[Webkit-unassigned] [Bug 85752] New: Truncating multiplication on integers should not OSR exit every time

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=85752

           Summary: Truncating multiplication on integers should not OSR
                    exit every time
           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


Say you do a * b where the result is larger than what can fit in an int32, but you use the result in integer contexts only (like [_] | 0).  Currently the DFG will "prove" that the multiplication will produce integers despite overflowing.  But then the multiplication will speculate no overflow, which will typically fail, since in this case a * b does overflow.

Multiplication should be smarter about this.  If you do a * b and we know that it overflows then in the general case our best bet is to perform the multiplication using the FPU.  But if we know something about a or b, for example if we really have something like a * 42, then we know that integer truncation would be equivalent to double truncation - and hence we can perform an integer multiplication and skip the overflow check.

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