Victor Costan e1353b9fa8 Remove ARCH_* guards around Bits::FindLSBSetNonZero64(). ...

Bits::FindLSBSetNonZero64() is now available unconditionally, and it
should be easier to reason about the code included in each build
configuration.

This reduces the amount of conditional compiling going on, which makes
it easier to reason about what stubs are a used in each build
configuration.

The guards were added to work around the fact that MSVC has a
_BitScanForward64() intrinsic, but the intrinsic is only implemented on
64-bit targets, and causes a compilation error on 32-bit targets errors.
By contrast, Clang and GCC support __builtin_ctzll() everywhere, and
implement it with varying efficiency.

This CL reworks the conditional compilation directives so that
Bits::FindLSBSetNonZero64() uses the _BitScanForward64() intrinsic on
MSVC when available, and the portable implementation otherwise.
PiperOrigin-RevId: 310007748

2020-05-05 20:18:12 +00:00

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