linux-kernel - Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu

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Date:   Thu, 19 Oct 2023 20:16:24 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     peterz@...radead.org, Nadav Amit <namit@...are.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Thu, Oct 19, 2023 at 8:06 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, 19 Oct 2023 at 10:21, Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > > A compiler that were to rematerializes an inline asm (instead of
> > > spilling) would be a bad joke. That's not an optimization, that's just
> > > a crazy bad compiler with a code generation bug.
> >
> > But that is what the compiler does without volatile.
>
> Do you actually have a real case of that, or are basing it purely off
> insane documentation?
>
> Because remat of inline asm really _is_ insane.

The following testcase pushes the compiler to the limit:

--cut here--
extern void ex (int);

static int read (void)
{
  int ret;

  asm ("# -> %0" : "=r"(ret));
  return ret;
}

int foo (void)
{
  int ret = read ();

  ex (ret);
  asm volatile ("clobber" : : : "ax", "cx", "dx", "bx", "bp", "si", "di");

  return ret;
}

extern int m;

int bar (void)
{
  int ret = m;

  ex (ret);
  asm volatile ("clobber" : : : "ax", "cx", "dx", "bx", "bp", "si", "di");

  return ret;
}
--cut here--

Please compile the above with -S -O2 -m32 (so we don't have to list
all 16 x86_64 registers).

And NO (whee...), there is no rematerialization of asm (foo() ). OTOH,
there is also no rematerialization from non-volatile memory (bar() ),
although it would be more optimal than spill to/fill from a frame pair
of moves. I wonder what are "certain circumstances" that the
documentation is referring to.

Uros.


Uros.