* Erik Faye-Lund <kusmabite@xxxxxxxxx> wrote:
> 2011/4/28 Ingo Molnar <mingo@xxxxxxx>:
> >
> > * Erik Faye-Lund <kusmabite@xxxxxxxxx> wrote:
> >
> >> > Secondly, the combined speedup of the cached case with my two patches
> >> > appears to be more than 30% on my testbox so it's a very nifty win from two
> >> > relatively simple changes.
> >>
> >> That speed-up was on ONE test vector, no? There are a lot of other uses of
> >> hash-comparisons in Git, did you measure those?
> >
> > I picked this hash function because it showed up in the profile (see the
> > profile i posted). There's one other hash that mattered as well in the profile,
> > see the lookup_object() patch i sent yesterday.
>
> My point was that the 30% improvement was in "git gc", which is not
> the only important use-case. How does this affect other git commands?
In a followup mail i measured git fsck, which showed a speedup too. (despite
being mostly dependent on external libraries to do most of the processing)
If you'd like to see other things tested please suggest a testcase that you
think uses these hashes extensively, i don't really know what the slowest (and
affected) Git commands are - git gc is the one *i* notice as being pretty slow
(for good reasons).
> >> from the exception handler, others doesn't. So this patch is pretty much
> >> guaranteed to cause a crash in some setups.
> >
> > If unsigned char arrays are allocated unaligned then that's another bug i
> > suspect that should be fixed.
>
> We can't. The compiler decides the alignment of variables on the stack. Some
> compilers / compiler-setting pairs might align char-arrays, while others
> might not.
Even if that were true it can be solved: you'd need to declare the sha1 not as
a char array but as a u32 * array or so. We do have control over the alignment
of data structures, obviously.
> > Unaligned access on x86 is not free either - there's cycle penalties.
> >
> > Alas, i have not seen these sha1 hash buffers being allocated unaligned (in
> > my very limited testing). In which spots are they allocated unaligned?
>
> Like I said above, it can happen when allocated on the stack. But it can also
> happen in malloc'ed structs, or in global variables. An array is aligned to
> the size of it's base member type. But malloc does worst-case-allignment,
> because it happens at run-time without type-information.
Well, should we ready be ready to throw up our hands as if we didnt have
control over the alignment of objects and have to accept suboptimal code as a
result? We do have control over that.
In any case, i'll retract the null case as it really isnt called that often in
the tests i've done - updated patch below - it simply falls back on to hashcmp.
Thanks,
Ingo
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
diff --git a/cache.h b/cache.h
index 2674f4c..39fa9cd 100644
--- a/cache.h
+++ b/cache.h
@@ -675,14 +675,24 @@ 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);
+ return !hashcmp(sha1, null_sha1);
}
+
static inline void hashcpy(unsigned char *sha_dst, const unsigned char *sha_src)
{
memcpy(sha_dst, sha_src, 20);
--
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