[Interpreter] Optimize code of the form 'if (x === undefined)'.

test/cctest/interpreter/test-bytecode-generator.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <fstream>
 
 #include "src/v8.h"
 
 #include "src/interpreter/bytecode-array-iterator.h"
 #include "src/interpreter/bytecode-generator.h"
@@ skipping 1009 matching lines @@
 
       "return 'string' === typeof(undefined);\n",
 
       "return 'unknown' === typeof(undefined);\n",
   };
 
   CHECK(CompareTexts(BuildActual(printer, snippets),
                      LoadGolden("CompareTypeOf.golden")));
 }
 
+TEST(CompareNil) {
+  InitializedIgnitionHandleScope scope;
+  BytecodeExpectationsPrinter printer(CcTest::isolate());
+
+  const char* snippets[] = {
+      "var a = 1;\n"
+      "return a === null;\n",
+
+      "var a = undefined;\n"
+      "return undefined === a;\n",
+
+      "var a = undefined;\n"
+      "return undefined !== a;\n",
+
+      "var a = 2;\n"
+      "return a != null;\n",
+
+      "var a = undefined;\n"
+      "return undefined == a;\n",
+
+      "var a = undefined;\n"
+      "return undefined === a ? 1 : 2;\n",
+
+      "var a = 0;\n"
+      "return null == a ? 1 : 2;\n",
+
+      "var a = 0;\n"
+      "return undefined !== a ? 1 : 2;\n",
+
+      "var a = 0;\n"
+      "return a === null ? 1 : 2;\n",
+
+      "var a = 0;\n"
+      "if (a === null) {\n"
+      "  return 1;\n"
+      "} else {\n"
+      "  return 2;\n"
+      "}\n",
+
+      "var a = 0;\n"
+      "if (a != undefined) {\n"
+      "  return 1;\n"
+      "}\n",
+
+      "var a = undefined;\n"
+      "var b = 0;\n"
+      "while (a !== undefined) {\n"
+      "  b++;\n"
+      "}\n",
+  };
+
+  CHECK(CompareTexts(BuildActual(printer, snippets),
+                     LoadGolden("CompareNil.golden")));
+}
+
 TEST(Delete) {
   InitializedIgnitionHandleScope scope;
   BytecodeExpectationsPrinter printer(CcTest::isolate());
 
   const char* snippets[] = {
       "var a = {x:13, y:14}; return delete a.x;\n",
 
       "'use strict'; var a = {x:13, y:14}; return delete a.x;\n",
 
       "var a = {1:13, 2:14}; return delete a[2];\n",
@@ skipping 1386 matching lines @@
 
   CHECK(CompareTexts(BuildActual(printer, snippets),
                      LoadGolden("ForAwaitOf.golden")));
 
   i::FLAG_harmony_async_iteration = old_flag;
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8