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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 15 Dec 2010 15:15:09 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Nick Piggin <npiggin@...il.com>
CC:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	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

On 12/15/2010 06:06 AM, Nick Piggin wrote:
> 
> 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.
> 

I agree that the byte-compare or long-compare should give you very close
results in modern pipeline CPUs. But surly 12 increments-and-test should
show up against 3 (or even 2). I would say it must be a better plan.

BTW the long approach if you assume that the beginning of the string
is long aligned than it is only a matter of comparing the last byte
with a mask, no branches. But I'm not saying, just make sure they are
padded.

Just my $0.017
Thanks
Boaz
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/