[Webkit-unassigned] [Bug 83234] New: DFG InstanceOf should not uselessly speculate cell

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=83234

           Summary: DFG InstanceOf should not uselessly speculate cell
           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


A useless speculation is one that:

1) Is performed on a value that dies at the point of speculation.

2) Handling the case where the speculation would fail is trivial and takes fewer instructions than an OSR exit trampoline (i.e. <12 bytes on x86).

InstanceOf has a SpeculateCell optimization, which is probably great most of the time, but there are times when you do something like:

if (o.f instanceof p)

The temporary result of o.f dies immediately. Handling the case where it's not a cell is super easy: just return false. So there's little use in speculating cell here.

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