linux-kernel - Re: [RFC PATCH 0/4] x86/percpu: Use segment qualifiers

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Message-ID: <CAFULd4Y-gr+UAvi4m1-p4MnJyMv3NRcyH=TFLZfFfNngnE_Kpw@mail.gmail.com>
Date: Tue, 29 Apr 2025 18:31:33 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: X86 ML <x86@...nel.org>, LKML <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: [RFC PATCH 0/4] x86/percpu: Use segment qualifiers

[Dead thread resurrection]

On Sun, Oct 1, 2023 at 10:21 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:

> And indeed, the *very* first thing I tried shows that this is all very
> very buggy in gcc.
>
> What did I try? A simple memory copy with a structure assignment.
>
> Try to compile this:
>
>     #include <string.h>
>     struct a { long arr[30]; };
>
>     __seg_fs struct a m;
>     void foo(struct a *dst) { *dst = m; }
>
> using the kernel compiler options (it's the "don't use sse/avx" ones
> that matter):
>
>     gcc -mno-avx -mno-sse -O2 -S t.c
>
> and look at the end result. It's complete and utter sh*t:
>
>         foo:
>                 xorl    %eax, %eax
>                 cmpq    $240, %rax
>                 jnb     .L5
>         .L2:
>                 movzbl  %fs:m(%rax), %edx
>                 movb    %dl, (%rdi,%rax)
>                 addq    $1, %rax
>                 cmpq    $240, %rax
>                 jb      .L2
>         .L5:
>                 ret
>
> to the point that I can only go "WTF"?
>
> I mean, it's not just that it does the copy one byte at a time. It
> literally compares %rax to $240 just after it has cleared it. I look
> at that code, and I go "a five-year old with a crayon could have done
> better".
>
> In other words, no, we're not using this thing that generates that
> kind of garbage.
>
> Somebody needs to open a bugzilla entry for this kind of code generation.

FYI, after GCC PR 111657 [1] was fixed, gcc-16 will generate the following code:

foo:
       movl    $m, %esi
       movl    $30, %ecx
       rep movsq       %fs:(%rsi), (%rdi)
       ret

And adding -mtune=skylake:

foo:
       movl    $m, %esi
       movl    $240, %ecx
       rep movsb       %fs:(%rsi), (%rdi)
       ret

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111657

Uros.