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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/107115] Wrong codegen from TBAA under stores that
 change effective type?
Date: Thu, 06 Oct 2022 09:44:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107115

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
So possibly expand_assignment is eliding the store somewhere downthread
(it's operand_equal_p check shouldn't fire).  It's

rtx
store_expr (tree exp, rtx target, int call_param_p,
            bool nontemporal, bool reverse)
{
...
  if ((! rtx_equal_p (temp, target)
       || (temp !=3D target && (side_effects_p (temp)
                              || side_effects_p (target))))

which doesn't fire.  We have

(gdb) p debug_rtx (temp)
(mem:DI (reg/f:DI 87 [ _11 ]) [2 *_11+0 S8 A64])
(gdb) p debug_rtx (target)
(mem:DI (reg/f:DI 87 [ _11 ]) [1 MEM[(long int *)_11]+0 S8 A64])

Note that even just matching up MEM_ALIAS_SET isn't enough.  I'd call the
above optimization premature at least.

(gdb) p mems_same_for_tbaa_p (temp, target)
$13 =3D false

so

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 80bb1b8a4c5..71a7fc19c42 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -6207,7 +6207,8 @@ store_expr (tree exp, rtx target, int call_param_p,

   if ((! rtx_equal_p (temp, target)
        || (temp !=3D target && (side_effects_p (temp)
-                             || side_effects_p (target))))
+                             || side_effects_p (target)
+                             || !mems_same_for_tbaa_p (temp, target))))
       && TREE_CODE (exp) !=3D ERROR_MARK
       /* If store_expr stores a DECL whose DECL_RTL(exp) =3D=3D TARGET,
         but TARGET is not valid memory reference, TEMP will differ

fixes this mistake.  But then we run into the sched issue which
-fno-schedule-insns2 "fixes".=