--- old/src/share/vm/opto/stringopts.cpp	2015-09-18 11:03:45.557264949 +0200
+++ new/src/share/vm/opto/stringopts.cpp	2015-09-18 11:03:45.469264947 +0200
@@ -822,11 +822,12 @@
             }
           } 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");
+            if (iff->outcnt() == 2) {
+              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 uncommon trap");
+            }
             ctrl = iff->in(0);
           } else {
             break;
@@ -911,9 +912,18 @@
       assert(ctrl_path.member(ptr), "should be a known piece of control");
     } else if (ptr->is_IfTrue()) {
       IfNode* iff = ptr->in(0)->as_If();
-      BoolNode* b = iff->in(1)->isa_Bool();
+      // Skip dead test
+      if (iff->in(1)->is_Con()) {
+        ptr = ptr->in(0)->in(0);
+        continue;
+      }
 
+      BoolNode* b = iff->in(1)->isa_Bool();
       if (b == NULL) {
+#ifndef PRODUCT
+        iff->in(1)->dump();
+        assert(false, "Unexpected input to IfNode");
+#endif
         fail = true;
         break;
       }
--- /dev/null	2015-09-18 08:01:11.771996015 +0200
+++ new/test/compiler/stringopts/TestPresizedStringBuilder.java	2015-09-18 11:03:45.781264956 +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();
+  }
+}
+