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From: "gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/101188] [postreload] Uses content of a
 clobbered register
Date: Mon, 29 May 2023 19:35:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101188

--- Comment #10 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Created attachment 55189
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D55189&action=3Dedit
Proposed patch for postreload.cc so analysis is not bypassing "next" insn.

(In reply to Georg-Johann Lay from comment #9)
> postreload.cc::reload_cse_move2add() loops over all insns, and at some po=
int
> it encounters
>=20
> (insn 44 14 15 2 (set (reg/f:HI 14 r14 [58])
>         (reg/v/f:HI 16 r16 [orig:51 self ] [51])) "fail1.c":28:5 101
> {*movhi_split}
>      (nil))
>=20
> During the analysis for that insn, it executes
>=20
> 	  rtx_insn *next =3D next_nonnote_nondebug_insn (insn);
> 	  rtx set =3D NULL_RTX;
> 	  if (next)
> 	    set =3D single_set (next);
>=20
> where next is
>=20
> (insn 15 44 16 2 (parallel [
>             (set (reg/f:HI 14 r14 [58])
>                 (plus:HI (reg/f:HI 14 r14 [58])
>                     (const_int 68 [0x44])))
>             (clobber (reg:QI 31 r31))
>         ]) "fail1.c":28:5 175 {addhi3_clobber}
>      (nil))
>=20
> Further down, it continues with success =3D 0:
>=20
>       if (success)
> 	delete_insn (insn);
>       changed |=3D success;
>       insn =3D next;
>       [...]
>       continue;
>=20
> The scan then continues with NEXT_INSN (insn), which is the insn AFTER in=
sn
> 15, so the CLOBBER of QI:31 in insn 15 is bypassed, and note_stores or
> similar is never executed on insn 15.

A simple solution is to not "insn =3D next;" and just to pseudo-delete insn=
 so
that NEXT_INSN (insn) works in the for loop:

diff --git a/gcc/postreload.cc b/gcc/postreload.cc
index fb392651e1b..388b8c0a506 100644
--- a/gcc/postreload.cc
+++ b/gcc/postreload.cc
@@ -2031,9 +2031,8 @@ reload_cse_move2add (rtx_insn *first)
                            }
                        }
                      if (success)
-                       delete_insn (insn);
+                       SET_INSN_DELETED (insn);
                      changed |=3D success;
-                     insn =3D next;
                      move2add_record_mode (reg);
                      reg_offset[regno]
                        =3D trunc_int_for_mode (added_offset + base_offset,=