Use the property load IC for accessing the array length.

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 6595 matching lines @@
   ASSERT(current_block() != NULL);
   ASSERT(current_block()->HasPredecessor());
   expr->RecordTypeFeedback(oracle(), zone());
 
   if (TryArgumentsAccess(expr)) return;
 
   CHECK_ALIVE(VisitForValue(expr->obj()));
 
   HInstruction* instr = NULL;
   if (expr->AsProperty()->IsArrayLength()) {
+    // Note that in the monomorphic case IsArrayLength() is false because we
+    // handle that it with a regular property load IC.
     HValue* array = Pop();
     AddInstruction(new(zone()) HCheckNonSmi(array));
     HInstruction* mapcheck =
         AddInstruction(HCheckInstanceType::NewIsJSArray(array, zone()));
     instr = new(zone()) HJSArrayLength(array, mapcheck);
   } else if (expr->IsStringLength()) {
     HValue* string = Pop();
     AddInstruction(new(zone()) HCheckNonSmi(string));
     AddInstruction(HCheckInstanceType::NewIsString(string, zone()));
     instr = new(zone()) HStringLength(string);
@@ skipping 15 matching lines @@
   } else if (expr->key()->IsPropertyName()) {
     Handle<String> name = expr->key()->AsLiteral()->AsPropertyName();
     SmallMapList* types = expr->GetReceiverTypes();
 
     bool monomorphic = expr->IsMonomorphic();
     Handle<Map> map;
     if (expr->IsMonomorphic()) {
       map = types->first();
       if (map->is_dictionary_map()) monomorphic = false;
     }
-    if (monomorphic) {
-      Handle<JSFunction> getter;
-      Handle<JSObject> holder;
-      if (LookupGetter(map, name, &getter, &holder)) {
-        AddCheckConstantFunction(holder, Top(), map);
-        if (FLAG_inline_accessors && TryInlineGetter(getter, expr)) return;
-        AddInstruction(new(zone()) HPushArgument(Pop()));
-        instr = new(zone()) HCallConstantFunction(getter, 1);
+
+    // Try to see if this is an array length access.
+    if (name->Equals(isolate()->heap()->length_symbol())) {
+      bool is_array = false;
+      bool fast_mode = false;
+      bool map_mode = false;
+      HInstruction* mapcheck = NULL;
+
+      if (expr->IsMonomorphic()) {
+        // Even if there is more than one map they all must be element
+        // transition maps, so checking just one is ok.
+        if (map->instance_type() == JS_ARRAY_TYPE) {
+          is_array = true;
+          map_mode = true;
+          fast_mode = IsFastElementsKind(map->elements_kind());
+        }
       } else {
-        instr = BuildLoadNamedMonomorphic(Pop(), name, expr, map);
+        // Since we will emit an instance check we have to conservatively
+        // assume that some arrays will contain slow elements, so we set
+        // fast_mode to false.
+        map_mode = false;
+        fast_mode = false;
+        is_array = true;
+        for (int i = 0; i < types->length(); i++) {
+          Handle<Map> current_map = types->at(i);
+          if (!current_map->instance_type() == JS_ARRAY_TYPE) {
+            is_array = false;
+            break;
+          }
+        }
       }
-    } else if (types != NULL && types->length() > 1) {
-      return HandlePolymorphicLoadNamedField(expr, Pop(), types, name);
-    } else {
-      instr = BuildLoadNamedGeneric(Pop(), name, expr);
+
+      // It is an array length access so we produce a HJSArrayLength.
+      if (is_array) {
+        HValue* array = Pop();
+        AddInstruction(new(zone()) HCheckNonSmi(array));
+        mapcheck = map_mode ?
+            HInstruction::cast(
+                new(zone()) HCheckMaps(array, map, zone(), NULL,
+                                       ALLOW_ELEMENT_TRANSITION_MAPS)) :
+            HInstruction::cast(
+                HCheckInstanceType::NewIsJSArray(array, zone()));
+        AddInstruction(mapcheck);
+        instr = new(zone()) HJSArrayLength(
+            array, mapcheck, fast_mode ? HType::Smi() : HType::Tagged());
+      }
     }
 
+    // We cannot know if it was an array length access so we handle it as
+    // a regular property access.
+    if (instr == NULL) {
+      if (monomorphic) {
+        Handle<JSFunction> getter;
+        Handle<JSObject> holder;
+        if (LookupGetter(map, name, &getter, &holder)) {
+          AddCheckConstantFunction(holder, Top(), map);
+          if (FLAG_inline_accessors && TryInlineGetter(getter, expr)) return;
+          AddInstruction(new(zone()) HPushArgument(Pop()));
+          instr = new(zone()) HCallConstantFunction(getter, 1);
+        } else {
+          instr = BuildLoadNamedMonomorphic(Pop(), name, expr, map);
+        }
+      } else if (types != NULL && types->length() > 1) {
+        return HandlePolymorphicLoadNamedField(expr, Pop(), types, name);
+      } else {
+        instr = BuildLoadNamedGeneric(Pop(), name, expr);
+      }
+    }
   } else {
     CHECK_ALIVE(VisitForValue(expr->key()));
 
     HValue* key = Pop();
     HValue* obj = Pop();
 
     bool has_side_effects = false;
     HValue* load = HandleKeyedElementAccess(
         obj, key, NULL, expr, expr->id(), expr->position(),
         false,  // is_store
@@ skipping 3333 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal