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

src/interpreter/mkpeephole.cc

Index: src/interpreter/mkpeephole.cc
diff --git a/src/interpreter/mkpeephole.cc b/src/interpreter/mkpeephole.cc
index 62d3a77e02c7faf513f1d8ff020217b6fd4785dd..81f35f670112ccf002c5c971cdbc14bb590a9dbc 100644
--- a/src/interpreter/mkpeephole.cc
+++ b/src/interpreter/mkpeephole.cc
@@ -192,6 +192,19 @@ PeepholeActionAndData PeepholeActionTableWriter::LookupActionAndData(
     }
   }
 
+  // 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) {