[FFmpeg-devel] Don't use _tzcnt instrinics with clang for windows w/o BMI.

Dale Curtis dalecurtis at chromium.org
Tue Oct 24 23:15:33 EEST 2017

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:

Signed-off-by: Dale Curtis <dalecurtis at chromium.org>
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