[gcc r12-5572] tree-optimization/103458 - avoid creating new loops in CD-DCE

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:85e91ad55a69282c1b0e34569836a026a1a954d1

commit r12-5572-g85e91ad55a69282c1b0e34569836a026a1a954d1
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Nov 29 09:15:47 2021 +0100

    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:
---
 gcc/testsuite/gcc.dg/torture/pr103458.c | 21 +++++++++++++++++++++
 gcc/tree-ssa-dce.c                      |  8 ++++++--
 2 files changed, 27 insertions(+), 2 deletions(-)

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