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: <CA+55aFxz46_efiQw9Nk_pFbX3AYx+oehPQOk-BDd1Xpbr2Oa3w@mail.gmail.com>
Date:   Tue, 5 Jun 2018 16:04:37 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
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 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?

               Linus