Re: [ANNOUNCE] Git v2.19.0-rc0

From: Duy Nguyen <pclouds@gmail.com>
To: Jeff King <peff@peff.net>
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 18:08:38 +0200	[thread overview]
Message-ID: <CACsJy8B+FN6W32ZUgetXbEJ=Ld5ZDHHhNWVz2aiZLZG1_qA+FQ@mail.gmail.com> (raw)
In-Reply-To: <20180822151703.GB32630@sigill.intra.peff.net>

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.. Is that what you
meant by "write it out"?
-- 
Duy