On Sun, Mar 10, 2024 at 2:09 PM Jeff Law <j...@ventanamicro.com> wrote: > > > > On 3/10/24 3:05 PM, Andrew Pinski wrote: > > 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 > Yea, realized it as I pushed the send button. Then t-bird crashed, > repeatedly. > > Attached this time..
One minor comment on it The comment: > return true for EQ and false for NE. Seems to be the opposite for what the code does: > return code == EQ_EXPR ? boolean_false_node : boolean_true_node; Thanks, Andrew > > jeff >
