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 20:22:43 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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")

On Tue, Jun 05, 2018 at 05:05:14PM +0200, Ingo Molnar wrote:
> 
> * 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.

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

Tested? :^) I had P4 maybe ~15(?) years ago.

godbolt.org earliest compiler is 4.1.2 and it generates "movb [r32], imm8"
with "-m32 -O2 -march=pentium4" for simple memset-style loop
if it counts for something.

Actually I think __clear_user should be rewritten in C with assembly.
It's biggest user is probably ELF loader and those partial page .bss
clears should be noticeable.