Re: [GIT PULL] x86/asm changes for v4.18

[Linus Torvalds]

On Mon, Jun 4, 2018 at 5:21 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>  - __clear_user() micro-optimization (Alexey Dobriyan)

Was this actually tested?

I think one reason people avoided the constant was that on some
microarchitecture it ended up being a separate uop just for the
constant generation, because it wouldn't fit in a single uop.

I'm pretty sure that used to be the case for P4, for example.

Afaik there have also been issues with decoding instructions that have
both an immediate and a memory offset.

I suspect none of this is an issue on modern cores, but there really
at least historically were cases where

   mov %reg,mem

was better than

   mov $imm,mem

if %reg already had the right value, so it's not at all 100% obvious
that the micro-optimization really _optimizes_ anything.

Any time people do this, they should add numbers.

            Linus