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

src/interpreter/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 "src/interpreter/bytecode-generator.h"
 
 #include "src/ast/compile-time-value.h"
 #include "src/ast/scopes.h"
 #include "src/code-stubs.h"
 #include "src/compilation-info.h"
@@ skipping 1813 matching lines @@
       // We need to load the variable into the accumulator, even when in a
       // VisitForRegisterScope, in order to avoid register aliasing if
       // subsequent expressions assign to the same variable.
       builder()->LoadAccumulatorWithRegister(source);
       if (hole_check_mode == HoleCheckMode::kRequired) {
         BuildThrowIfHole(variable->name());
       }
       break;
     }
     case VariableLocation::UNALLOCATED: {
-      builder()->LoadGlobal(variable->name(), feedback_index(slot),
-                            typeof_mode);
+      // The global identifier "undefined" is immutable. Everything
+      // else could be reassigned.
+      if (variable->raw_name()->IsOneByteEqualTo("undefined")) {

[mythria, 2016/12/05 17:48:12]

I took this logic from here:
https://cs.chromium.org/chromium/src/v8/src/ast/ast.cc?sq=package:chromium&rc...

I think it is safe to do this optimization right? or Am I missing something?

[rmcilroy, 2016/12/05 19:08:09]

It's possibly slow to check IsOneByteEqualTo each time here. Could you instead
cache the AstRawString value of  ast_value_factory->undefined_string() in the
constructor and do a straight pointer comparison with that instead here (I think
that should catch all the occurrences that this would).

[mythria, 2016/12/06 10:13:27]

Nice one. Done.

+        builder()->LoadUndefined();
+      } else {
+        builder()->LoadGlobal(variable->name(), feedback_index(slot),
+                              typeof_mode);
+      }
       break;
     }
     case VariableLocation::CONTEXT: {
       int depth = execution_context()->ContextChainDepth(variable->scope());
       ContextScope* context = execution_context()->Previous(depth);
       Register context_reg;
       if (context) {
         context_reg = context->reg();
         depth = 0;
       } else {
@@ skipping 1352 matching lines @@
 }
 
 Runtime::FunctionId BytecodeGenerator::StoreKeyedToSuperRuntimeId() {
   return is_strict(language_mode()) ? Runtime::kStoreKeyedToSuper_Strict
                                     : Runtime::kStoreKeyedToSuper_Sloppy;
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8