diff -r 8e7adf982378 -r 7c57aead6d3e src/share/vm/opto/parseHelper.cpp
--- a/src/share/vm/opto/parseHelper.cpp	Fri Nov 27 07:56:58 2009 -0800
+++ b/src/share/vm/opto/parseHelper.cpp	Thu Nov 12 09:24:21 2009 -0800
@@ -221,6 +221,14 @@
 
   // Push resultant oop onto stack
   push(obj);
+
+  // Keep track of whether opportunities exist for StringBuilder
+  // optimizations.
+  if (OptimizeStringConcat &&
+      (klass == C->env()->StringBuilder_klass() ||
+       klass == C->env()->StringBuffer_klass())) {
+    C->set_has_stringbuilder(true);
+  }
 }
 
 #ifndef PRODUCT