[Webkit-unassigned] [Bug 71202] New: DFG OSR exits should add to value profiles

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=71202

           Summary: DFG OSR exits should add to value profiles
           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


Value profiles are stochastic and imprecise.  In return, we get some nice properties, such as that they are really cheap - so even if code never gets optimized, such as if it is short running, it will still run quickly despite being profiled.  But, that imprecision often leads to misspeculations and missed opportunities.

An easy way to side-step this is if code frequently fails speculation by performing OSR exit, the OSR exit code should augment the value profile associated with the value that failed speculation (i.e. the value profile that contained the bad information) with that value.  This way, if recompilation is triggered due to frequent speculation failures, the DFG will have not just the normal value profiles but also a snapshot of why speculation was failing previously.

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