Re: Slow memcmp for aligned strings on Pentium 3

[Roger Sayle <roger@www.eyesopen.com>]

Hi Kevin,
> I did some tests and discovered that using cmps was rather slow,
> compared to a simple loop and then a bswap and subtract at the end.

I'm sure that GCC's memcmp implementations could be improved, but
from reading the code examples in your patch it looks like you
are always assuming that either the length is a multiple of four,
or that the bytes following the memory sections to be compared
contain identical values (i.e. you're hoping they're all zero).

i.e., if p and q are suitably 4-byte aligned

  memset(p,"abcd",4);
  memset(q,"abef",4);
  memcmp(p,q,2)

should compare equal but don't using bswaps and subtractions.
Similarly, when two words mismatch their return value <0 or
>0 should depend upon the first byte that differs, not the
values of the bytes that come after it.

I suspect it should be possible to fix your code to handle these
termination conditions correctly, and a comparison of your
routine's performance with these fixes vs. __builtin_memcmp
would be of interest.

Roger
--
Roger Sayle,                         E-mail: roger@eyesopen.com
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