Hi Matt, Thanks for the quick reply.
On Mon, Nov 16, 2015 at 11:02 PM, Matt Oliver <protogo...@gmail.com> wrote: > On 17 November 2015 at 12:12, Hans Wennborg <h...@chromium.org> wrote: >> This commit [1] is causing problems when compiling with Clang on Windows: >> >> ..\..\third_party\ffmpeg\libavutil/x86/intmath.h(53,33) : error: >> always_inline function '__tzcnt_u32' requires target feature 'bmi', >> but would be inlined into function 'ff_ctzll_x86' that is compiled >> without support for 'bmi' >> return ((uint32_t)v == 0) ? _tzcnt_u32((uint32_t)(v >> 32)) + 32 : >> _tzcnt_u32((uint32_t)v); >> ^ >> >> Essentially the compiler is saying that it won't allow using this >> intrinsic unless compiling for a target that supports BMI. >> >> Is there a performance reason for using __tzcnt_u32 instead of >> _BitScanForward, or was it mainly to simplify the code? >> >> We're working around this in Chromium by #define'ing __tzcnt_u32 to >> __builtin_ctz at the moment, but it would be good if we could find a >> nicer solution that could be applied upstream. >> >> Cheers, >> Hans >> >> [1]. >> https://github.com/FFmpeg/FFmpeg/commit/6c6ac9cb17c4944514bde833f2fa8aa8dafa974a > > > tzcnt was used instead of bsf as it has performance advantages on any cpu > that supports the tzcnt instruction. Although tzcnt is a newer instruction > that is part of the BMI instruction set the actual opcode generated by this > instruction is the equivalent to 'rep bsf'. So this instruction will still > execute on any older cpu that doesnt support tzcnt as just the bsf > instruction instead. So it was used as it provides optimum performance on > both newer and older cpus. > > Clang is clearly not allowing this particular optimisation. I dont know of a > way to ignore this using a command line option so probably the best way is > to just disable this code when using clang. This will go back to using the > previous behavior which assuming clang on windows still defines the gcc > version check macros will use the __builtin_ctz. > > The attached patch should fix it but I dont have clang for windows to test > it. This got fixed in Clang instead: http://llvm.org/viewvc/llvm-project?rev=253358&view=rev It will now allow the intrinsic even if the target doesn't have BMI. Sorry for the noise. Cheers, Hans _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
