On Tue, Jul 22, 2014 at 5:17 PM, Jan Hubicka <hubi...@ucw.cz> wrote: >> I don't see why >> >> long x[1024]; >> >> Q *q = new (x) Q; >> q->~Q (); >> new (x) T; >> >> would be invalid. I also don't see why >> >> Q q; >> q.~Q (); >> new (&q) T; >> >> would be. Object lifetime is precisely specified and I don't see where it is >> tied to (static) storage lifetime. > > This is precisely the testcase I posted on beggining of this thread. > > I do not see how the testcases can work with aliasing rules in the case Q's > and T's > memory is known to not alias.
It works because of the well-defined memory model (with regarding to TBAA) in the middle-end. Every store changes the dynamic type of a memory location which means that you can only use TBAA for true-dependence checks (not anti-dependence or write-dependence checks). That has been the way we operate since GCC 4.3 (if I remember correctly). That's also the reason we don't have to special-case unions in any tricky way (yeah, we still do - because of that type-punning special case and RTL alias analysis not dealing with it). > Either we need to define what is and is not supported or go for speculative > devirt more often. The GCC middle-end (which also has to deal with cross-language cases!) has this specified very clearly. Richard. > Honza
