[Mesa-dev] Building with -fno-builtin-memcmp for improved performance

[Keith Whitwell]

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.

Keith