--- a/src/share/vm/opto/block.cpp	Wed Aug 27 16:33:34 2008 -0700
+++ b/src/share/vm/opto/block.cpp	Thu Aug 28 10:22:12 2008 -0700
@@ -467,6 +467,10 @@
   // get successor block succ_no
   assert(succ_no < in->_num_succs, "illegal successor number");
   Block* out = in->_succs[succ_no];
+  // Compute frequency of the new block. Do this before inserting
+  // new block in case succ_prob() needs to infer the probability from
+  // surrounding blocks.
+  float freq = in->_freq * in->succ_prob(succ_no);
   // get ProjNode corresponding to the succ_no'th successor of the in block
   ProjNode* proj = in->_nodes[in->_nodes.size() - in->_num_succs + succ_no]->as_Proj();
   // create region for basic block
@@ -491,6 +495,8 @@
   }
   // remap predecessor's successor to new block
   in->_succs.map(succ_no, block);
+  // Set the frequency of the new block
+  block->_freq = freq;
   // add new basic block to basic block list
   _blocks.insert(block_no + 1, block);
   _num_blocks++;

--- a/src/share/vm/opto/gcm.cpp	Wed Aug 27 16:33:34 2008 -0700
+++ b/src/share/vm/opto/gcm.cpp	Thu Aug 28 10:22:12 2008 -0700
@@ -1609,7 +1609,30 @@
 float Block::succ_prob(uint i) {
   int eidx = end_idx();
   Node *n = _nodes[eidx];  // Get ending Node
-  int op = n->is_Mach() ? n->as_Mach()->ideal_Opcode() : n->Opcode();
+
+  int op = n->Opcode();
+  if (n->is_Mach()) {
+    if (n->is_MachNullCheck()) {
+      // Can only reach here if called after lcm. The original Op_If is gone,
+      // so we attempt to infer the probability from one or both of the
+      // successor blocks.
+      assert(_num_succs == 2, "expecting 2 successors of a null check");
+      // If either successor has only one predecessor, then the
+      // probabiltity estimate can be derived using the
+      // relative frequency of the successor and this block.
+      if (_succs[i]->num_preds() == 2) {
+        return _succs[i]->_freq / _freq;
+      } else if (_succs[1-i]->num_preds() == 2) {
+        return 1 - (_succs[1-i]->_freq / _freq);
+      } else {
+        // Estimate using both successor frequencies
+        float freq = _succs[i]->_freq;
+        return freq / (freq + _succs[1-i]->_freq);
+      }
+    }
+    op = n->as_Mach()->ideal_Opcode();
+  }
+
 
   // Switch on branch type
   switch( op ) {