Group lexical context variables for faster look up.

src/runtime/runtime-scopes.cc

Index: src/runtime/runtime-scopes.cc
diff --git a/src/runtime/runtime-scopes.cc b/src/runtime/runtime-scopes.cc
index ff7e783393081cbe731a8d5f265a5b0fa852b773..8b1b90ee2bf0ae4b2bdccee2f405a7b24407ca20 100644
--- a/src/runtime/runtime-scopes.cc
+++ b/src/runtime/runtime-scopes.cc
@@ -36,9 +36,10 @@ static Object* DeclareGlobals(Isolate* isolate, Handle<GlobalObject> global,
                               bool is_const, bool is_function) {
   Handle<ScriptContextTable> script_contexts(
       global->native_context()->script_context_table());
-  ScriptContextTable::LookupResult lookup;
-  if (ScriptContextTable::Lookup(script_contexts, name, &lookup) &&
-      IsLexicalVariableMode(lookup.mode)) {
+  // We use LookupLexical to limit lookup to lexical variables. As long as
+  // lexical variables are not used extensively, this is a performance win.
+  // TODO(yangguo): reconsider this shortcut.
+  if (ScriptContextTable::LookupLexical(script_contexts, name)) {
     return ThrowRedeclarationError(isolate, name);
   }
 
@@ -624,9 +625,13 @@ static Object* FindNameClash(Handle<ScopeInfo> scope_info,
   for (int var = 0; var < scope_info->ContextLocalCount(); var++) {
     Handle<String> name(scope_info->ContextLocalName(var));
     VariableMode mode = scope_info->ContextLocalMode(var);
-    ScriptContextTable::LookupResult lookup;
-    if (ScriptContextTable::Lookup(script_context, name, &lookup)) {
-      if (IsLexicalVariableMode(mode) || IsLexicalVariableMode(lookup.mode)) {
+    if (IsLexicalVariableMode(mode)) {
+      ScriptContextTable::LookupResult lookup;
+      if (ScriptContextTable::Lookup(script_context, name, &lookup)) {
+        return ThrowRedeclarationError(isolate, name);
+      }
+    } else {
+      if (ScriptContextTable::LookupLexical(script_context, name)) {
         return ThrowRedeclarationError(isolate, name);
       }
     }