Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons

> Ingo Molnar wrote:
> 
> > Most overhead is in hashcmp(), which uses memcmp(), which falls back to 
> > assembly string operations.
> >
> > But we know that hashcmp() compares hashes, which if they do not match, the first byte
> > will differ in 99% of the cases.
> >
> > So i tried the patch below: instead of relying on GCC putting in the string 
> > ops, i used an open-coded loop for this relatively short comparison, which does 
> > not go beyond the first byte in 99% of the cases.
> [...]
> > --- a/cache.h
> > +++ b/cache.h
> > @@ -675,14 +675,33 @@ extern char *sha1_pack_name(const unsigned char *sha1);
> [...]
> > +static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> >  {
> > -	return memcmp(sha1, sha2, 20);
> > +	int i;
> > +
> > +	for (i = 0; i < 20; i++, sha1++, sha2++) {
> 
> Hm.  This would be very sensitive to the compiler, since a too-smart 
> optimizer could take this loop and rewrite it back to memcmp! So I wonder if 
> it's possible to convey this to the compiler more precisely:
> 
> 	return memcmp_probably_differs_early(sha1, sha2, 20);
> 
> E.g., how would something like
> 
> 	const unsigned int *start1 = (const unsigned int *) sha1;
> 	const unsigned int *start2 = (const unsigned int *) sha2;
> 
> 	if (likely(*start1 != *start2)) {
> 		if (*start1 < *start2)
> 			return -1;
> 		return +1;
> 	}
> 	return memcmp(sha1 + 4, sha2 + 4, 16);
>
> perform?