> On July 23, 2014 4:42:22 PM CEST, Jan Hubicka <hubi...@ucw.cz> wrote: > >> 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). > > > >I see, I did not notice this change - it seems like quite a big hammer > >though, > >limiting scheduling (and loop opts) quite noticeably for all languages. > >Are > >there any other motivations for this besides placement new? > > Aggregate copies and memcpy transferring the dynamic type for example. Being > able to tbaa union accesses for another. And yes, placement new. > > It's not so much an optimization preventing thing as you still can move loads > up and stores down with the help of tbaa.
well, but you lose extra parallelism like *shortptr = exp <longer dependency chain with shortptr> var = *shortptr *intptr = exp <longer dependency chain with intptr> var = *intptr Honza