Re: [PATCH v2 4/4] x86/percpu: Use C for percpu read/write accessors

[Linus Torvalds <torvalds@linux-foundation.org>]

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