diff --git a/compiler/optimizing/constant_folding_test.cc b/compiler/optimizing/constant_folding_test.cc
index 422223f..11f6362 100644
--- a/compiler/optimizing/constant_folding_test.cc
+++ b/compiler/optimizing/constant_folding_test.cc
@@ -627,8 +627,8 @@
     "  9: If(8)\n"
     "BasicBlock 2, pred: 1, succ: 3\n"
     "  12: Goto 3\n"
-    "BasicBlock 3, pred: 2, 5, succ: 4\n"
-    "  22: Phi(3, 5) [15]\n"
+    "BasicBlock 3, pred: 5, 2, succ: 4\n"
+    "  22: Phi(5, 3) [15]\n"
     "  15: Add(22, 3)\n"
     "  17: ReturnVoid\n"
     "BasicBlock 4, pred: 3\n"
diff --git a/compiler/optimizing/dead_code_elimination_test.cc b/compiler/optimizing/dead_code_elimination_test.cc
index 3209d3e..ee3a61a 100644
--- a/compiler/optimizing/dead_code_elimination_test.cc
+++ b/compiler/optimizing/dead_code_elimination_test.cc
@@ -89,8 +89,8 @@
     "  9: If(8)\n"
     "BasicBlock 2, pred: 1, succ: 3\n"
     "  12: Goto 3\n"
-    "BasicBlock 3, pred: 2, 5, succ: 4\n"
-    "  22: Phi(3, 5) [15]\n"
+    "BasicBlock 3, pred: 5, 2, succ: 4\n"
+    "  22: Phi(5, 3) [15]\n"
     "  15: Add(22, 3)\n"
     "  17: ReturnVoid\n"
     "BasicBlock 4, pred: 3\n"
@@ -101,7 +101,7 @@
   // Expected difference after dead code elimination.
   diff_t expected_diff = {
     { "  3: IntConstant [15, 22, 8]\n", "  3: IntConstant [22, 8]\n" },
-    { "  22: Phi(3, 5) [15]\n",         "  22: Phi(3, 5)\n" },
+    { "  22: Phi(5, 3) [15]\n",         "  22: Phi(5, 3)\n" },
     { "  15: Add(22, 3)\n",             removed }
   };
   std::string expected_after = Patch(expected_before, expected_diff);
diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index 68c197e..01eb2d7 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -180,8 +180,9 @@
   HBasicBlock* new_block = new (arena_) HBasicBlock(this, successor->GetDexPc());
   AddBlock(new_block);
   new_block->AddInstruction(new (arena_) HGoto());
-  block->ReplaceSuccessor(successor, new_block);
-  new_block->AddSuccessor(successor);
+  // Use `InsertBetween` to ensure the predecessor index and successor index of
+  // `block` and `successor` are preserved.
+  new_block->InsertBetween(block, successor);
   if (successor->IsLoopHeader()) {
     // If we split at a back edge boundary, make the new block the back edge.
     HLoopInformation* info = successor->GetLoopInformation();
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 9443653..edd66ec 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -625,6 +625,20 @@
     predecessors_.Put(predecessor_index, new_block);
   }
 
+  // Insert `this` between `predecessor` and `successor. This method
+  // preserves the indicies, and will update the first edge found between
+  // `predecessor` and `successor`.
+  void InsertBetween(HBasicBlock* predecessor, HBasicBlock* successor) {
+    size_t predecessor_index = successor->GetPredecessorIndexOf(predecessor);
+    DCHECK_NE(predecessor_index, static_cast<size_t>(-1));
+    size_t successor_index = predecessor->GetSuccessorIndexOf(successor);
+    DCHECK_NE(successor_index, static_cast<size_t>(-1));
+    successor->predecessors_.Put(predecessor_index, this);
+    predecessor->successors_.Put(successor_index, this);
+    successors_.Add(successor);
+    predecessors_.Add(predecessor);
+  }
+
   void RemovePredecessor(HBasicBlock* block) {
     predecessors_.Delete(block);
   }
diff --git a/compiler/optimizing/ssa_test.cc b/compiler/optimizing/ssa_test.cc
index fb3e7d7..0e8c058 100644
--- a/compiler/optimizing/ssa_test.cc
+++ b/compiler/optimizing/ssa_test.cc
@@ -115,7 +115,7 @@
     "  3: If(2)\n"
     "BasicBlock 2, pred: 1, succ: 3\n"
     "  4: Goto\n"
-    "BasicBlock 3, pred: 2, 5, succ: 4\n"
+    "BasicBlock 3, pred: 5, 2, succ: 4\n"
     "  5: ReturnVoid\n"
     "BasicBlock 4, pred: 3\n"
     "  6: Exit\n"
@@ -145,8 +145,8 @@
     "  4: If(3)\n"
     "BasicBlock 2, pred: 1, succ: 3\n"
     "  5: Goto\n"
-    "BasicBlock 3, pred: 2, 5, succ: 4\n"
-    "  6: Phi(1, 0) [7]\n"
+    "BasicBlock 3, pred: 5, 2, succ: 4\n"
+    "  6: Phi(0, 1) [7]\n"
     "  7: Return(6)\n"
     "BasicBlock 4, pred: 3\n"
     "  8: Exit\n"
@@ -428,8 +428,8 @@
     "  10: Goto\n"
     "BasicBlock 5, pred: 3, succ: 2\n"
     "  11: Goto\n"
-    "BasicBlock 6, pred: 4, 8, succ: 7\n"
-    "  12: Phi(2, 5) [13]\n"
+    "BasicBlock 6, pred: 8, 4, succ: 7\n"
+    "  12: Phi(5, 2) [13]\n"
     "  13: Return(12)\n"
     "BasicBlock 7, pred: 6\n"
     "  14: Exit\n"
@@ -480,7 +480,7 @@
     "  4: If(3)\n"
     "BasicBlock 2, pred: 1, succ: 3\n"
     "  5: Goto\n"
-    "BasicBlock 3, pred: 2, 5, succ: 4\n"
+    "BasicBlock 3, pred: 5, 2, succ: 4\n"
     "  6: ReturnVoid\n"
     "BasicBlock 4, pred: 3\n"
     "  7: Exit\n"
@@ -517,7 +517,7 @@
     "  8: Add(0, 0)\n"
     "  9: Goto\n"
     // This block should not get a phi for local 1.
