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    SubjectRe: [PATCH] x86/asm/entry/64: Use shorter MOVs from segmers registers
    On 05/14/2015 07:08 PM, Linus Torvalds wrote:
    > On Thu, May 14, 2015 at 10:05 AM, Linus Torvalds
    > <torvalds@linux-foundation.org> wrote:
    >>
    >> I don't object to the patch, but did we actually confirm that it
    >> always overwrites all of %ecx?
    >
    > Just to clarify: I don't object to the patch because the code doesn't
    > actually end up *depending* on the high bits anyway, and does
    > word-sized compares etc. And the instruction size and speed things I
    > don't doubt. So it's just the commit message I wanted to check wrt
    > that whole "always overwrites all of %ecx". Because older CPU's didn't
    > necessarily (things like partial register writes are much less of an
    > issue when you're in-order and stupid ;)

    This is 64-bit code, and all 64-bit CPUs zero-extend moves from
    segment registers. As you said, in this particular code it wouldn't
    matter anyway since subsequent code doesn't care about high bits of %ecx...


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