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

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To: Avi Kivity <avi@xxxxxxxxxx>
Subject: Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()
From: David Howells <dhowells@xxxxxxxxxx>
Date: Thu, 15 Apr 2010 09:48:12 +0100
Cc: dhowells@xxxxxxxxxx, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Matthew Wilcox <matthew@xxxxxx>, Ralf Baechle <ralf@xxxxxxxxxxxxxx>, mingo@xxxxxxx, tglx@xxxxxxxxxxxxx, linux-arch@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
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References: <4BC5D19A.8000605@xxxxxxxxxx> <20100406133026.GD20577@xxxxxxxxxxxxxxxx> <alpine.LFD.2.00.1003261005490.3721@xxxxxxxxxxxxxxxxxxxxxxx> <20100326144241.8583.95617.stgit@xxxxxxxxxxxxxxxxxxxxxx> <28287.1269625325@xxxxxxxxxx> <alpine.LFD.2.00.1003261042580.3721@xxxxxxxxxxxxxxxxxxxxxxx> <20100326175827.GD20055@xxxxxxxxxxxxxx> <alpine.LFD.2.00.1003261100440.3721@xxxxxxxxxxxxxxxxxxxxxxx> <17213.1271245760@xxxxxxxxxx>

Avi Kivity <avi@xxxxxxxxxx> 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.

David
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