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fpizlo@apple.com authored
https://bugs.webkit.org/show_bug.cgi?id=121371 Reviewed by Sam Weinig. Forward rewiring is a tricky part of OSR that handles the following: a: Something(...) SetLocal(@a, locX) b: Int32ToDouble(@a) c: SomethingThatExits(@b) <no further uses of @a or @b> Note that at @c, OSR will think that locX->@a, but @a will be dead. So it must be smart enough to find @b, which contains an equivalent value. It must do this for any identity functions we support. Currently we support four such functions. Currently the code for doing this is basically duplicated between the DFG and the FTL. Also both versions of the code have some really weirdly written logic for picking the "best" identity function to use. We should fix this by simply having a way to ask "is this node an identity function, and if so, then how good is it?" Then both the DFG and FTL could use this and have no hard-wired knowledge of those identity functions. While we're at it, this also changes some terminology because I found the use of the word "needs" confusing. Note that this retains the somewhat confusing behavior that we don't search all possible forward/backward uses. We only search one step in each direction. This is because we only need to handle cases that FixupPhase and the parser insert. All other code that tries to insert intermediate conversion nodes should ensure to Phantom the original node. For example, the following transformation is illegal: Before: x: SomethingThatExits(@a) After: w: Conversion(@a) x: SomethingThatExits(@w) The correct form of that transformation is one of these: Correct #1: v: DoAllChecks(@a) // exit here w: Conversion(@a) x: Something(@w) // no exit Correct #2: w: Conversion(@a) x: SomethingThatExits(@w) y: Phantom(@a) Correct #3: w: Conversion(@a) x: SomethingThatExits(@w, @a) Note that we use #3 for some heap accesses, but of course it requires that the node you're using has an extra slot for a "dummy" use child. Broadly speaking though, such transformations should be relegated to something below DFG IR, like LLVM IR. * dfg/DFGNodeType.h: (JSC::DFG::forwardRewiringSelectionScore): (JSC::DFG::needsOSRForwardRewiring): * dfg/DFGVariableEventStream.cpp: (JSC::DFG::VariableEventStream::reconstruct): * ftl/FTLLowerDFGToLLVM.cpp: (JSC::FTL::LowerDFGToLLVM::addExitArgumentForNode): git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155793 268f45cc-cd09-0410-ab3c-d52691b4dbfc
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