndexToNodeOrdinal[postOrderIndex] = nodeOrdinal;
>+                postOrderIndex++;
>+                stackTop--;
>+            }
>+        }
>+
>+        // Unvisited nodes.
>+        // This can happen if the parent node was disallowed on the backend, but other nodes
>+        // that were only referenced from that disallowed node were eventually allowed because
>+        // they may be generic system objects. Give these nodes a postOrderIndex anyways.
>+        if (postOrderIndex !== this._nodeCount) {
>+            // Root was the last node visited. Revert assigning it an index, add it back at the end.
>+            postOrderIndex--;
>+
>+            // Visit unvisited nodes.
>+            for (let nodeOrdinal = 1; nodeOrdinal < this._nodeCount; ++nodeOrdinal) {
>+                if (visited[nodeOrdinal])
>+                    continue;
>+                nodeOrdinalToPostOrderIndex[nodeOrdinal] = postOrderIndex;
>+                postOrderIndexToNodeOrdinal[postOrderIndex] = nodeOrdinal;
>+                postOrderIndex++;
>+            }
>+
>+            // Visit root again.
>+            nodeOrdinalToPostOrderIndex[rootNodeOrdinal] = postOrderIndex;
>+            postOrderIndexToNodeOrdinal[postOrderIndex] = rootNodeOrdinal;
>+            postOrderIndex++;
>+        }
>+
>+        console.assert(postOrderIndex === this._nodeCount, "All nodes were visited");
>+        console.assert(nodeOrdinalToPostOrderIndex[rootNodeOrdinal] === this._nodeCount - 1, "Root node should have the last possible postOrderIndex");
>+
>+        return {nodeOrdinalToPostOrderIndex, postOrderIndexToNodeOrdinal};
>+    }
>+
>+    _buildDominatorIndexes(nodeOrdinalToPostOrderIndex, postOrderIndexToNodeOrdinal)
>+    {
>+        // The algorithm is based on the article:
>+        // K. Cooper, T. Harvey and K. Kennedy "A Simple, Fast Dominance Algorithm"
>+
>+        let rootPostOrderIndex = this._nodeCount - 1;
>+        let noEntry = this._nodeCount;
>+
>+        let affected = new Uint8Array(this._nodeCount);
>+        let dominators = new Uint32Array(this._nodeCount);
>+
>+        // Initialize with unset value.
>+        dominators.fill(noEntry);
>+
>+        // Mark the root's dominator value.
>+        dominators[rootPostOrderIndex] = rootPostOrderIndex;
>+
>+        // Affect the root's children. Also use this opportunity to mark them as GC roots.
>+        let rootEdgeIndex = this._nodeOrdinalToFirstOutgoingEdge[rootNodeOrdinal];
>+        for (let edgeIndex = rootEdgeIndex; this._edges[edgeIndex + edgeFromIdOffset] === rootNodeIdentifier; edgeIndex += edgeFieldCount) {
>+            let toIdentifier = this._edges[edgeIndex + edgeToIdOffset];
>+            let toNodeOrdinal = this._nodeIdentifierToOrdinal.get(toIdentifier);
>+            let toPostOrderIndex = nodeOrdinalToPostOrderIndex[toNodeOrdinal];
>+            affected[toPostOrderIndex] = 1;
>+            this._nodeOrdinalIsGCRoot[toNodeOrdinal] = 1;
>+        }
>+
>+        let changed = true;
>+        while (changed) {
>+            changed = false;
>+
>+            for (let postOrderIndex = rootPostOrderIndex - 1; postOrderIndex >= 0; --postOrderIndex) {
>+                if (!affected[postOrderIndex])
>+                    continue;
>+                affected[postOrderIndex] = 0;
>+
>+                // The dominator is already the root, nothing to do.
