Re: [RFC] [PR tree-optimization/92539] Optimize away tests against invalid pointers

[Andrew Pinski <pinskia@gmail.com>]

On Sun, Mar 10, 2024 at 2:09 PM Jeff Law <jlaw@ventanamicro.com> wrote:
>
>
>
> On 3/10/24 3:05 PM, Andrew Pinski wrote:
> > On Sun, Mar 10, 2024 at 2:04 PM Jeff Law <jlaw@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
>