Fix a bug in optimizing when the null constant has been DCE.

If it has been DCE, we should create a new one, instead of
using the old one.

Also move the first DCE to a place where it could actually
be useful.

bug:21870788

(cherry picked from commit 18e6873c469b48aaed22148451523479eece98e3)

Change-Id: I3b3ab2dafe8ce5fb60868fd1a6ef0eeefe666e0c

6 files changed, 76 insertions, 2 deletions

diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index c294294..fb150c3 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -287,7 +287,10 @@ void HGraph::InsertConstant(HConstant* constant) {
 }
 
 HNullConstant* HGraph::GetNullConstant() {
-  if (cached_null_constant_ == nullptr) {
+  // For simplicity, don't bother reviving the cached null constant if it is
+  // not null and not in a block. Otherwise, we need to clear the instruction
+  // id and/or any invariants the graph is assuming when adding new instructions.
+  if ((cached_null_constant_ == nullptr) || (cached_null_constant_->GetBlock() == nullptr)) {
     cached_null_constant_ = new (arena_) HNullConstant();
     InsertConstant(cached_null_constant_);
   }
diff --git a/compiler/optimizing/optimizing_compiler.cc b/compiler/optimizing/optimizing_compiler.cc
index 5632434..64d6023 100644
--- a/compiler/optimizing/optimizing_compiler.cc
+++ b/compiler/optimizing/optimizing_compiler.cc
@@ -342,9 +342,9 @@ static void RunOptimizations(HGraph* graph,
 
   HOptimization* optimizations[] = {
     &intrinsics,
-    &dce1,
     &fold1,
     &simplify1,
+    &dce1,
     &inliner,
     // BooleanSimplifier depends on the InstructionSimplifier removing redundant
     // suspend checks to recognize empty blocks.
diff --git a/test/501-null-constant-dce/expected.txt b/test/501-null-constant-dce/expected.txt
new file mode 100644
index 0000000..ccaf6f8
--- /dev/null
+++ b/test/501-null-constant-dce/expected.txt
@@ -0,0 +1 @@
+Enter
diff --git a/test/501-null-constant-dce/info.txt b/test/501-null-constant-dce/info.txt
new file mode 100644
index 0000000..2c4a686
--- /dev/null
+++ b/test/501-null-constant-dce/info.txt
@@ -0,0 +1 @@
+Regression test for the optimizing compiler. See comment in smali file.
diff --git a/test/501-null-constant-dce/smali/DCE.smali b/test/501-null-constant-dce/smali/DCE.smali
new file mode 100644
index 0000000..4a1765e
--- /dev/null
+++ b/test/501-null-constant-dce/smali/DCE.smali
@@ -0,0 +1,37 @@
+# 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 LDCE;
+
+.super Ljava/lang/Object;
+
+.method public static method([I)LDCE;
+   .registers 2
+   const/4 v0, 0
+   # Jump over the code that requires the null constant
+   # so that the compiler sees the null constant as dead code.
+   if-eq v0, v0, :end
+   invoke-static {v0}, LDCE;->method([I)LDCE;
+   :end
+   invoke-static {}, LDCE;->$inline$returnNull()LDCE;
+   move-result-object v0
+   return-object v0
+.end method
+
+.method public static $inline$returnNull()LDCE;
+   .registers 2
+   const/4 v0, 0
+   # Return null to make `method` call GetConstantNull again.
+   return-object v0
+.end method
diff --git a/test/501-null-constant-dce/src/Main.java b/test/501-null-constant-dce/src/Main.java
new file mode 100644
index 0000000..3a2d491
--- /dev/null
+++ b/test/501-null-constant-dce/src/Main.java
@@ -0,0 +1,32 @@
+/*
+ * 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("DCE");
+    Method m = c.getMethod("method", int[].class);
+    int[] array = new int[7];
+    Object[] arguments = { array };
+    Object result = m.invoke(null, arguments);
+    if (result != null) {
+      throw new Error("Expected null, got " + result);
+    }
+  }
+}