>+                if (dominators[postOrderIndex] === rootPostOrderIndex)
>+                    continue;
>+
>+                let newDominatorIndex = noEntry;
>+                let nodeOrdinal = postOrderIndexToNodeOrdinal[postOrderIndex];
>+                let incomingEdgeIndex = this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal];
>+                let incomingEdgeIndexEnd = this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal + 1];
>+                for (let edgeIndex = incomingEdgeIndex; edgeIndex < incomingEdgeIndexEnd; ++edgeIndex) {
>+                    let fromNodeOrdinal = this._incomingNodes[edgeIndex];
>+                    let fromPostOrderIndex = nodeOrdinalToPostOrderIndex[fromNodeOrdinal];
>+                    if (dominators[fromPostOrderIndex] !== noEntry) {
>+                        if (newDominatorIndex === noEntry)
>+                            newDominatorIndex = fromPostOrderIndex;
>+                        else {
>+                            while (fromPostOrderIndex !== newDominatorIndex) {
>+                                while (fromPostOrderIndex < newDominatorIndex)
>+                                    fromPostOrderIndex = dominators[fromPostOrderIndex];
>+                                while (newDominatorIndex < fromPostOrderIndex)
>+                                    newDominatorIndex = dominators[newDominatorIndex];
>+                            }
>+                        }
>+                    }
>+                    if (newDominatorIndex === rootPostOrderIndex)
>+                        break;
>+                }
>+
>+                // Changed. Affect children.
>+                if (newDominatorIndex !== noEntry && dominators[postOrderIndex] !== newDominatorIndex) {
>+                    dominators[postOrderIndex] = newDominatorIndex;
>+                    changed = true;
>+
>+                    let outgoingEdgeIndex = this._nodeOrdinalToFirstOutgoingEdge[nodeOrdinal];
>+                    let nodeIdentifier = this._nodes[(nodeOrdinal * nodeFieldCount) + nodeIdOffset];
>+                    for (let edgeIndex = outgoingEdgeIndex; this._edges[edgeIndex + edgeFromIdOffset] === nodeIdentifier; edgeIndex += edgeFieldCount) {
>+                        let toNodeIdentifier = this._edges[edgeIndex + edgeToIdOffset];
>+                        let toNodeOrdinal = this._nodeIdentifierToOrdinal.get(toNodeIdentifier);
>+                        let toNodePostOrder = nodeOrdinalToPostOrderIndex[toNodeOrdinal];
>+                        affected[toNodePostOrder] = 1;
>+                    }
>+                }
>+            }
>+        }
>+
>+        for (let postOrderIndex = 0; postOrderIndex < this._nodeCount; ++postOrderIndex) {
>+            let nodeOrdinal = postOrderIndexToNodeOrdinal[postOrderIndex];
>+            let dominatorNodeOrdinal = postOrderIndexToNodeOrdinal[dominators[postOrderIndex]];
>+            this._nodeOrdinalToDominatorNodeOrdinal[nodeOrdinal] = dominatorNodeOrdinal;
>+        }
>+    }
>+
>+    _buildRetainedSizes(postOrderIndexToNodeOrdinal)
>+    {
>+        // Self size.
>+        for (let nodeIndex = 0, nodeOrdinal = 0; nodeOrdinal < this._nodeCount; nodeIndex += nodeFieldCount, nodeOrdinal++)
>+            this._nodeOrdinalToRetainedSizes[nodeOrdinal] = this._nodes[nodeIndex + nodeSizeOffset];
>+
>+        // Attribute size to dominator.
>+        for (let postOrderIndex = 0; postOrderIndex < this._nodeCount - 1; ++postOrderIndex) {
>+            let nodeOrdinal = postOrderIndexToNodeOrdinal[postOrderIndex];
>+            let nodeRetainedSize = this._nodeOrdinalToRetainedSizes[nodeOrdinal];
>+            let dominatorNodeOrdinal = this._nodeOrdinalToDominatorNodeOrdinal[nodeOrdinal];
>+            this._nodeOrdinalToRetainedSizes[dominatorNodeOrdinal] += nodeRetainedSize;
>+        }
>+    }
>+
>+    _markDeadNodes(collectedNodesList)
>+    {
>+        let affected = false;
>+
>+        for (let i = 0; i < collectedNodesList.length; ++i) {
>+            let nodeIdentifier = collectedNodesList[i];
>+            if (nodeIdentifier > this._lastNodeIdentifier)
>+                continue;
>+            let nodeOrdinal = this._nodeIdentifierToOrdinal.get(nodeIdentifier);
>+            this._nodeOrdinalIsDead[nodeOrdinal] = 1;
>+            affected = true;
>+        }
>+
>+        return affected;
>+    }
>+
>+    _isNodeGlobalObject(nodeIndex)
>+    {
>+        let className = this._nodeClassNamesTable[this._nodes[nodeIndex + nodeClassNameOffset]];
>+        return className === "Window"
>+            || className === "JSWindowProxy"
>+            || className === "GlobalObject";
>+    }
>+
>+    _gcRootPaths(nodeIdentifier)
>+    {
>+        let targetNodeOrdinal = this._nodeIdentifierToOrdinal.get(nodeIdentifier);
>+
>+        if (this._nodeOrdinalIsGCRoot[targetNodeOrdinal])
>+            return [];
>+
>+        // FIXME: Array push/pop can affect performance here, but in practice it hasn't been an issue.
