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

src/interpreter/bytecode-generator.cc

Index: src/interpreter/bytecode-generator.cc
diff --git a/src/interpreter/bytecode-generator.cc b/src/interpreter/bytecode-generator.cc
index d611cafd3a3b8bc18281cef25cdff6cd3242fd4b..f406b245652cccaf6f3d63245f6b4bdbb876537a 100644
--- a/src/interpreter/bytecode-generator.cc
+++ b/src/interpreter/bytecode-generator.cc
@@ -1831,8 +1831,14 @@ void BytecodeGenerator::BuildVariableLoad(Variable* variable,
       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: {