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

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Message-ID: <4BC6D334.8050607@redhat.com>
Date:	Thu, 15 Apr 2010 11:49:56 +0300
From:	Avi Kivity <avi@...hat.com>
To:	David Howells <dhowells@...hat.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Wilcox <matthew@....cx>,
	Ralf Baechle <ralf@...ux-mips.org>, 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 04/15/2010 11:48 AM, David Howells wrote:
> Avi Kivity<avi@...hat.com>  wrote:
>
>    
>> Even if Intel processors behave that way, other processors (real and
>> emulated) use those manuals as a specification.  Emulated processors are
>> unlikely to touch an undefined register, but real processors may.
>>
>> (qemu tcg appears not to touch the output)
>>      
> Possibly because the AMD64 spec specifies that the destination will be
> unchanged if the source was 0.
>    

Likely.  But we haven't tested all current and future x86 clones, and 
they may be based off the Intel documentation instead of the AMD 
documentation.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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