Subject: Re: [PATCH] Make strbuf_cmp inline, constify its arguments and optimize it a bit

From: Pierre Habouzit <[email-masked]>
Date: Mon, 08 Oct 2007 09:23:12 +0200

On Mon, Oct 08, 2007 at 01:45:27AM +0000, Miles Bader wrote:
> Alex Riesen <[email-masked]> writes:
> > int strbuf_cmp2(struct strbuf *a, struct strbuf *b)
> > {
> > 	int len = a->len < b->len ? a->len: b->len;
> > 	int cmp = memcmp(a->buf, b->buf, len);
> > 	if (cmp)
> > 		return cmp;
> > 	return a->len < b->len ? -1: a->len != b->len;
> > }
> 
> BTW, why are you making such effort to return only -1, 0, or 1 in the
> last line?  memcmp/strcmp make no such guarantee; e.g. glibc says:
> 
>      The `strcmp' function compares the string S1 against S2, returning
>      a value that has the same sign as the difference between the first
>      differing pair of characters (interpreted as `unsigned char'
>      objects, then promoted to `int').
> 
>      If the two strings are equal, `strcmp' returns `0'.
> 
>      A consequence of the ordering used by `strcmp' is that if S1 is an
>      initial substring of S2, then S1 is considered to be "less than"
>      S2.
> 
> So I think the last line can just be:
> 
>    return a->len - b->len;

  Won't work because ->len are size_t and return value is int, so on 64
bits platform, this has chances to overflow.

  FWIW I believe we are doing micro-benchs in a function that is used in
2 places in git right now.

-- 
·O·  Pierre Habouzit
··O                                                [email-masked]
OOO                                                http://www.madism.org