>+
>+        let paths = [];
>+        let currentPath = [];
>+        let visited = new Uint8Array(this._nodeCount);
>+
>+        function visitNode(nodeOrdinal)
>+        {
>+            if (this._nodeOrdinalIsGCRoot[nodeOrdinal]) {
>+                let fullPath = currentPath.slice();
>+                let nodeIndex = nodeOrdinal * nodeFieldCount;
>+                fullPath.push({node: nodeIndex});
>+                paths.push(fullPath);
>+                return;
>+            }
>+
>+            if (visited[nodeOrdinal])
>+                return;
>+            visited[nodeOrdinal] = 1;
>+
>+            let nodeIndex = nodeOrdinal * nodeFieldCount;
>+            currentPath.push({node: nodeIndex});
>+
>+            // Loop in reverse order because edges were added in reverse order.
>+            // It doesn't particularly matter other then consistency with previous code.
>+            let incomingEdgeIndexStart = this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal];
>+            let incomingEdgeIndexEnd = this._nodeOrdinalToFirstIncomingEdge[nodeOrdinal + 1];
>+            for (let incomingEdgeIndex = incomingEdgeIndexEnd - 1; incomingEdgeIndex >= incomingEdgeIndexStart; --incomingEdgeIndex) {
>+                let fromNodeOrdinal = this._incomingNodes[incomingEdgeIndex];
>+                let fromNodeIndex = fromNodeOrdinal * nodeFieldCount;
>+                // let fromNodeIsInternal = this._nodes[fromNodeIndex + nodeInternalOffset];
>+                // if (fromNodeIsInternal)
>+                //     continue;
>+
>+                let edgeIndex = this._incomingEdges[incomingEdgeIndex];
>+                currentPath.push({edge: edgeIndex});
>+                visitNode.call(this, fromNodeOrdinal);
>+                currentPath.pop();
>+            }
>+
>+            currentPath.pop();
>+        }
>+
>+        visitNode.call(this, targetNodeOrdinal);
>+
>+        return paths;
>+    }
>+};
>diff --git a/Tools/GCHeapInspector/script/interface.js b/Tools/GCHeapInspector/script/interface.js
>new file mode 100644
>index 0000000000000000000000000000000000000000..ae3ad5b00b29551da9d210e2edc7f59376278a3c
>--- /dev/null
>+++ b/Tools/GCHeapInspector/script/interface.js
>@@ -0,0 +1,515 @@
>+
>+
>+// DOM Helpers
>+class DOMUtils
>+{
>+    static removeAllChildren(node)
>+    {
>+        while (node.lastChild)
>+             node.removeChild(node.lastChild);
>+    }
>+    
>+    static createDetails(summaryText)
>+    {
>+        let summary = document.createElement('summary');
>+        summary.textContent = summaryText;
>+    
>+        let details = document.createElement('details');
>+        details.appendChild(summary);
>+        
>+        details.addEventListener('keydown', function(event) {
>+            const rightArrowKeyCode = 39;
>+            const leftArrowKeyCode = 37;
>+            if (event.keyCode == rightArrowKeyCode)
>+                this.open = true;
>+            else if (event.keyCode == leftArrowKeyCode)
>+                this.open = false;
>+        }, false);
>+
>+        return details;
>+    }
>+};
>+
>+// Heap Inspector Helpers
>+class HeapInspectorUtils
>+{
>+    static humanReadableSize(sizeInBytes)
>+    {
>+        var i = -1;
>+        if (sizeInBytes < 512)
>+            return sizeInBytes + '&thinsp;B';
>+        var byteUnits = ['&thinsp;KB', '&thinsp;MB', '&thinsp;GB'];
>+        do {
>+            sizeInBytes = sizeInBytes / 1024;
