On Fri, Dec 2, 2011 at 1:59 PM, Andrew Haley <a...@redhat.com> wrote: > On Fri, Dec 2, 2011 at 5:46 AM, <paul_kon...@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.
I see. As we do not explicitely model this dependency we probably get lucky by the if (gimple_has_volatile_ops ()) bail-out; most passes do. Richard. > Andrew.
