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

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Message-ID: <6910.1292392625@jrobl>
Date:	Wed, 15 Dec 2010 14:57:05 +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


Thank you for explanation.
I understood about padding zero tail bytes as an improvement or a
variation of the 'long *' approach. But I didn't write about it in my
previous mail. If I had written, you might not need to describe again.

I have also compared "repe; cmpsb" and __builtin_memcmp "in userspace".
And __builtin_memcmp shows faster and it looks like an approach of
mixing pointers. Although the code size grows, in source it will be
simple #define and it is already done in x86_32. Do you (or any other
x86 experts) think it is worth to try?


J. R. Okajima

Nick Piggin:
> What I meant is that a "normal" memcmp that does long * memory
> operations is not a good idea, because it requires code and branches
> to handle the tail of the string. When average string lengths are less
> than 16 bytes, it hardly seems wothwhile. It will just get more
> mispredicts and bigger icache footprint.
>
> However instead of a normal memcmp, we could actually pad dentry
> names out to sizeof(long) with zeros, and take advantage of that with
> a memcmp that does not have to handle tails -- it would operate
> entirely with longs.
>
> That would avoid icache and branch regressions, and might speed up
> the operation on some architectures. I just doubted whether it would
> show an improvement to be worth doing at all. If it does, I'm all for it.
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