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

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Message-ID: <20100406133026.GD20577@parisc-linux.org>
Date:	Tue, 6 Apr 2010 07:30:26 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	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, Mar 26, 2010 at 11:03:09AM -0700, Linus Torvalds wrote:
> On Fri, 26 Mar 2010, Ralf Baechle wrote:
> > "The documentation on the 80386 and 80486 states that op1 is undefined if
> > op2 is 0.  In reality the 80386 will leave the value in op1 unchanged.
> > The first versions of the 80486 will change op1 to an undefined value.
> > Later version again will leave it unchanged."
> > 
> > [1] Die Intel Familie in German language, by Robert Hummel, 1992
> 
> Ok, that explains my memory of us having tried this, at least.
> 
> But I do wonder if any of the people working for Intel could ask the CPU 
> architects whether we could depend on the "don't write" for 64-bit mode. 
> If AMD already documents the don't-touch semantics, and if Intel were to 
> be ok with documenting it for their 64-bit capable CPU's, we wouldn't then 
> need to rely on undefined behavior.

I don't know whether we can get it /documented/, but the architect I
asked said "We'll never get away with reverting to the older behavior,
so in essence the architecture is set to not overwrite."

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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