Re: Why doesn't combine like volatiles? (volatile_ok again, sorry!)

[Richard Earnshaw <rearnsha at gcc dot gnu dot org>]

On Tue, 2005-11-22 at 19:10, Robert Dewar wrote:
> Dave Korn wrote:
> > Robert Dewar wrote:
> > 
> 
> >   Isn't it pretty much implied by point 1, "Not more than one volatile memory
> > ref appears in the instructions being considered"?  
> 
> No, that allows a volatile reference to be combined with something else.
> I think this is a mistake, because people often think of volatile as
> guaranteeing what Ada would call an atomic access, one instruction
> accessing just the variable and nothing else.

If there are N volatile accesses in the sequence under consideration,
there must be exactly N volatile accesses in the resulting combination
(the only thing I've missed in my list is to say that the volatile
reference can't be eliminated -- I've already restricted N to 1, and
rule 4 already said that the mode of the access couldn't change).

There is, however, a further restriction that I thought of last night. 
This is very hard to maintain generally, and may therefore be a near
show-stopper:

5) The instruction must never need to be restarted after the volatile
access has been accepted by the memory system.

This restriction rules out, for example, using a volatile value as an
input in many floating point operations (since the operations may trap
depending on the values read).

R.