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

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Date:	Fri, 26 Mar 2010 11:03:09 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ralf Baechle <ralf@...ux-mips.org>
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, Ralf Baechle wrote:
> 
> My trusty old 486 book [1] in the remarks about the BSF instruction:
> 
> "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.

		Linus
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