2015-03-13  Filip Pizlo  <fpizlo@apple.com>

        DFG::PutStackSinkingPhase should eliminate GetStacks that have an obviously known source

        https://bugs.webkit.org/show_bug.cgi?id=141624

        Reviewed by NOBODY (OOPS!).

        This was an obvious omission from the original PutStackSinkingPhase. Previously, we would treat

        GetStacks conservatively and assume that the stack slot escaped. That's pretty dumb, since a

        GetStack is a local load of the stack. This change makes GetStack a no-op from the standpoint of

        this phase's deferral analysis. At the end we either keep the GetStack (if there was no concrete

        deferral) or we replace it with an identity over the value that would have been stored by the

        deferred PutStack. Note that this might be a Phi that the phase creates, so this is strictly

        stronger than what GCSE could do.

        This is probably not a speed-up now, but it will be very useful for the varargs simplification

        done in bug 141174.

        * dfg/DFGPutStackSinkingPhase.cpp:

                    if (node->op() == GetStack) {

                        // A GetStack doesn't affect anything, since we know which local we are reading

                        // from.

                        continue;

                    }

                case GetStack: {

                    StackAccessData* data = node->stackAccessData();

                    FlushFormat format = deferred.operand(data->local);

                    if (!isConcrete(format)) {

                        // This means there is no deferral. No deferral means that the most

                        // authoritative value for this stack slot is what is stored in the stack. So,

                        // keep the GetStack.

                        break;

                    }

                    // We have a concrete deferral, which means a PutStack that hasn't executed yet. It

                    // would have stored a value with a certain format. That format must match our

                    // format. But more importantly, we can simply use the value that the PutStack would

                    // have stored and get rid of the GetStack.

                    DFG_ASSERT(m_graph, node, format == data->format);

                    Node* incoming = mapping.operand(data->local);

                    node->convertToIdentity();

                    node->child1() = incoming->defaultEdge();

                    break;

                }