[Interpreter] Optimize equality check with null/undefined with a check on the map.

src/interpreter/mkpeephole.cc

 // Copyright 2016 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 <array>
 #include <fstream>
 #include <map>
 #include <string>
 #include <vector>
 
@@ skipping 174 matching lines @@
             Bytecode::kShiftLeftSmi};
       case Bytecode::kShiftRight:
         return {
             PeepholeAction::kTransformLdaZeroBinaryOpToBinaryOpWithZeroAction,
             Bytecode::kShiftRightSmi};
       default:
         break;
     }
   }
 
+  // Fuse LdaNull/LdaUndefined followed by a equality comparison with test
+  // undetectable. Testing undetectable is a simple check on the map which is
+  // more efficient than the full comparison operation.
+  // Note: StrictEquals cannot use this, they need to compare it with the
+  // Null/undefined map.
+  if (last == Bytecode::kLdaNull || last == Bytecode::kLdaUndefined) {
+    if (current == Bytecode::kTestEqual) {
+      return {PeepholeAction::
+                  kTransformEqualityWithNullOrUndefinedToTestUndetectableAction,
+              Bytecode::kTestUndetectable};

[rmcilroy, 2016/12/05 10:57:19]

Could we also fuse NotEquals too (with another bytecode for
TestNotUndetectable)? Would that be worth it? 

[mythria, 2016/12/05 14:53:11]

Full-codegen does not do this, so I wasn't sure we need this. Should I do it in
a follow-up cl?

+    }
+  }
+
   // If there is no last bytecode to optimize against, store the incoming
   // bytecode or for jumps emit incoming bytecode immediately.
   if (last == Bytecode::kIllegal) {
     if (Bytecodes::IsJump(current)) {
       return {PeepholeAction::kUpdateLastJumpAction, Bytecode::kIllegal};
     } else if (current == Bytecode::kNop) {
       return {PeepholeAction::kUpdateLastIfSourceInfoPresentAction,
               Bytecode::kIllegal};
     } else {
       return {PeepholeAction::kUpdateLastAction, Bytecode::kIllegal};
@@ skipping 145 matching lines @@
 
   std::ofstream ofs(argv[1], std::ofstream::trunc);
   v8::internal::interpreter::PeepholeActionTableWriter writer;
   writer.BuildTable();
   writer.Write(ofs);
   ofs.flush();
   ofs.close();
 
   return 0;
 }