On Sun, Mar 10, 2024 at 2:04 PM Jeff Law <j...@ventanamicro.com> wrote: > > Here's a potential approach to fixing PR92539, a P2 -Warray-bounds false > positive triggered by loop unrolling. > > As I speculated a couple years ago, we could eliminate the comparisons > against bogus pointers. Consider: > > > <bb 8> [local count: 30530247]: > > if (last_12 != &MEM <const char> [(void *)"aa" + 3B]) > > goto <bb 9>; [54.59%] > > else > > goto <bb 12>; [45.41%] > > > That's a valid comparison as ISO allows us to generate, but not > dereference, a pointer one element past the end of the object. > > But +4B is a bogus pointer. So given an EQ comparison against that > pointer we could always return false and for NE always return true. > > VRP and DOM seem to be the most natural choices for this kind of > optimization on the surface. However DOM is actually not viable because > the out-of-bounds pointer warning pass is run at the end of VRP. So > we've got to take care of this prior to the end of VRP. > > > > I haven't done a bootstrap or regression test with this. But if it > looks reasonable I can certainly push on it further. I have confirmed it > does eliminate the tests and shuts up the bogus warning. > > The downside is this would also shut up valid warnings if user code did > this kind of test. > > Comments/Suggestions?
ENOPATCH > > Jeff
