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From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Jeff King <peff@peff.net>
Cc: "brian m. carlson" <sandals@crustytoothpaste.net>,
	Derrick Stolee <stolee@gmail.com>,
	Junio C Hamano <gitster@pobox.com>,
	Git Mailing List <git@vger.kernel.org>
Subject: Re: [ANNOUNCE] Git v2.19.0-rc0
Date: Wed, 22 Aug 2018 09:39:57 +0200	[thread overview]
Message-ID: <CACBZZX7Cmp8d=UKF2nk36fL7mR+umdKwKZAKNZSkyP0NXvquhw@mail.gmail.com> (raw)
In-Reply-To: <20180822060735.GA13195@sigill.intra.peff.net>

On Wed, Aug 22, 2018 at 8:20 AM Jeff King <peff@peff.net> wrote:
>
> On Wed, Aug 22, 2018 at 05:36:26AM +0000, brian m. carlson wrote:
>
> > On Tue, Aug 21, 2018 at 11:03:44PM -0400, Jeff King wrote:
> > > So I wonder if there's some other way to tell the compiler that we'll
> > > only have a few values. An enum comes to mind, though I don't think the
> > > enum rules are strict enough to make this guarantee (after all, it's OK
> > > to bitwise-OR enums, so they clearly don't specify all possible values).
> >
> > I was thinking about this:
> >
> > diff --git a/cache.h b/cache.h
> > index 1398b2a4e4..1f5c6e9319 100644
> > --- a/cache.h
> > +++ b/cache.h
> > @@ -1033,7 +1033,14 @@ extern const struct object_id null_oid;
> >
> >  static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> >  {
> > -     return memcmp(sha1, sha2, the_hash_algo->rawsz);
> > +     switch (the_hash_algo->rawsz) {
> > +             case 20:
> > +                     return memcmp(sha1, sha2, 20);
> > +             case 32:
> > +                     return memcmp(sha1, sha2, 32);
> > +             default:
> > +                     assert(0);
> > +     }
> >  }
>
> Unfortunately this version doesn't seem to be any faster than the status
> quo. And looking at the generated asm, it still looks to be calling
> memcpy(). Removing the "case 32" branch switches it back to fast
> assembly (this is all using gcc 8.2.0, btw). So I think we're deep into
> guessing what the optimizer is going to do, and there's a good chance
> that other versions are going to optimize it differently.
>
> We might be better off just writing it out manually. Unfortunately, it's
> a bit hard because the neg/0/pos return is more expensive to compute
> than pure equality. And only the compiler knows at each inlined site
> whether we actually want equality. So now we're back to switching every
> caller to use hasheq() if that's what they want.
>
> But _if_ we're OK with that, and _if_ we don't mind some ifdefs for
> portability, then this seems as fast as the original (memcmp+constant)
> code on my machine:
>
> diff --git a/cache.h b/cache.h
> index b1fd3d58ab..c406105f3c 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1023,7 +1023,16 @@ extern const struct object_id null_oid;
>
>  static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
>  {
> -       return memcmp(sha1, sha2, the_hash_algo->rawsz);
> +       switch (the_hash_algo->rawsz) {
> +       case 20:
> +               if (*(uint32_t *)sha1 == *(uint32_t *)sha2 &&
> +                   *(unsigned __int128 *)(sha1+4) == *(unsigned __int128 *)(sha2+4))
> +                       return 0;
> +       case 32:
> +               return memcmp(sha1, sha2, 32);
> +       default:
> +               assert(0);
> +       }
>  }
>
>  static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
>
> Which is really no surprise, because the generated asm looks about the
> same. There are obviously alignment questions there. It's possible it
> could even be written portably as a simple loop. Or maybe not. We used
> to do that, but modern compilers were able to optimize the memcmp
> better. Maybe that's changed. Or maybe they were simply unwilling to
> unroll a 20-length loop to find out that it could be turned into a few
> quad-word compares.
>
> > That would make it obvious that there are at most two options.
> > Unfortunately, gcc for me determines that the buffer in walker.c is 20
> > bytes in size and steadfastly refuses to compile because it doesn't know
> > that the value will never be 32 in our codebase currently.  I'd need to
> > send in more patches before it would compile.
>
> Yeah, I see that warning all over the place (everywhere that calls
> is_null_oid(), which is passing in a 20-byte buffer).
>
> > I don't know if something like this is an improvement or now, but this
> > seems to at least compile:
> >
> > diff --git a/cache.h b/cache.h
> > index 1398b2a4e4..3207f74771 100644
> > --- a/cache.h
> > +++ b/cache.h
> > @@ -1033,7 +1033,13 @@ extern const struct object_id null_oid;
> >
> >  static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> >  {
> > -     return memcmp(sha1, sha2, the_hash_algo->rawsz);
> > +     switch (the_hash_algo->rawsz) {
> > +             case 20:
> > +             case 32:
> > +                     return memcmp(sha1, sha2, the_hash_algo->rawsz);
> > +             default:
> > +                     assert(0);
> > +     }
>
> I think that would end up with the same slow code, as gcc would rather
> call memcmp than expand out the two sets of asm.
>
> > I won't have time to sit down and test this out until tomorrow afternoon
> > at the earliest.  If you want to send in something in the mean time,
> > even if that limits things to just 20 for now, that's fine.
>
> 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

