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Message-ID: <CAFULd4YRHmQVnwaORm7=7kUs7DYG7SfwdTXAitDt=bxiMU5AoQ@mail.gmail.com>
Date: Wed, 4 Oct 2023 22:07:54 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Andy Lutomirski <luto@...nel.org>,
Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
Brian Gerst <brgerst@...il.com>,
Denys Vlasenko <dvlasenk@...hat.com>,
"H . Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>,
Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 4/4] x86/percpu: Use C for percpu read/write accessors
On Wed, Oct 4, 2023 at 9:42 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Unrelated reaction..
>
> On Wed, 4 Oct 2023 at 12:24, Uros Bizjak <ubizjak@...il.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?
It does when LSB is accessed at the same address. For example:
int m;
_Bool foo (void) { return m & 0x0f; }
compiles to:
0: f6 05 00 00 00 00 0f testb $0xf,0x0(%rip) # 7 <foo+0x7>
>
> 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.
You get a store forwarding stall when you write a bigger operand to
memory and then read part of it, if the smaller part doesn't start at
the same
address.
Uros.
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