[PATCH] tree-optimization/103458 - avoid creating new loops in CD-DCE

[PATCH] tree-optimization/103458 - avoid creating new loops in CD-DCE

[Richard Biener]

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.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

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 00000000000..3fd3b5fd2ff
--- /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 e3e6f0955b7..1f817b95fab 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
-- 
2.31.1