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?
next prev parent 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
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