linux-kernel - Re: Big git diff speedup by avoiding x86 "fast string" memcmp

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Message-ID: <12853.1292353313@jrobl>
Date:	Wed, 15 Dec 2010 04:01:53 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	Nick Piggin <npiggin@...il.com>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Big git diff speedup by avoiding x86 "fast string" memcmp


Nick Piggin:
> Well, let's see what turns up. We certainly can try the long *
> approach. I suspect on architectures where byte loads are
> very slow, gcc will block the loop into larger loads, so it should
> be no worse than a normal memcmp call, but if we do explicit
> padding we can avoid all the problems associated with tail
> handling.

Thank you for your reply.
But unfortunately I am afraid that I cannot understand what you wrote
clearly due to my poor English. What I understood is,
- I suggested 'long *' approach
- You wrote "not bad and possible, but may not be worth"
- I agreed "the approach may not be effective"
And you gave deeper consideration, but the result is unchaged which
means "'long *' approach may not be worth". Am I right?


> In short, I think the change should be suitable for all x86 CPUs,
> but I would like to hear more opinions or see numbers for other
> cores.

I'd like to hear from other x86 experts too.
Also I noticed that memcmp for x86_32 is defined as __builtin_memcmp
(for x86_64 is "rep cmp"). Why does x86_64 doesn't use __builtin_memcmp?
Is it really worse?


J. R. Okajima
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