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

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Date:   Tue, 5 Jun 2018 17:05:14 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL]
 x86/asm changes for v4.18")


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

> On Mon, Jun 4, 2018 at 5:21 AM Ingo Molnar <mingo@...nel.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