Re: volatile correctness: combine vs. target.md

[Andrew Haley <aph at redhat dot com>]

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.

On 12/02/2011 12:27 PM, Richard Guenther wrote:
> It's ok in terms of GCC internals.  Whether it's ok in terms
> of the C language specification I don't know (but I think it is ok).

It's not.

5.1.2.3 Program execution

Accessing a volatile object, modifying an object, modifying a file, or
calling a function that does any of those operations are all side
effects, which are changes in the state of the execution
environment. Evaluation of an expression may produce side effects. At
certain specified points in the execution sequence called sequence
points, all side effects of previous evaluations shall be complete and
no side effects of subsequent evaluations shall have taken place. (A
summary of the sequence points is given in annex C.)

...

The least requirements on a conforming implementation are: — At
sequence points, volatile objects are stable in the sense that
previous accesses are complete and subsequent accesses have not yet
occurred.

So, in here:

       Volatile int *a, *b;
       Int j, k;
       J = *a; k = *b;

the fetch from *a must complete before the sequence point that is the
semicolon.

Andrew.