Re: basic asm and memory clobbers - Proposed solution

[David Wohlferd <dw at LimeGreenSocks dot com>]

On 11/28/2015 10:30 AM, Paul_Koning@Dell.com wrote:
> > On Nov 28, 2015, at 2:02 AM, Bernd Edlinger <bernd.edlinger@hotmail.de> wrote:
> >
> > ...
> > Well, I start to think that Jeff is right, and we should treat a asm ("") as if it
> > were asm volatile ("" ::: ) but if the asm ("nonempty with optional %") we should
> > treat it as asm volatile ("nonempty with optional %%" ::: "memory").
> I agree.  Even if that goes beyond the letter of what the manual has promised before, it is the cautious answer, and it matches expectations of a lot of existing code.

Trying to guess what people might have been expecting is a losing game.  
There is a way for people to be clear about what they want to clobber, 
and that's to use extended asm.  The way to clear up the ambiguity is to 
start deprecating basic asm, not to add to the confusion by changing its 
behavior after all these years.

And the first step to do that is to provide a means of finding them.  
That's what the patch at 
https://gcc.gnu.org/ml/gcc/2015-11/msg00198.html does.

Once they are located, people can decide for themselves what to do. If 
they favor the 'cautious' approach, they can change their asms to use 
:::"memory" (or start clobbering registers too, to be *really* safe).  
For people who require maximum backward compatibility and/or minimum 
impact, they can use :::.

Have you tried that patch?  How many warnings does it kick out for your 
projects?

dw