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

[Maciej W. Rozycki]

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 <macro@xxxxxxxxxxx>

  Maciej