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

[Fabio]

>----Messaggio originale----
>Da: keithw at vmware.com
>Data: 20/09/2011 16.45
>A: "Roland Scheidegger"<sroland at vmware.com>
>Cc: "Fabio"<fabio.ped at libero.it>, <mesa-dev at 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.