Messages in this thread | | | Date | Thu, 14 May 2015 19:41:42 +0200 | From | Denys Vlasenko <> | Subject | Re: [PATCH] x86/asm/entry/64: Use shorter MOVs from segmers registers |
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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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