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Date: Fri, 26 Mar 2010 10:42:46 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Scott Lurndal <scott.lurndal@3leafsystems.com>
cc: David Howells <dhowells@redhat.com>, mingo@elte.hu, tglx@linutronix.de,
       linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()
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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