On Wed, Jul 08, 2015 at 04:14:20PM -0500, Segher Boessenkool wrote:
> On Wed, Jul 08, 2015 at 03:51:12PM -0500, Josh Poimboeuf wrote:
> > > > > > For other archs, e.g. x86-64, you can do
> > > > > >
> > > > > > register void *sp asm("%sp");
> > > > > > asm volatile("call func" : "+r"(sp));
> >
> > I've found that putting "sp" in the clobber list also seems to work:
> >
> > asm volatile("call func" : : : "sp");
> >
> > This syntax is nicer because it doesn't need a local variable associated
> > with the register. Do you see any issues with this approach?
>
> Like I said, that forces the function to have a frame pointer. That might
> not matter for x86 and your application (because you already have a frame
> pointer for other reasons).
>
> Clobbering sp in inline asm isn't a terribly sane thing to do (and neither
> is writing to it, as in my code above), of course; we don't *actually*
> clobber it here (that is, write an unspecified value to it), but still.
> Just reading it would be better for your sanity, and is also enough to
> prevent shrink-wrapping the asm.
>
> I do believe it should work though, I see no issues with it _that_ way.
> You'll have to test and see (it works fine in trivial tests).
My apologies, I misread your earlier comments. You did mention
clobbering as a possibility.
I'll do some testing with it as an input operand to see if that
suffices.
Thanks again.
--
Josh