-    "BasicBlock 5, pred: 2, 4, 7, succ: 6\n"
+    "BasicBlock 5, pred: 2, 7, 4, succ: 6\n"
     "  10: ReturnVoid\n"
     "BasicBlock 6, pred: 5\n"
     "  11: Exit\n"
diff --git a/test/485-checker-dce-loop-update/smali/TestCase.smali b/test/485-checker-dce-loop-update/smali/TestCase.smali
index da27bf6..ab4afdb 100644
--- a/test/485-checker-dce-loop-update/smali/TestCase.smali
+++ b/test/485-checker-dce-loop-update/smali/TestCase.smali
@@ -141,7 +141,7 @@
 ## CHECK-DAG:                    If [<<ArgY>>]                              loop:<<HeaderY>>
 ## CHECK-DAG:                    If [<<ArgZ>>]                              loop:<<HeaderY>>
 ## CHECK-DAG:     <<Mul9:i\d+>>  Mul [<<PhiX1>>,<<Cst9>>]                   loop:<<HeaderY>>
-## CHECK-DAG:     <<PhiX2:i\d+>> Phi [<<Mul9>>,<<PhiX1>>]                   loop:<<HeaderY>>
+## CHECK-DAG:     <<PhiX2:i\d+>> Phi [<<PhiX1>>,<<Mul9>>]                   loop:<<HeaderY>>
 ## CHECK-DAG:                    If [<<Cst1>>]                              loop:<<HeaderY>>
 ## CHECK-DAG:     <<Add5>>       Add [<<PhiX2>>,<<Cst5>>]                   loop:<<HeaderY>>
 ## CHECK-DAG:     <<Add7>>       Add [<<PhiX1>>,<<Cst7>>]                   loop:<<HeaderY>>
@@ -158,7 +158,7 @@
 ## CHECK-DAG:     <<Add7>>       Add [<<PhiX1>>,<<Cst7>>]                   loop:<<HeaderY>>
 ## CHECK-DAG:                    If [<<ArgZ>>]                              loop:none
 ## CHECK-DAG:     <<Mul9:i\d+>>  Mul [<<PhiX1>>,<<Cst9>>]                   loop:none
-## CHECK-DAG:     <<PhiX2:i\d+>> Phi [<<Mul9>>,<<PhiX1>>]                   loop:none
+## CHECK-DAG:     <<PhiX2:i\d+>> Phi [<<PhiX1>>,<<Mul9>>]                   loop:none
 ## CHECK-DAG:                    Return [<<PhiX2>>]                         loop:none
 
 .method public static testExitPredecessors(IZZ)I
diff --git a/test/509-pre-header/expected.txt b/test/509-pre-header/expected.txt
new file mode 100644
index 0000000..ccaf6f8
--- /dev/null
+++ b/test/509-pre-header/expected.txt
@@ -0,0 +1 @@
+Enter
diff --git a/test/509-pre-header/info.txt b/test/509-pre-header/info.txt
new file mode 100644
index 0000000..e9d8b94
--- /dev/null
+++ b/test/509-pre-header/info.txt
@@ -0,0 +1,3 @@
+Regression test for the SimplifyCFG phase of optimizing.
+The invariant that the pre header of a loop header is the
+first predecessor was not preserved.
diff --git a/test/509-pre-header/smali/PreHeader.smali b/test/509-pre-header/smali/PreHeader.smali
new file mode 100644
index 0000000..04f4e49
--- /dev/null
+++ b/test/509-pre-header/smali/PreHeader.smali
@@ -0,0 +1,39 @@
+# Copyright (C) 2015 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+.class public LPreHeader;
+
+.super Ljava/lang/Object;
+
+# Label names in this method are taken from the original apk
+# that exposed the crash. The crash was due to fixing a critical
+# edge and not preserving the invariant that the pre header of a loop
+# is the first predecessor of the loop header.
+.method public static method()V
+   .registers 2
+   const/4 v0, 0
+   const/4 v1, 0
+   goto :b31
+   :b23
+   if-eqz v0, :b25
+   goto :b23
+   :b25
+   return-void
+   :b31
+   if-eqz v0, :b23
+   if-eqz v1, :bexit
+   goto :b31
+   :bexit
+   return-void
+.end method
diff --git a/test/509-pre-header/src/Main.java b/test/509-pre-header/src/Main.java
new file mode 100644
index 0000000..1eca419
--- /dev/null
+++ b/test/509-pre-header/src/Main.java
@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import java.lang.reflect.Method;
+
+public class Main {
+  public static void main(String[] args) throws Exception {
+    // Workaround for b/18051191.
+    System.out.println("Enter");
+    Class<?> c = Class.forName("PreHeader");
+    Method m = c.getMethod("method");
+    Object[] arguments = { };
+    m.invoke(null, arguments);
+  }
+}