  reply	other threads:[~2018-08-22  7:40 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-20 22:13 [ANNOUNCE] Git v2.19.0-rc0 Junio C Hamano
2018-08-20 22:41 ` Stefan Beller
2018-08-20 23:39   ` Jonathan Nieder
2018-08-21  0:27     ` Jonathan Nieder
2018-08-21  0:46       ` Stefan Beller
2018-08-21 20:41 ` Derrick Stolee
2018-08-21 21:29   ` Jeff King
2018-08-22  0:48     ` brian m. carlson
2018-08-22  3:03       ` Jeff King
2018-08-22  3:36         ` Jeff King
2018-08-22 11:11           ` Derrick Stolee
2018-08-22  5:36         ` brian m. carlson
2018-08-22  6:07           ` Jeff King
2018-08-22  7:39             ` Ævar Arnfjörð Bjarmason [this message]
2018-08-22 11:14               ` Derrick Stolee
2018-08-22 15:17                 ` Jeff King
2018-08-22 16:08                   ` Duy Nguyen
2018-08-22 16:14                     ` Duy Nguyen
2018-08-22 16:26                       ` Jeff King
2018-08-22 16:49                         ` Derrick Stolee
2018-08-22 16:58                           ` Duy Nguyen
2018-08-22 17:04                             ` Derrick Stolee
2018-08-22 16:59                           ` Jeff King
2018-08-22 17:02                             ` Junio C Hamano
2018-08-22 15:14               ` Jeff King
2018-08-22 14:28           ` Derrick Stolee
2018-08-22 15:24             ` Jeff King
2018-08-22 12:42         ` Paul Smith
2018-08-22 15:23           ` Jeff King
2018-08-23  1:23             ` Jonathan Nieder
2018-08-23  2:16               ` Jeff King
2018-08-23  2:27                 ` Jonathan Nieder
2018-08-23  5:02                   ` Jeff King
2018-08-23  5:09                     ` brian m. carlson
2018-08-23  5:10                     ` Jonathan Nieder
2018-08-23 13:20                     ` Junio C Hamano
2018-08-23 16:31                       ` wide t/perf output, was " Jeff King
2018-08-23  3:47                 ` brian m. carlson
2018-08-23  5:04                   ` Jeff King
2018-08-23 10:26                     ` Derrick Stolee
2018-08-23 13:16                       ` Junio C Hamano
2018-08-23 16:14                       ` Jeff King
2018-08-23 23:30                         ` Jacob Keller
2018-08-23 23:40                           ` Jeff King
2018-08-24  0:06                             ` Jeff King
2018-08-24  0:16                               ` Jeff King
2018-08-24  2:48                                 ` Jacob Keller
2018-08-24  2:59                                   ` Jeff King
2018-08-24  6:45                                     ` Jeff King
2018-08-24 11:04                                       ` Derrick Stolee
2018-08-27 19:36                                     ` Junio C Hamano
2018-08-23 18:53                       ` Jeff King
2018-08-23 20:59                         ` Derrick Stolee
2018-08-24  6:56                           ` Jeff King
2018-08-24  7:57                             ` Ævar Arnfjörð Bjarmason
2018-08-24 16:45                           ` Derrick Stolee
2018-08-25  8:26                             ` Jeff King
2018-09-02 18:53                       ` Kaartic Sivaraam

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