Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18")

[Ingo Molnar <mingo@kernel.org>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, Jun 4, 2018 at 5:21 AM Ingo Molnar <mingo@kernel.org> wrote:
> >
> >  - __clear_user() micro-optimization (Alexey Dobriyan)
> 
> Was this actually tested?

I'm not sure - Alexey?

> 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.

Ok, fair point and agreed - if Alexey sends some measurements to back the change 
I'll keep this, otherwise queue up a revert.

Thanks,

	Ingo