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    SubjectRe: [Q] Looking for an emulation for CMOV* instructions.
    On Wed, 16 Jan 2002, Jamie Lokier wrote:

    > Alan Cox wrote:
    > > > What's the point of optimizing an IF to a cmov if I have
    > > > to insert another IF to see if I can use cmov?
    > >
    > > I've always wondered. Intel made the instruction optional yet there isnt
    > > an obvious way to do runtime fixups on it
    >
    > Yes there is -- emulation! :-)
    >

    It's just as bad, probably worse! You trap on an invalid op-code. The
    trap-handler checks the op-code and if it's emulated, it emulates it
    and returns to the executing task. This takes many instruction cycles,
    certainly more than `if(cmov) doit; else do_something_else;` --which,
    itself, takes many more instruction cycles than cmov is supposed to
    reduce. It's a no-win situation. The only way to win is a compile-time
    choice. This means customizing for your CPU IFF it has the cmov
    instruction.

    Cheers,
    Dick Johnson

    Penguin : Linux version 2.4.1 on an i686 machine (797.90 BogoMips).

    I was going to compile a list of innovations that could be
    attributed to Microsoft. Once I realized that Ctrl-Alt-Del
    was handled in the BIOS, I found that there aren't any.


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