https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107115
--- 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 != 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 = 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 != target && (side_effects_p (temp)
- || side_effects_p (target))))
+ || side_effects_p (target)
+ || !mems_same_for_tbaa_p (temp, target))))
&& TREE_CODE (exp) != ERROR_MARK
/* If store_expr stores a DECL whose DECL_RTL(exp) == 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".
