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Re: Can shrink-wrapping ever move prologue past an ASM statement?


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


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