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 16:27:24 -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 4:20 PM Alexey Dobriyan <adobriyan@...il.com> wrote:
>
> This is Broadwell Xeon E5-2620 v4.
> Which is somewhat strange indeed because it should be modern enough.

Yeah, odd.

Here's the benchmark I used:

  #define SIZE 4068

  int main(int argc, char **argv)
  {
    int i;
    unsigned char buffer[SIZE], *p;

    for (i = 0; i < 1000000; i++)
        asm volatile(
            "1: movq %[zero],(%[mem]); addq %[eight],%[mem]; decl
%[count]; jne 1b"
            : [mem] "=r" (p)
            : [zero] "i" (0l), [eight] "i" (8l),
             "0" (buffer), [count] "r" (SIZE/8));
  }

where you can change that "i" for [zero] and [eight] to be "r" to get
the register version.

I just timed it, because I'm lazy and perf seemed to be overkill.

It might be some very specific loop buffer issue or something.

Or maybe my benchmark above is broken, I didn't really verify that the
end result was any good (I just did an objdump to verify the asm code
superficially).

                 Linus