Bug ID: 37062
           Summary: Clang should support _BitscanForward and
                    _BitscanForward64 in constant expressions
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++

In MSVC++'s STL, we implemented binary GCD as this can be faster, as it
replaces divisions with bsf. Unfortunately, right now we had to fake bsf with a
loop, resulting in std::gcd actually being slower than a naïve Euclid's
algorithm implementation. This could be substantially faster with real bsf.

Note that C1XX doesn't currently support _BitscanForward in constexpr contexts;
this bug is being filed at the same time it's being filed against C1XX (and

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