2013-10-11  Filip Pizlo  <fpizlo@apple.com>

        FTL OSR exit should perform zero extension on values smaller than 64-bit

        https://bugs.webkit.org/show_bug.cgi?id=122688

        Reviewed by NOBODY (OOPS!).

        In the DFG we usually make the simplistic assumption that a 32-bit value in a 64-bit

        register will have zeros on the high bits.  In the few cases where the high bits are

        non-zero, the DFG sort of tells us this explicitly.

        But when working with llvm.webkit.stackmap, it doesn't work that way.  Consider we might

        emit LLVM IR like:

            %2 = trunc i64 %1 to i32

            stuff %2

            call @llvm.webkit.stackmap(...., %2)

        LLVM may never actually emit a truncation instruction of any kind.  And that's great - in

        many cases it won't be needed, like if %2 is a 32-bit op that ignores the high bits

        anyway.  Hence LLVM may tell us that %2 is in the register that still had the value from

        before truncation, and that register may have garbage in the high bits.

        This means that on our end, if we want a 32-bit value and we want that value to be

        zero-extended, we should zero-extend it ourselves.  This is pretty easy and should be

        cheap, so we should just do it and not make it a requirement that LLVM does it on its

        end.

        This makes all tests pass with JSC_ftlOSRExitUsesStackmap=true.

        * ftl/FTLOSRExitCompiler.cpp:

        (JSC::FTL::compileStubWithOSRExitStackmap):

        * ftl/FTLValueFormat.cpp:

        (JSC::FTL::reboxAccordingToFormat):

        jit.zeroExtend32ToPtr(value, value);

        jit.zeroExtend32ToPtr(value, value);

        jit.zeroExtend32ToPtr(value, value);