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 <dalecur...@chromium.org>
From 123a60bdcf99cab6b6e6bd31a463a29bc49598f0 Mon Sep 17 00:00:00 2001 From: Dale Curtis <dalecur...@chromium.org> Date: Tue, 24 Oct 2017 13:03:59 -0700 Subject: [PATCH] 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 <dalecur...@chromium.org> --- libavutil/x86/intmath.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/libavutil/x86/intmath.h b/libavutil/x86/intmath.h index e83971c084..40743fd13e 100644 --- a/libavutil/x86/intmath.h +++ b/libavutil/x86/intmath.h @@ -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 -- 2.15.0.rc0.271.g36b669edcc-goog
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