On Fri, Jan 24, 2020 at 07:03:36AM +0000, Christophe Leroy wrote:
> >Le 24/01/2020 à 06:46, Michael Ellerman a écrit :
> >>
> >>If I do this it seems to work, but feels a little dicey:
> >>
> >> asm ("" : "=r" (r1));
> >> sp = r1 & (THREAD_SIZE - 1);
> >
> >
> >Or we could do add in asm/reg.h what we have in boot/reg.h:
> >
> >register void *__stack_pointer asm("r1");
> >#define get_sp() (__stack_pointer)
> >
> >And use get_sp()
> >
>
> It works, and I guess doing it this way is acceptable as it's exactly
> what's done for current in asm/current.h with register r2.
That is a *global* register variable. That works. We still need to
document a bit better what it does exactly, but this is the expected
use case, so that will work.
> Now I (still) get:
>
> sp = get_sp() & (THREAD_SIZE - 1);
> b9c: 54 24 04 fe clrlwi r4,r1,19
> if (unlikely(sp < 2048)) {
> ba4: 2f 84 07 ff cmpwi cr7,r4,2047
>
> Allthough GCC 8.1 what doing exactly the same with the form CLANG don't
> like:
>
> register unsigned long r1 asm("r1");
> long sp = r1 & (THREAD_SIZE - 1);
> b84: 54 24 04 fe clrlwi r4,r1,19
> if (unlikely(sp < 2048)) {
> b8c: 2f 84 07 ff cmpwi cr7,r4,2047
Sure, if it did what you expected, things will usually work out fine ;-)
(Pity that the compiler didn't come up with
rlwinm. r4,r1,0,19,20
bne bad
Or are the low bits of r4 used later again?)
Segher