>+            i++;
>+        } while (sizeInBytes > 1024);
>+
>+        return Math.max(sizeInBytes, 0.1).toFixed(1) + byteUnits[i];
>+    }
>+
>+    static addressForNode(node)
>+    {
>+        return node.wrappedAddress ? node.wrappedAddress : node.address;
>+    }
>+
>+    static nodeName(snapshot, node)
>+    {
>+        if (node.type == "Internal")
>+            return 'Internal node';
>+
>+        let result = node.className + ' @' + node.id + ' (' + HeapInspectorUtils.addressForNode(node) + ' ' + node.label + ')';
>+        if (node.gcRoot || node.markedRoot)
>+            result += ' (GC rootâ' + snapshot.reasonNamesForRoot(node.id).join(', ') + ')';
>+    
>+        return result;
>+    }
>+
>+    static spanForNode(inspector, node, showPathButton)
>+    {
>+        let nodeSpan = document.createElement('span');
>+
>+        if (node.type == "Internal") {
>+            nodeSpan.textContent = 'Internal node';
>+            return nodeSpan;
>+        }
>+
>+        let wrappedAddressString = node.wrappedAddress ? `wrapped ${node.wrappedAddress}` : '';
>+
>+        let nodeHTML = node.className + ` <span class="node-id">${node.id}</span> <span class="node-address">cell ${node.address} ${wrappedAddressString}</span> <span class="node-size">(retains ${HeapInspectorUtils.humanReadableSize(node.retainedSize)})</span>`;
>+    
>+        if (node.label.length)
>+            nodeHTML += ` <span class="node-label">â${node.label}â</span>`;
>+    
>+        nodeSpan.innerHTML = nodeHTML;
>+
>+        if (node.gcRoot || node.markedRoot) {
>+            let gcRootSpan = document.createElement('span');
>+            gcRootSpan.className = 'node-gc-root';
>+            gcRootSpan.textContent = ' (GC rootâ' + inspector.snapshot.reasonNamesForRoot(node.id).join(', ') + ')';
>+            nodeSpan.appendChild(gcRootSpan);
>+        } else if (showPathButton) {
>+            let showAllPathsAnchor = document.createElement('button');
>+            showAllPathsAnchor.className = 'node-show-all-paths';
>+            showAllPathsAnchor.textContent = 'Show all paths';
>+            showAllPathsAnchor.addEventListener('click', (e) => {
>+                inspector.showAllPathsToNode(node);
>+            }, false);
>+            nodeSpan.appendChild(showAllPathsAnchor);
>+        }
>+    
>+        return nodeSpan;
>+    }
>+
>+    static spanForEdge(snapshot, edge)
>+    {
>+        let edgeSpan = document.createElement('span');
>+        edgeSpan.className = 'edge';
>+        edgeSpan.innerHTML = '<span class="edge-type">' + edge.type + '</span> <span class="edge-data">' + edge.data + '</span>';
>+        return edgeSpan;
>+    }
>+
>+    static summarySpanForPath(inspector, path)
>+    {
>+        let pathSpan = document.createElement('span');
>+        pathSpan.className = 'path-summary';
>+    
>+        if (path.length > 0) {
>+            let pathLength = (path.length - 1) / 2;
>+            pathSpan.textContent = pathLength + ' step' + (pathLength > 1 ? 's' : '') + ' from ';
>+            pathSpan.appendChild(HeapInspectorUtils.spanForNode(inspector, path[0]), true);
>+        }
>+
>+        return pathSpan;
>+    }
>+
>+    static edgeName(edge)
>+    {
>+        return 'â ' + edge.type + ' ' + edge.data + ' â';
>+    }
>+};
>+
>+
>+// Manages a list of heap snapshot nodes that can dynamically build the contents of an HTMLListElement.
