From: Derrick Stolee <stolee@gmail.com>
To: Duy Nguyen <pclouds@gmail.com>
Cc: "Jeff King" <peff@peff.net>,
"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"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 13:04:08 -0400 [thread overview]
Message-ID: <387295e3-3112-ed09-d988-3d8156e3c7ce@gmail.com> (raw)
In-Reply-To: <CACsJy8By49nz6-q78GuMYP8ssrjhWYyhYrJcL8eWpi4-mZzzxA@mail.gmail.com>
On 8/22/2018 12:58 PM, Duy Nguyen wrote:
> On Wed, Aug 22, 2018 at 6:49 PM Derrick Stolee <stolee@gmail.com> wrote:
>> On 8/22/2018 12:26 PM, Jeff King wrote:
>>> On Wed, Aug 22, 2018 at 06:14:24PM +0200, Duy Nguyen wrote:
>>>
>>>> On Wed, Aug 22, 2018 at 6:08 PM Duy Nguyen <pclouds@gmail.com> wrote:
>>>>> On Wed, Aug 22, 2018 at 6:03 PM Jeff King <peff@peff.net> wrote:
>>>>>> On Wed, Aug 22, 2018 at 07:14:42AM -0400, Derrick Stolee wrote:
>>>>>>
>>>>>>> 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?
>>>>>> I tried various sprinkling of "const" around the declarations to make it
>>>>>> clear that the values wouldn't change once we saw them. But I couldn't
>>>>>> detect any difference. At most I think that would let us hoist the "if"
>>>>>> out of the loop, but gcc still seems unwilling to expand the memcmp when
>>>>>> there are other branches.
>>>>>>
>>>>>> I think if that's the thing we want to have happen, we really do need to
>>>>>> just write it out on that branch rather than saying "memcmp".
>>>>> This reminds me of an old discussion about memcpy() vs doing explicit
>>>>> compare loop with lots of performance measurements..
>>>> Ah found it. Not sure if it is still relevant in light of multiple hash support
>>>>
>>>> https://public-inbox.org/git/20110427225114.GA16765@elte.hu/
>>> Yes, that was what I meant. We actually did switch to that hand-rolled
>>> loop, but later we went back to memcmp in 0b006014c8 (hashcmp: use
>>> memcmp instead of open-coded loop, 2017-08-09).
>> Looking at that commit, I'm surprised the old logic was just a for loop, instead of a word-based approach, such as the following:
> Might work on x86 but it breaks on cpu architectures with stricter
> alignment. I don't think we have a guarantee that object_id is always
> 8 byte aligned.
You (and Peff) are probably correct here, which is unfortunate. I'm not
familiar with alignment constraints, but assume that such a word-based
approach is best.
Thanks,
-Stolee
next prev parent reply other threads:[~2018-08-22 17:04 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
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 [this message]
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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