[Webkit-unassigned] [Bug 89802] New: Value profiling should use tier-up threshold randomization to get more coverage

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=89802

           Summary: Value profiling should use tier-up threshold
                    randomization to get more coverage
           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 we often end up optimizing code with incomplete value profiling coverage - as in, the typical predicted type will be based on only one recorded value.  We're pretty good at making reasonable guesses even with incomplete information, but the fact that the code relies on this is kind of sad.  It would be better if we could reliably get multiple recorded values.

We can do this, if we LUB (least-upper-bound, or ValueProfile::computeUpdatedPrediction()) all value profiles a few times between when profiling starts and when OSR is triggered.  One way to make that happen is to make the execution counter have some random value that is smaller than the required threshold for OSR.  This will cause each function to take an OSR slow path without actually triggering optimization, which will in turn give us an opportunity to LUB all value profiles.

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