[i386] memcmp very slow

["Bonzini" <bonzini at gnu dot org>]

I think that the pattern that is used when inlining memcmp for the i386
should be disabled, except perhaps at -Os.  cmpsb is completely unoptimized
in the latest and not-so-latest processors (from the Pentium on).

Here are the results of timing a hand-written memcmp that I just submitted
to the libc-alpha mailing list (including call-return overheads) vs. the GCC
inline memcmp:

    utente@engineer:~/esperimenti$ gcc -g -O3 -fno-builtin test.c memcmp.S
    utente@engineer:~/esperimenti$ time ./a.out

    real    0m0.088s
    user    0m0.090s
    sys     0m0.000s

    utente@engineer:~/esperimenti$ gcc -g -O3 test.c
    utente@engineer:~/esperimenti$ time ./a.out

    real    0m0.102s
    user    0m0.100s
    sys     0m0.010s

The results that the inlined memcmp gives compared the current glibc memcmp
seem to be encouraging:

    utente@engineer:~/esperimenti$ gcc -g -O3 -fno-builtin test.c
    utente@engineer:~/esperimenti$ time ./a.out

    real    0m0.108s
    user    0m0.100s
    sys     0m0.010s

but this is only because glibc has a poor implementation of memcmp for the
i386 which also is nothing more than a cmpsb.

The handwritten assembly language memcmp implementation that I refer to is
available at http://sources.redhat.com/ml/libc-alpha/2002-10/msg00496.html

Paolo Bonzini