--- old/src/share/vm/opto/cfgnode.hpp	2015-09-29 14:27:50.587019342 +0200
+++ new/src/share/vm/opto/cfgnode.hpp	2015-09-29 14:27:50.451019348 +0200
@@ -395,10 +395,12 @@
 public:
   IfProjNode(IfNode *ifnode, uint idx) : CProjNode(ifnode,idx) {}
   virtual Node *Identity(PhaseTransform *phase);
+  virtual Node* Ideal(PhaseGVN* phase, bool can_reshape);
 
 protected:
   // Type of If input when this branch is always taken
   virtual bool always_taken(const TypeTuple* t) const = 0;
+  virtual bool never_taken(const TypeTuple* t) const = 0;
 
 #ifndef PRODUCT
 public:
@@ -415,6 +417,7 @@
 
 protected:
   virtual bool always_taken(const TypeTuple* t) const { return t == TypeTuple::IFTRUE; }
+  virtual bool never_taken(const TypeTuple* t)  const { return t == TypeTuple::IFFALSE; }
 };
 
 class IfFalseNode : public IfProjNode {
@@ -426,6 +429,7 @@
 
 protected:
   virtual bool always_taken(const TypeTuple* t) const { return t == TypeTuple::IFFALSE; }
+  virtual bool never_taken(const TypeTuple* t)  const { return t == TypeTuple::IFTRUE;  }
 };
 
 
--- old/src/share/vm/opto/ifnode.cpp	2015-09-29 14:27:51.319019308 +0200
+++ new/src/share/vm/opto/ifnode.cpp	2015-09-29 14:27:51.151019315 +0200
@@ -1625,6 +1625,19 @@
   return this;
 }
 
+Node* IfProjNode::Ideal(PhaseGVN* phase, bool can_reshape) {
+  if (in(0)->is_top()) {
+    return NULL;
+  }
+  const TypeTuple* t = phase->type(in(0))->is_tuple();
+  if (never_taken(t)) {
+    // Disconnect never taken branch
+    set_req(0, phase->C->top());
+    return this;
+  }
+  return NULL;
+}
+
 #ifndef PRODUCT
 //-------------------------------related---------------------------------------
 // An IfProjNode's related node set consists of its input (an IfNode) including
--- old/src/share/vm/opto/stringopts.cpp	2015-09-29 14:27:52.071019273 +0200
+++ new/src/share/vm/opto/stringopts.cpp	2015-09-29 14:27:51.899019281 +0200
@@ -822,11 +822,10 @@
             }
           } else if (ctrl->is_IfTrue()) { // null checks, class checks
             iff = ctrl->in(0)->as_If();
-            assert(iff->is_If(), "must be if");
             // Verify that the other arm is an uncommon trap
             Node* otherproj = iff->proj_out(1 - ctrl->as_Proj()->_con);
             CallStaticJavaNode* call = otherproj->unique_out()->isa_CallStaticJava();
-            assert(strcmp(call->_name, "uncommon_trap") == 0, "must be uncommond trap");
+            assert(strcmp(call->_name, "uncommon_trap") == 0, "must be uncommon trap");
             ctrl = iff->in(0);
           } else {
             break;
@@ -914,6 +913,13 @@
       BoolNode* b = iff->in(1)->isa_Bool();
 
       if (b == NULL) {
+#ifndef PRODUCT
+        if (PrintOptimizeStringConcat) {
+          tty->print_cr("unexpected input to IfNode");
+          iff->in(1)->dump();
+          tty->cr();
+        }
+#endif
         fail = true;
         break;
       }
--- /dev/null	2015-09-29 07:28:17.018874950 +0200
+++ new/test/compiler/stringopts/TestPresizedStringBuilder.java	2015-09-29 14:27:52.667019245 +0200
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8136469
+ * @summary C2's string concatenation optimization fails with pre-sized StringBuilder
+ * @run main/othervm -XX:CompileCommand=dontinline,TestPresizedStringBuilder::test TestPresizedStringBuilder
+ */
+public class TestPresizedStringBuilder {
+
+  static final boolean count;
+  static {
+    count = true;
+  }
+
+  public static void main(String[] args) {
+    for (int i = 0; i < 100_000; ++i) {
+      test();
+    }
+  }
+
+  public static String test() {
+    return new StringBuilder(count ? 1 : 2).toString();
+  }
+}
+