>+class InstanceList
>+{
>+    constructor(listElement, snapshot, listGeneratorFunc)
>+    {
>+        this.listElement = listElement;
>+        this.snapshot = snapshot;
>+        this.nodeList = listGeneratorFunc(this.snapshot);
>+        this.entriesAdded = 0;
>+        this.initialEntries = 100;
>+        this.entriesQuantum = 50;
>+        this.showMoreItem = undefined;
>+    }
>+    
>+    buildList(inspector)
>+    {
>+        DOMUtils.removeAllChildren(this.listElement);
>+        if (this.nodeList.length == 0)
>+            return;
>+
>+        let maxIndex = Math.min(this.nodeList.length, this.initialEntries);
>+        this.appendItemsForRange(inspector, 0, maxIndex);
>+        
>+        if (maxIndex < this.nodeList.length) {
>+            this.showMoreItem = this.makeShowMoreItem(inspector);
>+            this.listElement.appendChild(this.showMoreItem);
>+        }
>+        
>+        this.entriesAdded = maxIndex;
>+    }
>+    
>+    appendItemsForRange(inspector, startIndex, endIndex)
>+    {
>+        for (let index = startIndex; index < endIndex; ++index) {
>+            let instance = this.nodeList[index];
>+            let listItem = document.createElement('li');
>+            listItem.appendChild(HeapInspectorUtils.spanForNode(inspector, instance, true));
>+            this.listElement.appendChild(listItem);
>+        }
>+
>+        this.entriesAdded = endIndex;
>+    }
>+    
>+    appendMoreEntries(inspector)
>+    {
>+        let numRemaining = this.nodeList.length - this.entriesAdded;
>+        if (numRemaining == 0)
>+            return;
>+
>+        this.showMoreItem.remove();
>+
>+        let fromIndex = this.entriesAdded;
>+        let toIndex = Math.min(this.nodeList.length, fromIndex + this.entriesQuantum);
>+        this.appendItemsForRange(inspector, fromIndex, toIndex);
>+        
>+        if (toIndex < this.nodeList.length) {
>+            this.showMoreItem = this.makeShowMoreItem(inspector);
>+            this.listElement.appendChild(this.showMoreItem);
>+        }
>+    }
>+
>+    makeShowMoreItem(inspector)
>+    {
>+        let numberRemaining = this.nodeList.length - this.entriesAdded;
>+        
>+        let listItem = document.createElement('li');
>+        listItem.className = 'show-more';
>+        listItem.appendChild(document.createTextNode(`${numberRemaining} more `));
>+
>+        let moreButton = document.createElement('button');
>+        moreButton.textContent = `Show ${Math.min(this.entriesQuantum, numberRemaining)} more`;
>+
>+        let thisList = this;
>+        moreButton.addEventListener('click', function(e) {
>+            thisList.appendMoreEntries(inspector, 10);
>+        }, false);
>+        
>+        listItem.appendChild(moreButton);
>+        return listItem;
>+    }
>+};
>+
>+
>+class HeapSnapshotInspector
>+{
>+    constructor(containerElement, heapJSONData, filename)
>+    {
>+        this.containerElement = containerElement;
>+        this.resetUI();
>+
>+        this.snapshot = new HeapSnapshot(1, heapJSONData, filename);
>+        
>+        this.buildRoots();
>+        this.buildAllObjectsByType();
>+
>+        this.buildPathsToRootsOfType('Window');
>+        this.buildPathsToRootsOfType('HTMLDocument');
>+    }
>+    
>+    resetUI()
>+    {
>+        DOMUtils.removeAllChildren(this.containerElement);
>+        
>+        this.objectByTypeContainer = document.createElement('section');
>+        this.objectByTypeContainer.id = 'all-objects-by-type';
>+
>+        let header = document.createElement('h1');
>+        header.textContent = 'All Objects by Type'
>+        this.objectByTypeContainer.appendChild(header);
>+        
>+        this.rootsContainer =  document.createElement('section');
>+        this.rootsContainer.id = 'roots';
>+        header = document.createElement('h1');
>+        header.textContent = 'Roots'
>+        this.rootsContainer.appendChild(header);
>+
>+        this.pathsToRootsContainer =  document.createElement('section');
>+        this.pathsToRootsContainer.id = 'paths-to-roots';
>+        header = document.createElement('h1');
>+        header.textContent = 'Paths to roots'
>+        this.pathsToRootsContainer.appendChild(header);
>+
>+        this.allPathsContainer =  document.createElement('section');
>+        this.allPathsContainer.id = 'all-paths';
>+        header = document.createElement('h1');
>+        header.textContent = 'All paths toâ¦'
>+        this.allPathsContainer.appendChild(header);
>+
>+        this.containerElement.appendChild(this.pathsToRootsContainer);
>+        this.containerElement.appendChild(this.allPathsContainer);
>+        this.containerElement.appendChild(this.rootsContainer);
