On Wed, Feb 19, 2025 at 11:29 PM Jeff Law <jeffreya...@gmail.com> wrote:
>
>
> An interesting little bug. We're just emitting what appears to me to be
> a silly warning.
>
> By the time the array-bounds checker runs we have this statement in the IL:
>
> > MEM[(int *)_42 + -4B] ={v} {CLOBBER(eob)};
>
> Naturally that looks like an out of bounds array index and we warn.
> Which seems dubious at best. My first thought is we shouldn't be trying
> to analyze a gimple clobber for out of bounds array indexes.
OTOH all clobbers are inserted for actual allocated storage (even if not
accessed).
> But like most things in GCC, it's never that simple.
>
> It looks like we're relying on warning for that case for
> Warray-bounds-20.C.
>
>
> > <bb 2> [local count: 1073741824]:
> > MEM[(struct D1 *)&a + 1B] ={v} {CLOBBER(bob)};
> > MEM[(struct B *)&a + 1B]._vptr.B = &MEM <int (*) ()> [(void *)&_ZTC2D10_1B
> > + 24B];
> > MEM[(struct A *)&a + 9B]._vptr.A = &MEM <int (*) ()> [(void *)&_ZTC2D10_1B
> > + 64B];
> > C::C (&MEM[(struct D1 *)&a + 1B].D.3003, &MEM <const void *[8]> [(void
> > *)&_ZTT2D1 + 48B]);
>
> My sense is that this test is passing more by accident than by design
> and that the warning really should be triggered by one of the other
> statements.
And it isn't? So A and B are smaller than D1 and only D1 overruns?
But it's not
actually stored to?
> Martin just got it wrong here AFAICT. I did go back and
> check the original BZ (pr98266). It's unaffected as it doesn't have the
> gimple clobbers.
I'll note the unwanted diagnostic is on a EOB (end-of-object) while the
wanted is on a BOB (begin-of-object). Both are not begin/end-of-storage.
Now find a convincing argument that any of those are not worth diagnosing...
Richard.
>
> I'd propose a patch like the following, but I also realize this might be
> considered somewhat controversial, particularly since I think we could
> be missing diagnostics, particularly around placement-new that we now
> kind of pick up by accident (though not reliably as can be seen in other
> subtests withing Warray-bounds-20.C which were already xfail'd).
>
> Thoughts?
>
> Jeff
