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

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=160565

Jonathan Bedard <jbedard at apple.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Jonathan Bedard <jbedard at apple.com> ---
Here is the error message clang's undefined behavior sanitizer emits for this particular error:

/Volumes/Data/Code/UndefinedBehavior/OpenSource/WebKitBuild/Debug/usr/local/include/wtf/StdLibExtras.h:163:48: runtime error: unsigned integer overflow: 1025 * 16843009 cannot be represented in type 'unsigned int'

Further investigation on this has revealed that clang is combining some 'suspicious' behaviors with 'undefined' behaviors, and this is a case of 'suspicious' behavior, but, as per C++ 2014 standard 3.9.1, 4, unsigned types overflow with modulo 2^n, which is our desired behavior in this case.  In short, the behavior corrected here is both defined and desired.

Undefined behavior sanitizer information: http://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

C++ standard:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf

No fix is needed.  Marking as resolved.

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