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

[Roland Scheidegger]

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;
}

Roland