[Webkit-unassigned] [Bug 88284] New: DFG CFG simplification should correct the variables at the head of the predecessor block

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=88284

           Summary: DFG CFG simplification should correct the variables at
                    the head of the predecessor block
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: fpizlo at apple.com


Say that we have two blocks, B1 and B2.  B1 doesn't do anything to variable X.  B2 reads from variable X.  Say that CFG simplification decides to merge B1 and B2.  Currently, the new block, B1', will still say that the variable at head for X is Phi, which is wrong - it should be GetLocal, since that's the first thing that B1' does to X.

This is asymptomatic except if we then do constant folding on X.

I'm still trying to figure out a test case for this, but it's not looking so good.

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