diff --git a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/controlFlow/orderUtil.kt b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/controlFlow/orderUtil.kt
index 9d1a3c0dff31..0ea2dced77ac 100644
--- a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/controlFlow/orderUtil.kt
+++ b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/controlFlow/orderUtil.kt
@@ -1,37 +1,29 @@
-/*
- * Copyright 2000-2017 JetBrains s.r.o.
- *
- * 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.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 @file:JvmName("OrderUtil")
 
 package org.jetbrains.plugins.groovy.lang.psi.controlFlow
 
+import com.intellij.util.ArrayUtil.EMPTY_INT_ARRAY
 import org.jetbrains.plugins.groovy.lang.psi.controlFlow.impl.InstructionImpl
 import java.util.*
 
 private val fakeRoot = InstructionImpl(null)
 
-fun reversedPostOrder(flow: Array<Instruction>): IntArray = postOrder(flow).reversedArray()
+@JvmOverloads
+fun reversedPostOrder(flow: Array<Instruction>, reachable: Boolean = false): IntArray = postOrder(flow, reachable).reversedArray()
 
-fun postOrder(flow: Array<Instruction>): IntArray {
-  val N = flow.size
-  val result = IntArray(N)
+fun postOrder(flow: Array<Instruction>, reachable: Boolean): IntArray {
+  val n = flow.size
+  if (n == 0) return EMPTY_INT_ARRAY
+
+  val result = IntArray(n) { -1 }
   var resultIndex = 0
 
-  val visited = BooleanArray(N)
+  val visited = BooleanArray(n)
   val stack: Deque<Pair<Instruction, Iterator<Instruction>>> = LinkedList()
-  stack.push(fakeRoot to flow.iterator())
+
+  val rootIterator = if (reachable) listOf(flow[0]).iterator() else flow.iterator()
+  stack.push(fakeRoot to rootIterator)
 
   while (!stack.isEmpty()) {
     val (instruction, iterator) = stack.peek()
@@ -51,8 +43,13 @@ fun postOrder(flow: Array<Instruction>): IntArray {
     }
   }
 
-  assert(resultIndex == N)
-  return result
+  if (reachable) {
+    assert(resultIndex <= n)
+  }
+  else {
+    assert(resultIndex == n)
+  }
+  return if (resultIndex == n) result else result.take(resultIndex).toIntArray()
 }
 
 private inline fun <T> Iterator<T>.firstOrNull(predicate: (T) -> Boolean): T? {
diff --git a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DFAEngine.java b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DFAEngine.java
index 9d248e9bf8a3..45f0b73913ca 100644
--- a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DFAEngine.java
+++ b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DFAEngine.java
@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2017 JetBrains s.r.o.
- *
- * 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.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package org.jetbrains.plugins.groovy.lang.psi.dataFlow;
 
 import com.intellij.openapi.progress.ProgressManager;
@@ -84,7 +70,7 @@ public class DFAEngine<E> {
     final List<E> info = new ArrayList<>(Collections.nCopies(n, myDfa.initial()));
     final CallEnvironment env = new MyCallEnvironment(n);
 
-    final WorkList workList = new WorkList(getFlowOrder());
+    final WorkList workList = new WorkList(n, getFlowOrder());
 
     while (!workList.isEmpty()) {
       ProgressManager.checkCanceled();
@@ -108,10 +94,10 @@ public class DFAEngine<E> {
   @NotNull
   private int[] getFlowOrder() {
     if (myDfa.isForward()) {
-      return reversedPostOrder(myFlow);
+      return reversedPostOrder(myFlow, myDfa.isReachable());
     }
     else {
-      return postOrder(myFlow);
+      return postOrder(myFlow, myDfa.isReachable());
     }
   }
 
diff --git a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DfaInstance.java b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DfaInstance.java
index 057439e3bc70..f79357571960 100644
--- a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DfaInstance.java
+++ b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/DfaInstance.java
@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2017 JetBrains s.r.o.
- *
- * 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.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package org.jetbrains.plugins.groovy.lang.psi.dataFlow;
 
 import org.jetbrains.annotations.NotNull;
@@ -31,4 +17,11 @@ public interface DfaInstance<E> {
   default boolean isForward() {
     return true;
   }
+
+  /**
+   * @return {@code true} if this instance expects only instructions reachable from root
+   */
+  default boolean isReachable() {
+    return false;
+  }
 }
diff --git a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/WorkList.kt b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/WorkList.kt
index 4a15aafd0662..68878878871d 100644
--- a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/WorkList.kt
+++ b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/WorkList.kt
@@ -1,23 +1,9 @@
-/*
- * Copyright 2000-2017 JetBrains s.r.o.
- *
- * 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.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package org.jetbrains.plugins.groovy.lang.psi.dataFlow
 
 import java.util.*
 
-internal class WorkList(order: IntArray) {
+internal class WorkList(flowSize: Int, order: IntArray) {
 
   private val mySize: Int = order.size
   /**
@@ -27,7 +13,7 @@ internal class WorkList(order: IntArray) {
   /**
    * Mapping: instruction number -> index in [myOrder] array
    */
-  private val myInstructionToOrder: IntArray = IntArray(mySize)
+  private val myInstructionToOrder: IntArray = IntArray(flowSize)
   /**
    * Mapping: index -> whether instruction number needs to be processed
    *
diff --git a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/readWrite/ReadBeforeWriteInstance.kt b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/readWrite/ReadBeforeWriteInstance.kt
index 58613c4946b1..d4a594d20857 100644
--- a/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/readWrite/ReadBeforeWriteInstance.kt
+++ b/plugins/groovy/groovy-psi/src/org/jetbrains/plugins/groovy/lang/psi/dataFlow/readWrite/ReadBeforeWriteInstance.kt
@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2017 JetBrains s.r.o.
- *
- * 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.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package org.jetbrains.plugins.groovy.lang.psi.dataFlow.readWrite
 
 import gnu.trove.TObjectIntHashMap
@@ -41,4 +27,6 @@ class ReadBeforeWriteInstance(val nameIndex: TObjectIntHashMap<String>, val only
   }
 
   override fun initial(): ReadBeforeWriteState = ReadBeforeWriteState.bottom
+
+  override fun isReachable(): Boolean = true
 }
\ No newline at end of file