--- a/hotspot/src/share/vm/opto/loopnode.cpp	Mon Mar 21 08:42:20 2016 +0100
+++ b/hotspot/src/share/vm/opto/loopnode.cpp	Mon Mar 21 09:51:20 2016 +0100
@@ -3275,6 +3275,41 @@
   return LCA;
 }
 
+// Check the shape of the graph at the loop entry. In some cases,
+// the shape of the graph does not match the shape outlined below.
+// That is caused by the Opaque1 node "protecting" the shape of
+// the graph being removed by, for example, the IGVN performed
+// in PhaseIdealLoop::build_and_optimize().
+//
+// After the Opaque1 node has been removed, optimizations (e.g., split-if,
+// loop unswitching, and IGVN, or a combination of them) can freely change
+// the graph's shape. As a result, the graph shape outlined below cannot
+// be guaranteed anymore.
+bool PhaseIdealLoop::is_canonical_main_loop_entry(CountedLoopNode* cl) {
+  assert(cl->is_main_loop(), "check should be applied to main loops");
+  Node* ctrl = cl->in(LoopNode::EntryControl);
+  if (ctrl == NULL || (!ctrl->is_IfTrue() && !ctrl->is_IfFalse())) {
+    return false;
+  }
+  Node* iffm = ctrl->in(0);
+  if (iffm == NULL || !iffm->is_If()) {
+    return false;
+  }
+  Node* bolzm = iffm->in(1);
+  if (bolzm == NULL || !bolzm->is_Bool()) {
+    return false;
+  }
+  Node* cmpzm = bolzm->in(1);
+  if (cmpzm == NULL || !cmpzm->is_Cmp()) {
+    return false;
+  }
+  Node* opqzm = cmpzm->in(2);
+  if (opqzm == NULL || opqzm->Opcode() != Op_Opaque1) {
+    return false;
+  }
+  return true;
+}
+
 //------------------------------get_late_ctrl----------------------------------
 // Compute latest legal control.
 Node *PhaseIdealLoop::get_late_ctrl( Node *n, Node *early ) {