From: Richard Biener <rguenther@suse.de>
Date: Mon, 29 Nov 2021 08:15:47 +0000 (+0100)
Subject: tree-optimization/103458 - avoid creating new loops in CD-DCE
X-Git-Tag: basepoints/gcc-13~2741
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=85e91ad55a69282c1b0e34569836a026a1a954d1;p=thirdparty%2Fgcc.git

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.
---

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