Re: [ANNOUNCE] Git v2.19.0-rc0

[Jeff King <peff@peff.net>]

On Wed, Aug 22, 2018 at 09:39:57AM +0200, Ævar Arnfjörð Bjarmason wrote:

> > I don't have a good option. The assert() thing works until I add in the
> > "32" branch, but that's just punting the issue off until you add support
> > for the new hash.
> >
> > Hand-rolling our own asm or C is a portability headache, and we need to
> > change all of the callsites to use a new hasheq().
> >
> > Hiding it behind a per-hash function is conceptually cleanest, but not
> > quite as fast. And it also requires hasheq().
> >
> > So all of the solutions seem non-trivial.  Again, I'm starting to wonder
> > if it's worth chasing this few percent.
> 
> Did you try __builtin_expect? It's a GCC builtin for these sorts of
> situations, and sometimes helps:
> https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
> 
> I.e. you'd tell GCC we expect to have the 20 there with:
> 
>     if (__builtin_expect(the_hash_algo->rawsz == 20, 1)) { ... }
> 
> The perl codebase has LIKELY() and UNLIKELY() macros for this which if
> the feature isn't available fall back on just plain C code:
> https://github.com/Perl/perl5/blob/v5.27.7/perl.h#L3335-L3344

Sadly, no, this doesn't seem to change anything. We still end up with a
single call to memcmp.

I also tried "hiding" the fallback call like this:

diff --git a/cache.h b/cache.h
index b1fd3d58ab..7808bf3d6b 100644
--- a/cache.h
+++ b/cache.h
@@ -1021,9 +1021,13 @@ extern int find_unique_abbrev_r(char *hex, const struct object_id *oid, int len)
 extern const unsigned char null_sha1[GIT_MAX_RAWSZ];
 extern const struct object_id null_oid;
 
+int super_secret_memcmp(const void *a, const void *b, size_t len);
+
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
-	return memcmp(sha1, sha2, the_hash_algo->rawsz);
+	if (the_hash_algo->rawsz == 20)
+		return memcmp(sha1, sha2, 20);
+	return super_secret_memcmp(sha1, sha2, the_hash_algo->rawsz);
 }
 
 static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
diff --git a/sha1-file.c b/sha1-file.c
index 97b7423848..5cd0a4b73f 100644
--- a/sha1-file.c
+++ b/sha1-file.c
@@ -2280,3 +2280,8 @@ int read_loose_object(const char *path,
 		munmap(map, mapsize);
 	return ret;
 }
+
+int super_secret_memcmp(const void *a, const void *b, size_t len)
+{
+	return memcmp(a, b, len);
+}

but that just results in calling memcmp and super_secret_memcmp on the
two codepaths (with or without the __builtin_expect).

-Peff