[interpreter] Update ForInPrepare to conditionally use runtime.

src/interpreter/interpreter-assembler.cc

Index: src/interpreter/interpreter-assembler.cc
diff --git a/src/interpreter/interpreter-assembler.cc b/src/interpreter/interpreter-assembler.cc
index 8f5d941e5a2e6632fd93c8a0965fc5999375e629..00cfd165001281e71f640969f7a7e0df963fbf71 100644
--- a/src/interpreter/interpreter-assembler.cc
+++ b/src/interpreter/interpreter-assembler.cc
@@ -630,6 +630,76 @@ Node* InterpreterAssembler::CallRuntimeN(Node* function_id, Node* context,
                   first_arg, function_entry, result_size);
 }
 
+Node* InterpreterAssembler::EnumLength(Node* map) {
+  Node* bitfield_3 = LoadMapBitField3(map);
+  Node* enum_length = BitFieldDecode<Map::EnumLengthBits>(bitfield_3);
+  return SmiTag(enum_length);
+}
+
+void InterpreterAssembler::CheckEnumCache(Node* receiver, Label* use_cache,

[rmcilroy, 2016/07/19 11:08:54]

Could we move these both to CodeStubAssembler - they aren't specific to the
interpreter, and might be useful elsewhere.

(random thought - perhaps we want to make this a ForInPrepareStub which is
called by TF in the generic lowering, and inlined by the interpreter -  just a
thought, we don't need to do it here).

[oth, 2016/07/19 12:59:25]

Done.

+                                          Label* use_runtime) {
+  Variable current_js_object(this, MachineRepresentation::kTagged);
+  current_js_object.Bind(receiver);
+
+  Variable current_map(this, MachineRepresentation::kTagged);
+  current_map.Bind(LoadMap(current_js_object.value()));
+
+  // These variables are updated in the loop below.
+  Variable* loop_vars[2] = {&current_js_object, &current_map};
+  Label start(this, 2, loop_vars);

[rmcilroy, 2016/07/19 11:08:54]

nit - start->loop

[oth, 2016/07/19 12:59:25]

Done.

+
+  // Check if the enum length field is properly initialized, indicating that
+  // there is an enum cache.
+  {
+    Node* invalid_enum_cache_sentinel =
+        SmiConstant(Smi::FromInt(kInvalidEnumCacheSentinel));
+    Node* enum_length = EnumLength(current_map.value());
+
+    Node* condition = WordEqual(enum_length, invalid_enum_cache_sentinel);
+    BranchIf(condition, use_runtime, &start);
+  }
+
+  Label next(this);

[rmcilroy, 2016/07/19 11:08:54]

nit - move definition next to start

[oth, 2016/07/19 12:59:25]

Done.

+
+  // Check that there are no elements. prototype contains the current JS

[rmcilroy, 2016/07/19 11:08:54]

/s/prototype/|current_js_object| ?

[oth, 2016/07/19 12:59:25]

Done.

+  // object we've reached through the prototype chain.
+  Bind(&start);
+  {
+    Label if_elements(this), if_no_elements(this);
+    Node* elements = LoadElements(current_js_object.value());
+    Node* empty_fixed_array = LoadRoot(Heap::kEmptyFixedArrayRootIndex);
+    // Check that there are no elements.
+    BranchIf(WordEqual(elements, empty_fixed_array), &if_no_elements,

[rmcilroy, 2016/07/19 11:08:54]

You can do BranchIfWordEqual if you like.

[oth, 2016/07/19 12:59:25]

Done.

+             &if_elements);
+    Bind(&if_elements);
+    {
+      // Second chance, the object may be using the empty slow element
+      // dictionary.
+      Node* slow_empty_dictionary =
+          LoadRoot(Heap::kEmptySlowElementDictionaryRootIndex);
+      BranchIf(WordNotEqual(elements, slow_empty_dictionary), use_runtime,
+               &if_no_elements);
+    }
+
+    Bind(&if_no_elements);
+    {
+      // Update map prototype.
+      current_js_object.Bind(LoadMapPrototype(current_map.value()));
+      BranchIf(WordEqual(current_js_object.value(), NullConstant()), use_cache,
+               &next);
+    }
+  }
+
+  Bind(&next);
+  {
+    // For all objects but the receiver, check that the cache is empty.
+    current_map.Bind(LoadMap(current_js_object.value()));
+    Node* enum_length = EnumLength(current_map.value());
+    Node* zero_constant = SmiConstant(Smi::FromInt(0));
+    BranchIf(WordEqual(enum_length, zero_constant), &start, use_runtime);
+  }
+}
+
 void InterpreterAssembler::UpdateInterruptBudget(Node* weight) {
   Label ok(this), interrupt_check(this, Label::kDeferred), end(this);
   Node* budget_offset =