On Tue, Oct 13, 2015 at 8:58 PM, Ronald S. Bultje <rsbul...@gmail.com> wrote: > Hi, > > On Tue, Oct 13, 2015 at 8:09 PM, Ganesh Ajjanagadde <gajja...@mit.edu> > wrote: > >> 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? > > > You'll notice that Paul did it for the filters he maintains. I'm fine with > you doing this for any code I maintain (no further review required). You > can find maintainers for each piece of code in git log or MAINTAINERS. It > sounds like Paul is fine with this also. I think the general case, it'd be > nice to figure out why Carl's results are slightly different from yours (or > maybe it's noise?). If we can resolve that, I don't think there's any > further outstanding objections, right?
No other outstanding objections - the only serious concern is availability (which is a non-issue since we were already using fabs, fabsf sporadically). Carl's issues should be either noise, or a bad/slow libc fabs implementation. Hence I requested him for his config. I will give respective maintainers a week for slowly adding this stuff. To reiterate, I have not touched avcodec as it is mostly integer math anyway - if someone could point out some "hotspots" in avcodec with this issue, that would be great. > > Also, is a single push preferred, or one for each >> file (like the way it is being done)? > > > Whatever you prefer, really. Small commits have the advantage that they're > easier to bisect/revert if something breaks, but it's obviously a lot more > work to create N small patches than 1 big patch. So both approaches have > advantages and disadvantages and we just do whatever works best for each of > us. (Not very consistent, I admit.) I think giving a week will let me do a single patch "remaining floating point abs...". > > Ronald > _______________________________________________ > 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
