On Tue, Oct 13, 2015 at 2:45 AM, Clément Bœsch <u...@pkh.me> wrote: > On Tue, Oct 13, 2015 at 12:31:10AM -0400, Ganesh Ajjanagadde wrote: >> On Tue, Oct 13, 2015 at 12:26 AM, Ganesh Ajjanagadde <gajja...@mit.edu> >> wrote: >> > On Tue, Oct 13, 2015 at 12:16 AM, Carl Eugen Hoyos <ceho...@ag.or.at> >> > wrote: >> >> Ganesh Ajjanagadde <gajjanag <at> mit.edu> writes: >> >> >> >>> Bench from libavfilter/astats on a 15 min clip. >> >> >> >> I believe that your test would indicate that the >> >> old variant is faster or that no result can be >> >> given which is what my tests show. >> >> Also, how you can possibly believe that the old variant is faster is >> beyond me given the astonishing amount of work by Intel, Red Hat, and >> others to create the absolutely best performing libc. >> >> Just have a look at >> https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/ieee754/dbl-64/s_sin.c;hb=HEAD#l281, >> it gives an idea of the extreme lengths they go to. >> > > https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/ieee754/dbl-64/s_fabs.c;hb=HEAD > > [/tmp]☭ cat a.c > #include <math.h> > #include <stdlib.h> > > #define FFABS(a) ((a) >= 0 ? (a) : (-(a))) > > double f1d(double x) { return fabs(x); } > double f2d(double x) { return FFABS(x); } > > int f1i(int x) { return abs(x); } > int f2i(int x) { return FFABS(x); } > [/tmp]☭ gcc -O2 -c a.c && objdump -d -Mintel a.o > > a.o: file format elf64-x86-64 > > > Disassembly of section .text: > > 0000000000000000 <f1d>: > 0: f2 0f 10 0d 00 00 00 movsd xmm1,QWORD PTR [rip+0x0] # 8 > <f1d+0x8> > 7: 00 > 8: 66 0f 54 c1 andpd xmm0,xmm1 > c: c3 ret > d: 0f 1f 00 nop DWORD PTR [rax] > > 0000000000000010 <f2d>: > 10: 66 0f 2e 05 00 00 00 ucomisd xmm0,QWORD PTR [rip+0x0] # 18 > <f2d+0x8> > 17: 00 > 18: 72 06 jb 20 <f2d+0x10> > 1a: f3 c3 repz ret > 1c: 0f 1f 40 00 nop DWORD PTR [rax+0x0] > 20: f2 0f 10 0d 00 00 00 movsd xmm1,QWORD PTR [rip+0x0] # 28 > <f2d+0x18> > 27: 00 > 28: 66 0f 57 c1 xorpd xmm0,xmm1 > 2c: c3 ret > 2d: 0f 1f 00 nop DWORD PTR [rax] > > 0000000000000030 <f1i>: > 30: 89 fa mov edx,edi > 32: 89 f8 mov eax,edi > 34: c1 fa 1f sar edx,0x1f > 37: 31 d0 xor eax,edx > 39: 29 d0 sub eax,edx > 3b: c3 ret > 3c: 0f 1f 40 00 nop DWORD PTR [rax+0x0] > > 0000000000000040 <f2i>: > 40: 89 fa mov edx,edi > 42: 89 f8 mov eax,edi > 44: c1 fa 1f sar edx,0x1f > 47: 31 d0 xor eax,edx > 49: 29 d0 sub eax,edx > 4b: c3 ret > [/tmp]☭ > > So fabs() is inlined by the compiler (gcc 5.2.0 here), while abs() is > essentially identical to FFABS(). > > I have similar results with clang (3.7.0). > > Conclusion: using fabs() looks better with at least recent versions of clang > and GCC on x86-64 (but may introduce slight behaviour changes?) > > To be more rigorous, it would be interesting to compare on different arch & > compilers, but changing FFABS() with fabs() sounds OK to me.
I noticed that is being applied piecemeal, and some of it has been pushed. Does that mean I am free to push (with the reduced commit message) as well? Also, is a single push preferred, or one for each file (like the way it is being done)? > > -- > Clément B. > > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel > _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
