basic asm and memory clobbers - Proposed solution

[David Wohlferd]

On 11/30/2015 4:01 AM, Andrew Haley wrote:
 >> 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.
 >
 > Well, I disagree.  The warning is good, but so is the memory clobber.
 > They're not exclusive.

As Richard Henderson put it: "I'd be perfectly happy to deprecate and 
later completely remove basic asm within functions."

So my intent is that today's optional warning becomes tomorrow's default 
warning which eventually turns into a fatal error.  We could speed that 
up a bit by making it a default warning now and a fatal error as soon as 
we re-enter phase 1.  I just thought it might be better to update the 
docs now and delay the change to give more of a heads up.

Yes, we could pursue both clobbering and completely removing at the same 
time, but I don't think we should.  Since (currently) basic asm always 
has no clobbers describing how to update basic asm to extended is fairly 
simple: To get the same behavior in extended as you used to get from 
basic, just add a colon.  If some versions of gcc perform clobbers, then 
knowing how to convert gets way harder.

dw