VM: Correctly intrinsify implicit getters/setters when using field guards

runtime/vm/flow_graph_compiler.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_XXX.
 
 #include "vm/flow_graph_compiler.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
@@ skipping 1078 matching lines @@
       ASSERT(sequence_node.length() == 1);
       ASSERT(sequence_node.NodeAt(0)->IsReturnNode());
       const ReturnNode& return_node = *sequence_node.NodeAt(0)->AsReturnNode();
       ASSERT(return_node.value()->IsLoadInstanceFieldNode());
       const LoadInstanceFieldNode& load_node =
           *return_node.value()->AsLoadInstanceFieldNode();
       // Only intrinsify getter if the field cannot contain a mutable double.
       // Reading from a mutable double box requires allocating a fresh double.
       if (load_node.field().guarded_cid() == kDynamicCid) {
         GenerateInlinedGetter(load_node.field().Offset());
-        return true;
+        return !FLAG_use_field_guards;

[srdjan, 2016/02/12 17:03:50]

It would have been helpful to add comment why it depends on field guards

       }
       return false;
     }
     if (parsed_function().function().kind() == RawFunction::kImplicitSetter) {
       // An implicit setter must have a specific AST structure.
       // Sequence node has one store node and one return NULL node.
       const SequenceNode& sequence_node = *parsed_function().node_sequence();
       ASSERT(sequence_node.length() == 2);
       ASSERT(sequence_node.NodeAt(0)->IsStoreInstanceFieldNode());
       ASSERT(sequence_node.NodeAt(1)->IsReturnNode());
       const StoreInstanceFieldNode& store_node =
           *sequence_node.NodeAt(0)->AsStoreInstanceFieldNode();
       if (store_node.field().guarded_cid() == kDynamicCid) {
         GenerateInlinedSetter(store_node.field().Offset());
-        return true;
+        return !FLAG_use_field_guards;
       }
     }
   }
 
   EnterIntrinsicMode();
 
   Intrinsifier::Intrinsify(parsed_function(), this);
 
   ExitIntrinsicMode();
   // "Deoptimization" from intrinsic continues here. All deoptimization
@@ skipping 747 matching lines @@
 
 
 void FlowGraphCompiler::FrameStateClear() {
   ASSERT(!is_optimizing());
   frame_state_.TruncateTo(0);
 }
 #endif
 
 
 }  // namespace dart