linux-kernel - Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()

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Message-ID: <alpine.LFD.2.00.1003261038160.3721@i5.linux-foundation.org>
Date:	Fri, 26 Mar 2010 10:42:46 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Scott Lurndal <scott.lurndal@...afsystems.com>
cc:	David Howells <dhowells@...hat.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()



On Fri, 26 Mar 2010, Scott Lurndal wrote:
> 
> I wonder if Intel's EM64 stuff makes this more deterministic, perhaps
> David's implementation would work for x86_64 only?

Limiting it to x86-64 would certainly remove all the worries about all the 
historical x86 clones.

I'd still worry about it for future Intel chips, though. I absolutely 
_detest_ relying on undocumented features - it pretty much always ends up 
biting you eventually. And conditional writeback is actually pretty nasty 
from a microarchitectural standpoint.

			Linus
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