Re: [PATCH] x86/asm/entry/64: Use shorter MOVs from segmers registers

[Denys Vlasenko]

On 05/14/2015 07:08 PM, Linus Torvalds wrote:
> On Thu, May 14, 2015 at 10:05 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> 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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