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

Alex Riesen [off-list ref] 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;

-miles

-- 
Suburbia: where they tear out the trees and then name streets after them.

On Mon, Oct 08, 2007 at 01:45:27AM +0000, Miles Bader wrote:

Alex Riesen [off-list ref] writes:

quoted

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                                                madcoder@debian.org
OOO                                                http://www.madism.org