lkml.org 
[lkml]   [2010]   [Apr]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    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:
    > > "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."


    \
     
     \ /
      Last update: 2010-04-06 15:33    [W:0.020 / U:98.704 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site