>+        this.containerElement.appendChild(this.objectByTypeContainer);
>+    }
>+
>+    buildAllObjectsByType()
>+    {
>+        let categories = this.snapshot._categories;
>+        let categoryNames = Object.keys(categories).sort();
>+    
>+        for (var categoryName of categoryNames) {
>+            let category = categories[categoryName];
>+
>+            let details = DOMUtils.createDetails(`${category.className} (${category.count})`);
>+            
>+            let instanceListElement = document.createElement('ul');
>+            instanceListElement.className = 'instance-list';
>+            
>+            let instanceList = new InstanceList(instanceListElement, this.snapshot, function(snapshot) {
>+                return HeapSnapshot.instancesWithClassName(snapshot, categoryName);
>+            });
>+            
>+            instanceList.buildList(this);
>+
>+            details.appendChild(instanceListElement);
>+            this.objectByTypeContainer.appendChild(details);
>+        }
>+    }
>+    
>+    buildRoots()
>+    {
>+        let roots = this.snapshot.rootNodes();
>+        if (roots.length == 0)
>+            return;
>+
>+        let groupings = roots.reduce(function(accumulator, node) {
>+            var key = node.className;
>+                if (!accumulator[key]) {
>+                  accumulator[key] = [];
>+                }
>+                accumulator[key].push(node);
>+                return accumulator;        
>+        }, {});
>+    
>+        let rootNames = Object.keys(groupings).sort();
>+    
>+        for (var rootClassName of rootNames) {
>+            let rootsOfType = groupings[rootClassName];
>+        
>+            rootsOfType.sort(function(a, b) {
>+                let addressA = HeapInspectorUtils.addressForNode(a);
>+                let addressB = HeapInspectorUtils.addressForNode(b);
>+                return (addressA < addressB) ? -1 : (addressA > addressB) ? 1 : 0;
>+            })
>+
>+            let details = DOMUtils.createDetails(`${rootClassName} (${rootsOfType.length})`);
>+            let rootsTypeList = document.createElement('ul')
>+            rootsTypeList.className = 'instance-list';
>+
>+            for (let root of rootsOfType) {
>+                let rootListItem = document.createElement('li');
>+                rootListItem.appendChild(HeapInspectorUtils.spanForNode(this, root, true));
>+                rootsTypeList.appendChild(rootListItem);
>+            }
>+
>+            details.appendChild(rootsTypeList);
>+
>+            this.rootsContainer.appendChild(details);
>+        }
>+    }
>+    
>+    buildPathsToRootsOfType(type)
>+    {
>+        let instances = HeapSnapshot.instancesWithClassName(this.snapshot, type);
>+        if (instances.length == 0)
>+            return;
>+
>+        let header = document.createElement('h2');
>+        header.textContent = 'Shortest paths to all ' + type + 's';
>+    
>+        let detailsContainer = document.createElement('section')
>+        detailsContainer.className = 'path';
>+
>+        for (var instance of instances) {
>+            let shortestPath = this.snapshot.shortestGCRootPath(instance.id).reverse();
>+
>+            let details = DOMUtils.createDetails('');
>+            let summary = details.firstChild;
>+            summary.appendChild(HeapInspectorUtils.spanForNode(this, instance, true));
>+            summary.appendChild(document.createTextNode('â'));
>+            summary.appendChild(HeapInspectorUtils.summarySpanForPath(this, shortestPath));
>+
>+            let pathList = document.createElement('ul');
>+            pathList.className = 'path';
>+
>+            let isNode = true;
>+            let currItem = undefined;
>+            for (let item of shortestPath) {
>+                if (isNode) {
>+                    currItem = document.createElement('li');
>+                    currItem.appendChild(HeapInspectorUtils.spanForNode(this, item));
>+                    pathList.appendChild(currItem);
>+                } else {
>+                    currItem.appendChild(HeapInspectorUtils.spanForEdge(this.snapshot, item));
>+                    currItem = undefined;
>+                }
>+                isNode = !isNode;
>+            }
>+        
>+            details.appendChild(pathList);
>+            detailsContainer.appendChild(details);
>+        }
>+
>+        this.pathsToRootsContainer.appendChild(header);
>+        this.pathsToRootsContainer.appendChild(detailsContainer);
>+    }
>+
>+    showAllPathsToNode(node)
>+    {
>+        let paths = this.snapshot._gcRootPaths(node.id);
>+    
>+        let details = DOMUtils.createDetails('');
>+        let summary = details.firstChild;
