Initial support for length guards on final fields.

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
diff --git a/runtime/vm/code_generator.cc b/runtime/vm/code_generator.cc
index 1bfee23396d3931222eebda5d9f656a888fd7298..a5a597f39afe165340ba84cfffeb10ae51696528 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -1853,6 +1853,31 @@ double DartModulo(double left, double right) {
 }
 
 
+static intptr_t GetListLength(const Object& value) {
+  const intptr_t cid = value.GetClassId();
+  ASSERT(RawObject::IsBuiltinListClassId(cid));
+  // Extract list length.
+  if (value.IsTypedData()) {
+    const TypedData& list = TypedData::Cast(value);
+    return list.Length();
+  } else if (value.IsArray()) {
+    const Array& list = Array::Cast(value);
+    return list.Length();
+  } else if (value.IsGrowableObjectArray()) {
+    // List length is variable.
+    return Field::kNoFixedLength;
+  } else if (value.IsExternalTypedData()) {
+    // TODO(johnmccutchan): Enable for external typed data.
+    return Field::kNoFixedLength;
+  } else if (RawObject::IsTypedDataViewClassId(cid)) {
+    // TODO(johnmccutchan): Enable for typed data views.
+    return Field::kNoFixedLength;
+  }
+  UNIMPLEMENTED();
+  return Field::kNoFixedLength;
+}
+
+
 // Update global type feedback recorded for a field recording the assignment
 // of the given value.
 //   Arg0: Field object;
@@ -1861,8 +1886,14 @@ DEFINE_RUNTIME_ENTRY(UpdateFieldCid, 2) {
   ASSERT(arguments.ArgCount() == kUpdateFieldCidRuntimeEntry.argument_count());
   const Field& field = Field::CheckedHandle(arguments.ArgAt(0));
   const Object& value = Object::Handle(arguments.ArgAt(1));
-
-  field.UpdateCid(value.GetClassId());
+  const intptr_t cid = value.GetClassId();
+  field.UpdateCid(cid);
+  intptr_t list_length = Field::kNoFixedLength;
+  if ((field.guarded_cid() != kDynamicCid) &&
+      field.is_final() && RawObject::IsBuiltinListClassId(cid)) {
+    list_length = GetListLength(value);
+  }
+  field.UpdateLength(list_length);
 }
 
 }  // namespace dart