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
