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Don't use _tzcnt instrinics with clang for windows w/o BMI. 

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>
This commit is contained in:
Dale Curtis 2017-10-24 13:03:59 -07:00 committed by Michael Niedermayer
parent 953d55f443
commit 50e30d9bb7
1 changed files with 2 additions and 1 deletions
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3
libavutil/x86/intmath.h
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@ -47,7 +47,8 @@ static av_always_inline av_const int ff_log2_x86(unsigned int v)
# endif
# define ff_log2_16bit av_log2
#if defined(__INTEL_COMPILER) || (defined(_MSC_VER) && (_MSC_VER >= 1700))
#if defined(__INTEL_COMPILER) || (defined(_MSC_VER) && (_MSC_VER >= 1700) && \
(defined(__BMI__) || !defined(__clang__)))
# define ff_ctz(v) _tzcnt_u32(v)
# if ARCH_X86_64