>+        summary.appendChild(document.createTextNode(`${paths.length} path${paths.length > 1 ? 's' : ''} to `));
>+        summary.appendChild(HeapInspectorUtils.spanForNode(this, node, false));
>+
>+        let detailsContainer = document.createElement('section')
>+        detailsContainer.className = 'path';
>+    
>+        for (let path of paths) {
>+            let pathNodes = path.map((component) => {
>+                if (component.node)
>+                    return this.snapshot.serializeNode(component.node);
>+                return this.snapshot.serializeEdge(component.edge);
>+            }).reverse();
>+
>+            let pathDetails = DOMUtils.createDetails('');
>+            let pathSummary = pathDetails.firstChild;
>+            pathSummary.appendChild(HeapInspectorUtils.summarySpanForPath(this, pathNodes));
>+        
>+            let pathList = document.createElement('ul');
>+            pathList.className = 'path';
>+
>+            let isNode = true;
>+            let currItem = undefined;
>+            for (let item of pathNodes) {
>+                if (isNode) {
>+                    currItem = document.createElement('li');
>+                    currItem.appendChild(HeapInspectorUtils.spanForNode(this, item));
>+                    pathList.appendChild(currItem);
>+                } else {
>+                    currItem.appendChild(HeapInspectorUtils.spanForEdge(this.snapshot, item));
>+                    currItem = undefined;
>+                }
>+                isNode = !isNode;
>+            }
>+    
>+            pathDetails.appendChild(pathList);
>+            detailsContainer.appendChild(pathDetails);
>+        }
>+    
>+        details.appendChild(detailsContainer);
>+        this.allPathsContainer.appendChild(details);
>+    }
>+
>+};
>+
>+
>+function loadResults(dataString, filename)
>+{
>+    let inspectorContainer = document.getElementById('uiContainer');
>+    inspector = new HeapSnapshotInspector(inspectorContainer, dataString, filename)
>+}
>+
>+function filenameForPath(filepath)
>+{
>+    return filepath.match(/([^\/]+)(?=\.\w+$)/)[0];
>+}
>+
>+function hideDescription()
>+{
>+    document.getElementById('description').classList.add('hidden');
>+}
>+
>+var inspector;
>+
>+function setupInterface()
>+{
>+    // See if we have a file to load specified in the query string.
>+    var query_parameters = {};
>+    var pairs = window.location.href.slice(window.location.href.indexOf('?') + 1).split('&');
>+    var filename = "test-heap.json";
>+
>+    for (var i = 0; i < pairs.length; i++) {
>+        var pair = pairs[i].split('=');
>+        query_parameters[pair[0]] = decodeURIComponent(pair[1]);
>+    }
>+
>+    if ("filename" in query_parameters)
>+        filename = query_parameters["filename"];
>+
>+    fetch(filename)
>+      .then(function(response) {
>+          if (response.ok)
>+              return response.text();
>+          throw new Error('Failed to load data file ' + filename);
>+      })
>+      .then(function(dataString) {
>+          loadResults(dataString, filenameForPath(filename));
>+          hideDescription();
>+          document.getElementById('uiContainer').style.display = 'block';
>+      });
>+
>+    var drop_target = document.getElementById("dropTarget");
>+
>+    drop_target.addEventListener("dragenter", function (e) {
>+        drop_target.className = "dragOver";
>+        e.stopPropagation();
>+        e.preventDefault();
>+    }, false);
>+
>+    drop_target.addEventListener("dragover", function (e) {
>+        e.stopPropagation();
>+        e.preventDefault();
>+    }, false);
>+
>+    drop_target.addEventListener("dragleave", function (e) {
>+        drop_target.className = "";
>+        e.stopPropagation();
>+        e.preventDefault();
>+    }, false);
>+
>+    drop_target.addEventListener("drop", function (e) {
>+        drop_target.className = "";
>+        e.stopPropagation();
>+        e.preventDefault();
>+
>+        for (var i = 0; i < e.dataTransfer.files.length; ++i) {
>+            var file = e.dataTransfer.files[i];
>+
>+            var reader = new FileReader();
>+            reader.filename = file.name;
>+            reader.onload = function(e) {
>+                loadResults(e.target.result, filenameForPath(this.filename));
>+                hideDescription();
>+                document.getElementById('uiContainer').style.display = 'block';
>+            };
>+
>+            reader.readAsText(file);
>+            document.title = "GC Heap: " + reader.filename;
>+        }
>+    }, false);
>+}
>+
>+window.addEventListener('load', setupInterface, false);

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Attachments on bug 186416: 342231 | 347981 | 347982