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From: Jeff King <peff@peff.net>
To: Duy Nguyen <pclouds@gmail.com>
Cc: "Derrick Stolee" <stolee@gmail.com>,
	"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 12:26:09 -0400	[thread overview]
Message-ID: <20180822162609.GA11904@sigill.intra.peff.net> (raw)
In-Reply-To: <CACsJy8DifmYtDE4D58yVM7rYhfJFe8p-t9bXgMmMsw=txxmzHA@mail.gmail.com>

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).

-Peff

  reply	other threads:[~2018-08-22 16:26 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 [this message]
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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