Re: [PATCH 2/2] alpha: read $gp and $sp explicitly for clang

On Tue, Aug 11, 2026 at 1:55 PM Magnus Lindholm <[email protected]> wrote:
>
> On Tue, Aug 11, 2026 at 1:10 PM Maciej W. Rozycki <[email protected]> wrote:
> >
> > On Mon, 3 Aug 2026, Magnus Lindholm wrote:
> >
> > > > diff --git a/arch/alpha/kernel/traps.c b/arch/alpha/kernel/traps.c
> > > > index 7631129ac914..5b4f1ae2b74b 100644
> > > > --- a/arch/alpha/kernel/traps.c
> > > > +++ b/arch/alpha/kernel/traps.c
> > > > @@ -925,7 +925,9 @@ void
> > > >  trap_init(void)
> > > >  {
> > > >         /* Tell PAL-code what global pointer we want in the kernel.  */
> > > > -       register unsigned long gptr __asm__("$29");
> > > > +       unsigned long gptr;
> > > > +
> > > > +       __asm__ volatile("mov $29, %0" : "=r"(gptr));
> > > >         wrkgp(gptr);
> > > >
> > > >         wrent(entArith, 1);
> > >
> > > This looks good to me. The explicit $gp read and use of
> > > current_stack_pointer avoid relying on the unsupported local
> > > register-asm behavior.
> >
> >  It could have been worth mentioning in the change description that no
> > code quality regression results here with GCC:
> >
> > --- arch/alpha/kernel/traps-0.dump      2026-08-11 11:57:59.895247441 +0100
> > +++ arch/alpha/kernel/traps-1.dump      2026-08-11 11:56:31.788713285 +0100
> > @@ -2223,8 +2223,8 @@
> >                         1b70: GPDISP    .text+0x4
> >      1b74:      00 00 bd 23     lda     gp,0(gp)
> >      1b78:      f0 ff de 23     lda     sp,-16(sp)
> > -    1b7c:      10 04 fd 47     mov     gp,a0
> > -    1b80:      00 00 5e b7     stq     ra,0(sp)
> > +    1b7c:      00 00 5e b7     stq     ra,0(sp)
> > +    1b80:      10 04 fd 47     mov     gp,a0
> >      1b84:      37 00 00 00     call_pal        0x37
> >      1b88:      04 00 00 c2     br      a0,1b9c <trap_init+0x2c>
> >      1b8c:      08 00 1e a6     ldq     a0,8(sp)
> >
> >  Also I think our coding style would rather see the asm written as:
> >
> >         __asm__ __volatile__("mov $29, %0" : "=r" (gptr));
> >
> >  Otherwise:
> >
> > Reviewed-by: Maciej W. Rozycki <[email protected]>
> >
> >   Maciej
>
> Hi Matt,
>
> Given Maciej's comment on the inline asm style, do you want to send a v2
> of this patch, or are you happy for me to fold the
>
> __asm__ volatile(...)
>
> to
>
> __asm__ __volatile__(...)
>
> style change in when I apply it to the Alpha tree?
>
> Either is fine with me.
>
> Thanks,
> Magnus

Hey Magnus,

Feel free to just fix it up when you apply it.

Thanks,
Matt