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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103202] [12 regression] gcc miscompiles ed-1.17
Date: Fri, 12 Nov 2021 09:33:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103202

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Sergei Trofimovich from comment #3)
> Created attachment 51776 [details]
> main_loop_simpler.c
>=20
> Attaching seemingly simpler example. I applied most inlines and simplified
> loops as much as I could. It looks like VRP does something funny with:
>=20
>   while (n > 0)
>   {
>     while (n-- > 0)
>     {
>=20
> Looking at 039t.mergephi1
>=20
> with -fno-tree-vrp:
>=20
> --- no-vrp/ed-main_loop.c.039t.mergephi1    2021-11-12 09:07:55.120821967
> +0000
> +++ yes-vrp/ed-main_loop.c.039t.mergephi1    2021-11-12 09:07:55.186822973
> +0000
> @@ -93,7 +93,7 @@
>    # np_6 =3D PHI <np_7(10), np_40(6)>
>    # n_8 =3D PHI <n_9(10), n_23(6)>
>    n_23 =3D n_8 + -1;
> -  if (n_8 > 0)
> +  if (n_8 !=3D 0)
>      goto <bb 4>; [INV]
>    else
>      goto <bb 8>; [INV]
>=20
> I think our loop is executed with n =3D=3D -1 on real data and vrp provid=
ed
> incorrect range.

But how would the inner loop be reachable with n =3D=3D -1?=