On Fri, Dec 2, 2011 at 1:14 PM, Peter Bigot <big...@acm.org> 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.
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). Whether it is desirable is another question - and whether such desired behavior is easy to implement is another one. > 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. Yes, I think it is valid. Richard. > Peter
