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

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Message-ID: <20180605232022.GA4468@avx2>
Date:   Wed, 6 Jun 2018 02:20:22 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.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>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        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 04:04:37PM -0700, Linus Torvalds wrote:
> On Tue, Jun 5, 2018 at 4:01 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Tue, Jun 5, 2018 at 3:41 PM Alexey Dobriyan <adobriyan@...il.com> wrote:
> > >
> > > On my potato performance increase is 33%, sheesh.
> > > And CPU starts doing 3 instructions per cycle vs 2.
> >
> > Whee. That's a shockingly big difference. On my CPU (i7-6700K) it
> > makes absolutely no difference whether the values are integers or in
> > registers.
> 
> In fact, looking at Agner Fog's instruction lists, I don't see any CPU
> where it would make a difference, except for the P4 (where the
> immediate looks like it's a bad idea because it's an extra uop, but it
> might pack fine and not be noticeable).
> 
> But maybe I'm missing something subtle. What CPU, out of morbid interest?

This is Broadwell Xeon E5-2620 v4.
Which is somewhat strange indeed because it should be modern enough.