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Technically _tzcnt* intrinsics are only available when the BMI instruction set is present. However the instruction encoding degrades to "rep bsf" on older processors. Clang for Windows debatably restricts the _tzcnt* instrinics behind the __BMI__ architecture define, so check for its presence or exclude the usage of these intrinics when clang is present. See also: https://ffmpeg.org/pipermail/ffmpeg-devel/2015-November/183404.html https://bugs.llvm.org/show_bug.cgi?id=30506 http://lists.llvm.org/pipermail/cfe-dev/2016-October/051034.html Signed-off-by: Dale Curtis <dalecurtis@chromium.org> Reviewed-by: Matt Oliver <protogonoi@gmail.com> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>