[literals] Remove needless slack tracking for the empty object literal

Bug: v8:6211
Change-Id: If61c91e65abf0201651b894e348a7b342c5d0968
Reviewed-on: https://chromium-review.googlesource.com/654662Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47966}

src/builtins/builtins-constructor-gen.cc

@@ -756,10 +756,13 @@ Node* ConstructorBuiltinsAssembler::EmitCreateEmptyObjectLiteral(
   Node* map = LoadObjectField(object_function,
                               JSFunction::kPrototypeOrInitialMapOffset);
   CSA_ASSERT(this, IsMap(map));
+  // Ensure that slack tracking is disabled for the map.
+  STATIC_ASSERT(Map::kNoSlackTracking == 0);
+  CSA_ASSERT(this,
+             IsClearWord32<Map::ConstructionCounter>(LoadMapBitField3(map)));
   Node* empty_fixed_array = EmptyFixedArrayConstant();
   Node* result =
       AllocateJSObjectFromMap(map, empty_fixed_array, empty_fixed_array);
-  HandleSlackTracking(context, result, map, JSObject::kHeaderSize);
   return result;
 }
 

src/code-stub-assembler.cc

@@ -7327,6 +7327,7 @@ void CodeStubAssembler::HandleSlackTracking(Node* context, Node* object,
   // Perform in-object slack tracking if requested.
   Node* bit_field3 = LoadMapBitField3(initial_map);
   Label end(this), slack_tracking(this), finalize(this, Label::kDeferred);
+  STATIC_ASSERT(Map::kNoSlackTracking == 0);
   GotoIf(IsSetWord32<Map::ConstructionCounter>(bit_field3), &slack_tracking);
 
   // Initialize remaining fields.