EVALUATION

http://hg.openjdk.java.net/jdk7/hotspot-comp/hotspot/rev/6ca61c728c2d
                                     

SUGGESTED FIX

diff --git a/src/share/vm/opto/ifnode.cpp b/src/share/vm/opto/ifnode.cpp
--- a/src/share/vm/opto/ifnode.cpp
+++ b/src/share/vm/opto/ifnode.cpp
@@ -725,6 +725,11 @@ static Node *remove_useless_bool(IfNode 
   int true_path = phi->is_diamond_phi();
   if( true_path == 0 ) return NULL;
 
+  // Make sure that iff and the control of the phi are different. This
+  // should really only happen for dead control flow since it requires
+  // an illegal cycle.
+  if (phi->in(0)->in(1)->in(0) == iff) return NULL;
+
   // phi->region->if_proj->ifnode->bool->cmp
   BoolNode *bol2 = phi->in(0)->in(1)->in(0)->in(1)->as_Bool();
 
@@ -751,6 +756,7 @@ static Node *remove_useless_bool(IfNode 
   }
 
   Node* new_bol = (flip ? phase->transform( bol2->negate(phase) ) : bol2);
+  assert(new_bol != iff->in(1), "must make progress");
   iff->set_req(1, new_bol);
   // Intervening diamond probably goes dead
   phase->C->set_major_progress();
                                     

EVALUATION

The remove_useless_bool routine is signaling progress even though it not actually making any.  This is caused by a cycle in some dead code.