Re: [PATCH] x86/asm: Switch clflush alternatives to use %a address operand modifier

Uros Bizjak
2026-03-19T10:46:14Z

On Thu, Mar 19, 2026 at 11:20 AM David Laight
<david.laight.linux@gmail.com> wrote:
>
> On Wed, 18 Mar 2026 16:45:28 +0100
> Uros Bizjak <ubizjak@gmail.com> wrote:
>
> > On Wed, Mar 18, 2026 at 4:03 PM David Laight
> > <david.laight.linux@gmail.com> wrote:
> > >
> > > On Wed, 18 Mar 2026 10:08:11 +0100
> > > Uros Bizjak <ubizjak@gmail.com> wrote:
> > >
> > > > The inline asm used with alternative_input() specifies the address
> > > > operand for clflush with the "a" input operand constraint and
> > > > explicit "(%[addr])" dereference:
> > > >
> > > > "clflush (%[addr])", [addr] "a" (addr)
> > > >
> > > > This forces the pointer into %rax and manually encodes the memory
> > > > operand in the template. Instead, use the %a address operand
> > > > modifier and relax the constraint from "a" to "r":
> > > >
> > > > "clflush %a[addr]", [addr] "r" (addr)
> > > >
> > > > This lets the compiler choose the register while generating the
> > > > correct addressing mode.
> > >
> > > Aren't these two independent changes?
> >
> > I was hoping I can put a trivial "a" -> "r" change under the "also
> > ..." change. OTOH, let's change the summary to "x86/asm: Improve
> > clflush alternatives assembly", that will also handle your proposed
> > addition of "memory" clobber.
> >
> > > %a saves you having to know how to write the memory reference for the
> > > architecture - so is the same as (%[addr]) (assuming att syntax).
> > > I think the assembler handles the one 'odd' case of (%rbp).
> >
> > Yes, it does, and also fixes another 'odd' case of (%r13).
> >
> > > Was there ever a reason for using "a" rather than "r" - it seems an
> > > unusual choice.
> >
> > Probably just an oversight due to a follow-up __monitor() that wants
> > its operand in %rax.
>
> Actually gcc can be quite bad are reverse tracking register requirements.

This must be a very old GCC as I'm not aware of this deficiency.

--cut here--
void foo (int a)
{
asm volatile ("# 1" : : "r" (a));
asm volatile ("# 2" : : "a" (a));
}

void bar (int a)
{
asm volatile ("# 1" : : "a" (a));
asm volatile ("# 2" : : "a" (a));
}
--cut here--

foo:
movl %edi, %eax
# 1
# 2
ret

bar:
movl %edi, %eax
# 1
# 2
ret

Do you perhaps have a testcase to illustrate your claim?

> So forcing 'addr' into %rax for the cflush might actually remove
> a register move before the monitor.
> Indeed, were it to pick a different register there will always be a
> extra register move.
> If the value is in a different register (eg from a function call)
> then you'll move the register move instruction - but there'll still
> be one.
>
> So I suspect this change can never improve the code.

Of course, there will always be a register move in the above case, but
please look at [1].

[1] https://claude.ai/share/cf559f66-dfcf-451a-8260-6f687aead052

Uros.