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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 4 Oct 2023 12:42:19 -0700
Message-ID: <CAHk-=wjuRGzhuETLYDoi4hM6RAxHVL0ptuRb3TH-od+348Y8zA@mail.gmail.com>
Subject: Re: [PATCH v2 4/4] x86/percpu: Use C for percpu read/write accessors
To:     Uros Bizjak <ubizjak@gmail.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Nadav Amit <namit@vmware.com>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
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Unrelated reaction..

On Wed, 4 Oct 2023 at 12:24, Uros Bizjak <ubizjak@gmail.com> wrote:
>
> the code improves from:
>
>  65 8b 05 00 00 00 00    mov    %gs:0x0(%rip),%eax
>  a9 00 00 0f 00          test   $0xf0000,%eax
>
> to:
>
>  65 f7 05 00 00 00 00    testl  $0xf0000,%gs:0x0(%rip)
>  00 00 0f 00

Funky.

Why does gcc generate that full-width load from memory, and not demote
it to a byte test?

IOW, it should not be

  65 f7 05 00 00 00 00 testl  $0xf0000,%gs:0x0(%rip)
  00 00 0f 00

after optimizing it, it should be three bytes shorter at

  65 f6 05 00 00 00 00 testb  $0xf,%gs:0x0(%rip)
  0f

instead (this is "objdump", so it doesn't show that the relocation
entry has changed by +2 to compensate).

Now, doing the access narrowing is a bad idea for stores (because it
can cause subsequent loads to have conflicts in the store buffer), but
for loads it should always be a win to narrow the access.

I wonder why gcc doesn't do it. This is not related to __seg_gs - I
tried it with regular memory accesses too, and gcc kept those as
32-bit accesses too.

And no, the assembler can't optimize that operation either, since I
think changing the testl to a testb would change the 'P' bit in the
resulting eflags, so this is a "the compiler could pick a better
instruction choice" thing.

I'm probably missing some reason why gcc wouldn't do this. But clang
does seem to do this obvious optimization.

              Linus