Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
* Erik Faye-Lund <kusmabite@gmail.com> wrote:
Show 22 quoted lines
> 2011/4/28 Ingo Molnar <mingo@elte.hu>:
> >
> > * Junio C Hamano <gitster@pobox.com> wrote:
> >
> >> Ingo Molnar <mingo@elte.hu> writes:
> >>
> >> > +static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> >> > {
> >> > - return !memcmp(sha1, null_sha1, 20);
> >> > + int i;
> >> > +
> >> > + for (i = 0; i < 20; i++, sha1++, sha2++) {
> >> > + if (*sha1 != *sha2) {
> >> > + if (*sha1 < *sha2)
> >> > + return -1;
> >> > + return +1;
> >> > + }
>
> Why not just:
>
> if (*sha1 != *sha2)
> return *sha2 - *sha1;You mean "*sha1 - *sha2", right?
> memcmp isn't guaranteed to return onlt the values -1, 0, +1, it can
> return any value, just as long as it's sign of a non-zero return
> express the relationship between the first mis-matching byte.
Yeah, agreed, updated patch below. Seems to work fine here.
Ingo
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/cache.h b/cache.h
index 2674f4c..574c948 100644
--- a/cache.h
+++ b/cache.h
@@ -675,14 +675,30 @@ extern char *sha1_pack_name(const unsigned char *sha1);
extern char *sha1_pack_index_name(const unsigned char *sha1);
extern const char *find_unique_abbrev(const unsigned char *sha1, int);
extern const unsigned char null_sha1[20];
-static inline int is_null_sha1(const unsigned char *sha1)
+
+static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
{
- return !memcmp(sha1, null_sha1, 20);
+ int i;
+
+ for (i = 0; i < 20; i++, sha1++, sha2++) {
+ if (*sha1 != *sha2)
+ return *sha1 - *sha2;
+ }
+
+ return 0;
}
-static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
+
+static inline int is_null_sha1(const unsigned char *sha1)
{
- return memcmp(sha1, sha2, 20);
+ const unsigned long long *sha1_64 = (void *)sha1;
+ const unsigned int *sha1_32 = (void *)sha1;
+
+ if (sha1_64[0] || sha1_64[1] || sha1_32[4])
+ return 0;
+
+ return 1;
}
+
static inline void hashcpy(unsigned char *sha_dst, const unsigned char *sha_src)
{
memcpy(sha_dst, sha_src, 20);