From 85e91ad55a69282c1b0e34569836a026a1a954d1 Mon Sep 17 00:00:00 2001
From: Richard Biener <rguenther@suse.de>
Date: Mon, 29 Nov 2021 09:15:47 +0100
Subject: [PATCH] tree-optimization/103458 - avoid creating new loops in CD-DCE
When creating forwarders in CD-DCE we have to avoid creating loops
where we formerly did not consider those because of abnormal
predecessors. At this point simply excuse us when there are any
abnormal predecessors.
2021-11-29 Richard Biener <rguenther@suse.de>
PR tree-optimization/103458
* tree-ssa-dce.c (make_forwarders_with_degenerate_phis): Do not
create forwarders for blocks with abnormal predecessors.
* gcc.dg/torture/pr103458.c: New testcase.
---
gcc/testsuite/gcc.dg/torture/pr103458.c | 21 +++++++++++++++++++++
gcc/tree-ssa-dce.c | 8 ++++++--
2 files changed, 27 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/gcc.dg/torture/pr103458.c
diff --git a/gcc/testsuite/gcc.dg/torture/pr103458.c b/gcc/testsuite/gcc.dg/torture/pr103458.c
new file mode 100644
index 000000000000..3fd3b5fd2ffb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr103458.c
@@ -0,0 +1,21 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-Wno-div-by-zero" } */
+
+__attribute__ ((returns_twice)) int
+bar (void);
+
+void
+foo (int *p, int x)
+{
+ *p = 0;
+ while (*p < 1)
+ {
+ x = 0;
+ while (x < 1)
+ bar ();
+
+ x /= 0;
+ }
+
+ foo (p, x);
+}
diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
index e3e6f0955b7d..1f817b95faba 100644
--- a/gcc/tree-ssa-dce.c
+++ b/gcc/tree-ssa-dce.c
@@ -1650,8 +1650,12 @@ make_forwarders_with_degenerate_phis (function *fn)
/* Only PHIs with three or more arguments have opportunities. */
if (EDGE_COUNT (bb->preds) < 3)
continue;
- /* Do not touch loop headers. */
- if (bb->loop_father->header == bb)
+ /* Do not touch loop headers or blocks with abnormal predecessors.
+ ??? This is to avoid creating valid loops here, see PR103458.
+ We might want to improve things to either explicitely add those
+ loops or at least consider blocks with no backedges. */
+ if (bb->loop_father->header == bb
+ || bb_has_abnormal_pred (bb))
continue;
/* Take one PHI node as template to look for identical
--
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