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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104931] wrong-code with
 number_of_iterations_lt_to_ne
Date: Wed, 16 Mar 2022 13:09:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104931

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> The bug was made latent by g:51d464b608b38b9e2007948d10b1e0f1dcec142c whi=
ch
> ensured that
>=20
>   /* If the loop exits immediately, there is nothing to do.  */
>   tree tem =3D fold_binary (code, boolean_type_node, iv0->base, iv1->base=
);
>   if (tem && integer_zerop (tem))
>     {
>       if (!every_iteration)
>         return false;
>       niter->niter =3D build_int_cst (unsigned_type_for (type), 0);
>       niter->max =3D 0;
>       return true;
>     }
>=20
> triggered, folding (_2 + 4294967272) + 12 < _2 + 4294967272 to false.  Th=
at's
> the following part of the revision, and it probably triggers when doing
> expand_simple_operations.
>=20
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 84c9b918041..f5dcbf32bc7 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -2143,6 +2143,11 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>  (simplify
>    (pointer_plus (pointer_plus:s @0 @1) @3)
>    (pointer_plus @0 (plus @1 @3)))
> +#if GENERIC
> +(simplify
> +  (pointer_plus (convert:s (pointer_plus:s @0 @1)) @3)
> +  (convert:type (pointer_plus @0 (plus @1 @3))))
> +#endif
>=20=20
>  /* Pattern match
>=20
> It does seem to me that niter analysis relies on statically detecting
> not rolling loops, at least for the cases we assume no overflow happens
> and we use number_of_iterations_lt_to_ne.  I can't convince myself that
> only INTEGER_CST appearant negative delta are problematic for
> pointer types (which we always assume to have no overflow), but that would
> be the most simplistic solution here.  Still "negative" delta values shou=
ld
> be problematic for all cases, and since we only restrict us to constant
> modulo, delta can also be non-constant.

I verified backporting the above fixes the issue.  That's really the patch
I'm most comfortable with at this point ... :/=