Am Dienstag, den 11.01.2022, 10:13 +0100 schrieb Richard Biener: > On Tue, Jan 11, 2022 at 9:17 AM Martin Uecker <ma.uec...@gmail.com> wrote: > > Am Dienstag, den 11.01.2022, 08:11 +0100 schrieb Richard Biener: > > > On Mon, Jan 10, 2022 at 6:36 PM Martin Uecker <ma.uec...@gmail.com> wrote: > > > > Am Montag, den 10.01.2022, 10:04 +0100 schrieb Richard Biener:
... > Consider > > int a[1024]; > void foo (volatile int *p, float *q) > { > for (int i = 0; i < 1024; ++i) > { > *p = 1; > a[i] = *q; > } > } > > we happily apply invariant motion to the load from *q, making > it cross the store to the volatile object at *p. Now, q might be > NULL upon entry to the function and thus this transform > would violate the volatile "I/O" constraint (since I/O is observable) > and thus we will crash (which is UB) before doing the first I/O. > > That's an example I'd consider important for performance and > also a case that shows that usually the compiler will have a > very hard time proving UB cannot happen (as opposed to the > usual stance where it can assume it doesn't). I can see that the transformation in general is important, but why is it important if the "volatile" is there? I would assume that performance-sensitive code usually does not volatile. Or in other words, what is the purpose of volatile in this example if not either I/O or to prevent such optimizations? > The case we run into sth similar is with use of uninitialized > variables where proving some variable is initialized is nearly > impossible (copy initialization from a variable that is not > initialized is not initialization). These are the C++ rules. For C, an automatic variables which is not initialized and (as long as the address is not taken), it is directly UB. > We've mainly settled to the stance that only program termination > is observable which means if we do not know that a function > call will always return normally we have to avoid hoisting > observable UB across such function call (and I/O routines > usually fall into this category because they are not annotated > as always returning). Yes. > Handling all volatile accesses in the > very same way would be possible but quite some work I don't > see much value in. I see some value. But an alternative could be to remove volatile from the observable behavior in the standard or make it implementation-defined whether it is observable or not. Martin > Richard. > > > Martin > > > > > > > > I am trying to figure out whether this is feasible. > > > > > > > > > > For PRE yes, you'd just need to include the observable stmts you > > > > > care in the set of stmts that cause PRE to set BB_MAY_NOTRETURN. > > > > > In general this is of course harder. > > > > > > > > What other passes would need to be checked? > > > > > > All that do code motion by design or by accident. The difficulty is > > > that the resulting "wrong IL" is not wrong per se, just the difference is > > > which is hard to write a checker for (well, in priciple you could copy the > > > IL before passes and compare to the IL after) > > > > > > > And do you think there is any negative impact on > > > > an important optimization (considering this affects > > > > only volatile accesses)? > > > > > > Probably not. But then semantics of 'volatile' are very weak defined > > > so I'd like > > > to see a reference to a part of the standard that supports declaring this > > > (and only this - the 'volatile' case) a bug. > > > > > > > > > > GCC assumes by default that divide is trappable but stores not > > > > > > > are not > > > > > > > observable. This is where -fnon-call-exceptions come into play. > > > > > > > > > > > > Ok, thanks! I will look at this! > > > > > > > > > > > > > In the second case, GCC assumes reducing trappable instructions > > > > > > > are > > > > > > > fine. > > > > > > > > > > > > -fnon-call-exceptions would treat trapping instructions > > > > > > as defined (and trapping) instead of UB? This is > > > > > > then probably even stronger than the requirement above. > > > > > > > > > > No, I don't think it turns UB into defined behavior. Some frontends > > > > > might > > > > > expect that to some extent. So even with -fnon-call-exceptions we'd > > > > > happily do the re-ordering unless the exception is catched in the same > > > > > function. > > > > > > > > Thanks, > > > > Martin > > > > > > > > > > > Note I thought -fno-delete-dead-exceptions would fix the sink > > > > > > > but it didn't. > > > > > > > > > > > > Martin > > > > > > > > > > > >