On Wed, Oct 18, 2017 at 05:17:19PM +0200, Michael Matz wrote:
> On Wed, 18 Oct 2017, Segher Boessenkool wrote:
>
> > Certainly. And to work around the bug, it should work to mention some
> > hard register as asm input. Ideally something that is live anyway;
> > perhaps the stack pointer :-) Like so:
> >
> > __asm volatile ("mrs %0,PRIMASK" : "=r" (status) :: "sp");
> >
> > (I tested this, it does work around the bug).
> >
> > Or hey, why not the program counter, that should make it even clearer
> > something is funny here:
> >
> > __asm volatile ("mrs %0,PRIMASK" : "=r" (status) :: "pc");
> >
> > (also tested, works fine).
>
> Both of these are not asm inputs but clobbers, though :)
Yeah I typed (and tested) the code after writing that first sentence.
Clobbering was easier :-)
> And clobbering
> "pc" could have funny effects if any of our allocators ever would produce
> save/restore code around such asms for such clobbers.
Yeah -- but sp (and pc, if it exists) is a fixed register, so we don't
do such things :-)
Segher
