[Webkit-unassigned] [Bug 68294] New: method_check should repatch itself if it finds that the new structure(s) are the result of transitions from the old structure(s)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=68294

           Summary: method_check should repatch itself if it finds that
                    the new structure(s) are the result of transitions
                    from the old structure(s)
           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


The method_check optimization in the old JIT is now relied upon as a kind of profiling by the DFG JIT.  But this fails if the method_check takes the slow path simply because the structures it was caching on were transitioned.  This results in missed dead code elimination and inlining opportunities in the DFG.

The old JIT should simply repatch the fast path of method_check if it finds that the slow path was taken only because of structure transitions, but that the specific function optimization for that property is otherwise intact.

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