>----Messaggio originale---- >Da: kei...@vmware.com >Data: 20/09/2011 16.45 >A: "Roland Scheidegger"<srol...@vmware.com> >Cc: "Fabio"<fabio....@libero.it>, <mesa-dev@lists.freedesktop.org> >Ogg: Re: [Mesa-dev] Building with -fno-builtin-memcmp for improved performance > >On Tue, 2011-09-20 at 16:35 +0200, Roland Scheidegger wrote: >> Am 20.09.2011 16:15, schrieb Keith Whitwell: >> > On Tue, 2011-09-20 at 16:02 +0200, Roland Scheidegger wrote: >> >> Am 20.09.2011 12:35, schrieb Keith Whitwell: >> >>> On Tue, 2011-09-20 at 10:59 +0200, Fabio wrote: >> >>>> There was a discussion some months ago about using -fno-builtin-memcmp for >> >>>> improving memcmp performance: >> >>>> http://lists.freedesktop.org/archives/mesa-dev/2011-June/009078.html >> >>>> >> >>>> Since then, was it properly addressed in mesa or the flag is still >> >>>> recommended? If so, what about adding it in configure.ac? >> >>> >> >>> I've been meaning to follow up on this too. I don't know the answer, >> >>> but pinging Roland in case he does. >> >> >> >> I guess it is still recommended. >> >> Ideally this is really something which should be fixed in gcc - the >> >> compiler has all the knowledge about fixed alignment and size (if any) >> >> (and more importantly knows if only a binary answer is needed which >> >> makes this much easier) and doesn't need to do any function call. >> >> If you enable that flag and some platform just has the same primitive >> >> repz cmpsb sequence in the system library it will just get even slower, >> >> though I guess chances of that happening are slim (with the possible >> >> exception of windows). >> >> I think in most cases it won't make much difference, so nobody cared to >> >> implement that change. It is most likely still a good idea unless gcc >> >> addressed that in the meantime... >> > >> > Hmm, it seemed like it made a big difference in the earlier >> > discussion... >> Yes for llvmpipe and one app at least. >> But that struct being compared there is most likely the biggest (by far) >> anywhere (at least which is compared in a regular fashion). >> >> > I should take a look at reducing the size of the struct (as mentioned >> > before), but surely there's some way to pull in a better memcmp?? >> >> Well, apart from using -fno-builtin-memcmp we could build our own >> memcmpxx, though the version I did there (returning binary only result >> and assuming 32bit alignment/size allowing gcc to optimize it) was still >> slower for large sizes than -fno-builtin-memcmp. Of course we could >> optimize it more (e.g. for 64bit aligned/sized things, or using >> hand-coded sse2 versions using 128bit at-a-time comparisons) but then it >> gets more complicated, so I wasn't sure it was worth it. >> >> For reference here are the earlier numbers (ipers with llvmpipe): >> original ipers: 12.1 fps >> optimized struct compare: 16.8 fps >> -fno-builtin-memcmp: 18.1 fps >> >> And this was the function I used for getting the numbers: >> >> static INLINE int util_cmp_struct(const void *src1, const void *src2, >> unsigned count) >> { >> /* hmm pointer casting is evil */ >> const uint32_t *src1_ptr = (uint32_t *)src1; >> const uint32_t *src2_ptr = (uint32_t *)src2; >> unsigned i; >> assert(count % 4 == 0); >> for (i = 0; i < count/4; i++) { >> if (*src1_ptr != *src2_ptr) { >> return 1; >> } >> src1_ptr++; >> src2_ptr++; >> } >> return 0; >> } > >OK, maybe the first thing to do is fix the compared struct, then let's >see if there's anything significant left for a better memcmp to extract. > >I can find some time to do that in the next few days.
Was this problem ever reported to gcc devs BTW? I did a quick search and didn't find anything. Maybe it could be properly fixed there. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev