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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103359] [12 Regression] Dead Code Elimination
 Regression at -O3 (trunk vs 11.2.0)
Date: Tue, 23 Nov 2021 11:36:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103359

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Which means the CCP difference is that we're failing to propagate out copies
when they have a useful value:

Visiting statement:
_25 =3D _24;
which is likely CONSTANT
Lattice value changed to CONSTANT 0x0 (0xf).  Adding SSA edges to worklist.
ssa_edge_worklist: adding SSA use in _7 =3D _25;

Simulating statement: _7 =3D _25;

Visiting statement:
_7 =3D _25;
which is likely CONSTANT
Lattice value changed to CONSTANT 0x0 (0xf).  Adding SSA edges to worklist.

that's because the copy and constant lattice are unified and we cannot
track both.  And while forwprop will propagate out copies since it does
that when it visits uses it confuses itself with this.  Propagating them
out immediately fixes this but the obvious use of replace_uses_by causes
excessive folding and redundant work as well as out-of-order debug stmt
handling.

The following would fix the testcase.  Note there's still the appearant EVRP
regression compared to GCC 11.  The single_use test was added with
g:2fde61e3caf4c4660743e53497f52b65da1fe760 as a fix for PR66916.

diff --git a/gcc/match.pd b/gcc/match.pd
index f059b477f58..b32cc4a9368 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -5221,8 +5221,7 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
            && ((POINTER_TYPE_P (TREE_TYPE (@00))
                 && FUNC_OR_METHOD_TYPE_P (TREE_TYPE (TREE_TYPE (@00))))
                || (POINTER_TYPE_P (TREE_TYPE (@10))
-                   && FUNC_OR_METHOD_TYPE_P (TREE_TYPE (TREE_TYPE (@10))))=
))
-       && single_use (@0))
+                   && FUNC_OR_METHOD_TYPE_P (TREE_TYPE (TREE_TYPE (@10))))=
)))
    (if (TYPE_PRECISION (TREE_TYPE (@00)) =3D=3D TYPE_PRECISION (TREE_TYPE =
(@0))
        && (TREE_CODE (@10) =3D=3D INTEGER_CST
            || @1 !=3D @10)=