[Webkit-unassigned] [Bug 82155] New: DFG OSR exit value recoveries should be computed lazily

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=82155

           Summary: DFG OSR exit value recoveries should be computed
                    lazily
           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


Currently each OSR exit has a complete record of the value recoveries of all bytecode variables live at that exit. Each time a speculation check is emitted, we compute all of these value recoveries and save them.

This takes unnecessary space (since the value recovery for a variable at exit site K is likely to be identical to the value recovery for that variable at exit site K + 1) and unnecessary time (since we recompute those identical values repeatedly).

Instead, CodeBlock should record the sequence of variable-related changes in the DFG's representation of values. When an OSR exit is generated, the value recoveries can be lazily reconstructed from this sequence.

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