git@vger.kernel.org mailing list mirror (one of many)
 help / color / mirror / code / Atom feed
From: Derrick Stolee <stolee@gmail.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Jeff King" <peff@peff.net>
Cc: "brian m. carlson" <sandals@crustytoothpaste.net>,
	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 07:14:42 -0400	[thread overview]
Message-ID: <d1fafc75-d6e4-d363-d600-579c200aca43@gmail.com> (raw)
In-Reply-To: <CACBZZX7Cmp8d=UKF2nk36fL7mR+umdKwKZAKNZSkyP0NXvquhw@mail.gmail.com>


On 8/22/2018 3:39 AM, Ævar Arnfjörð Bjarmason wrote:
> 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:
>>> 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
The other thing I was going to recommend (and I'll try to test this out 
myself later) is to see if 'the_hash_algo->rawsz' is being treated as a 
volatile variable, since it is being referenced through a pointer. 
Perhaps storing the value locally and then casing on it would help?

  reply	other threads:[~2018-08-22 11:14 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
2018-08-22 11:14               ` Derrick Stolee [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=d1fafc75-d6e4-d363-d600-579c200aca43@gmail.com \
    --to=stolee@gmail.com \
    --cc=avarab@gmail.com \
    --cc=git@vger.kernel.org \
    --cc=gitster@pobox.com \
    --cc=peff@peff.net \
    --cc=sandals@crustytoothpaste.net \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

	https://80x24.org/mirrors/git.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).