[Webkit-unassigned] [Bug 160565] New: Undefined behavior in StdLibExtras.h, bitCount

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Thu Aug 4 13:45:46 PDT 2016


            Bug ID: 160565
           Summary: Undefined behavior in StdLibExtras.h, bitCount
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore Misc.
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jbedard at apple.com

In StdLibExtras.h bitCount, there is undefined behavior on return.  The line:
    return (((bits + (bits >> 4)) & 0xF0F0F0F) * 0x1010101) >> 24
almost always has undefined behavior.  In this case, the overflow is expected.  However, compiler optimization (given that this function is an inline) with constants may preform unexpected operations

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