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    SubjectRe: [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:
    > >
    > > 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.

    I'll drop one of them a note.

    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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