linux-kernel - Re: [GIT PULL] x86/asm changes for v4.18

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Message-ID: <CA+55aFx=zLkZx4h4rcWv79OKQSGJAwC3eS8=zSDgKjyrsbLrNA@mail.gmail.com>
Date:   Mon, 4 Jun 2018 18:58:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>
Cc:     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>
Subject: Re: [GIT PULL] x86/asm changes for v4.18

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 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.

            Linus