diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index e265311109d2..c0f554fed11e 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2017-02-21 Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/79621
+	* gimple-ssa-isolate-paths.c (find_implicit_erroneous_behavior): Ignore
+	blocks with edges to themselves.
+
 2017-02-21  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/79633
diff --git a/gcc/gimple-ssa-isolate-paths.c b/gcc/gimple-ssa-isolate-paths.c
index 25e8c8aad4cf..7babe0923625 100644
--- a/gcc/gimple-ssa-isolate-paths.c
+++ b/gcc/gimple-ssa-isolate-paths.c
@@ -35,6 +35,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-ssa.h"
 #include "cfgloop.h"
 #include "tree-cfg.h"
+#include "cfganal.h"
 #include "intl.h"
 
 
@@ -352,6 +353,16 @@ find_implicit_erroneous_behavior (void)
       if (has_abnormal_or_eh_outgoing_edge_p (bb))
 	continue;
 
+
+      /* If BB has an edge to itself, then duplication of BB below
+	 could result in reallocation of BB's PHI nodes.   If that happens
+	 then the loop below over the PHIs would use the old PHI and
+	 thus invalid information.  We don't have a good way to know
+	 if a PHI has been reallocated, so just avoid isolation in
+	 this case.  */
+      if (find_edge (bb, bb))
+	continue;
+
       /* First look for a PHI which sets a pointer to NULL and which
  	 is then dereferenced within BB.  This is somewhat overly
 	 conservative, but probably catches most of the interesting
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 9877624be836..9f278bb8b839 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2017-02-21  Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/79621
+	* gcc.c-torture/compile/pr79621.c: New test.
+
 2017-02-21  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/79655
diff --git a/gcc/testsuite/gcc.c-torture/compile/pr79621.c b/gcc/testsuite/gcc.c-torture/compile/pr79621.c
new file mode 100644
index 000000000000..f115c07a2a42
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr79621.c
@@ -0,0 +1,18 @@
+int b5;
+
+void
+h6 (int zb, int e7)
+{
+  while (b5 > 0)
+    {
+      int gv;
+
+      for (gv = 1; gv < 4; ++gv)
+        {
+          ((zb != 0) ? b5 : gv) && (b5 /= e7);
+          zb = 0;
+        }
+      e7 = 0;
+    }
+}
+