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

From: Matthew Wilcox
Date: Tue Apr 06 2010 - 09:30:37 EST

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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