volatile correctness: combine vs. target.md

[Peter Bigot]

On Fri, Dec 2, 2011 at 5:46 AM, <Paul_Koning@dell.com> wrote:
>
> ...
> >> It's never correct to exchange volatile accesses.
> >
> >That's not true.  volatile accesses to different memory locations
> >have no special dependence.  If it happens that GCC doesn't
> >do this kind of things then this is only because most passes
> >don't thouch volatile stmts at all (thus the reports for sub-optimal
> >code with volatile - we don't even try to do legal transformations).
>
> I'm confused.  Do you mean that in
>        Volatile int *a, *b;
>        Int j, k;
>        J = *a; k = *b;
> it is ok to fetch *b before *a?  I can't think of any device driver writer who would expect that.

While I don't have the reproducing case to hand, I did find a
situation where something like:

  volatile unsigned int counter; // tied to system clock
  volatile unsigned int ioport;
  t0 = counter;
  // stuff with ioport
  t1 = counter;

ended up being:

  //stuff with ioport
  t0 = counter;
  t1 = counter;

This might have been due to misbehavior of a machine role in my
target, but at the time I convinced myself that technically the move
was legal even though it completely invalidated the timing I was
trying to do.

Peter