Re: [gimple] assignments to volatile

[Mark Mitchell <mark@codesourcery.com>]

Michael Matz wrote:

>>> Sometimes we re-read the assigned-to object, and sometimes we do not.  For
>>> instance,
>>>   return vobj = data;
>>> will cause a reread of vobj, IF data is not a constant.
> 
> I'd consider the latter condition a bug.  I.e. the rereading must happen 
> in every case.

I think that it's without question that:

  vobj = data;

should not cause a read from vobj; the value is not used.  I think
real-world embedded programmers would be surprised to see a read from an
I/O register there.  Do you agree?

Similarly, I think that:

  cond ? vobj = data : ...

should not cause a read from vobj.  The conditional in this case is just
a form of if/then; it would surprise programmers if this behaved
differently from:

  if (cond)
    vobj = data;

Do you agree?

The case of:

  x = vobj = data;

is different because the value is being used.  In that case, I have no
strong opinion.

Rather than argue about this last case in theory, though, I'd be
inclined to look at existing practice.  What do RealView, CodeWarrior,
Green Hills or other long-standing compilers for embedded systems do?
If there's a clear consensus, we can follow that.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713