Fix bug in flow-graph loop detection.

runtime/vm/flow_graph.h

 // 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.
 
 #ifndef VM_FLOW_GRAPH_H_
 #define VM_FLOW_GRAPH_H_
 
 #include "vm/growable_array.h"
 #include "vm/intermediate_language.h"
 #include "vm/parser.h"
@@ skipping 229 matching lines @@
       const GrowableArray<BitVector*>& dom_frontier);
 
   void RemoveDeadPhis(GrowableArray<PhiInstr*>* live_phis);
 
   void ReplacePredecessor(BlockEntryInstr* old_block,
                           BlockEntryInstr* new_block);
 
   // Find the natural loop for the back edge m->n and attach loop
   // information to block n (loop header). The algorithm is described in
   // "Advanced Compiler Design & Implementation" (Muchnick) p192.
-  void FindLoop(BlockEntryInstr* m, BlockEntryInstr* n);
+  // Returns a BitVector indexed by block pre-order number where each bit
+  // indicates membership in the loop.
+  BitVector* FindLoop(BlockEntryInstr* m, BlockEntryInstr* n);
 
   // Finds natural loops in the flow graph and attaches a list of loop
   // body blocks for each loop header.
   ZoneGrowableArray<BlockEntryInstr*>* ComputeLoops();
 
   // DiscoverBlocks computes parent_ and assigned_vars_ which are then used
   // if/when computing SSA.
   GrowableArray<intptr_t> parent_;
   GrowableArray<BitVector*> assigned_vars_;
 
@@ skipping 120 matching lines @@
   // Per block sets of available blocks. Block A is available at the block B if
   // and only if A dominates B and all paths from A to B are free of side
   // effects.
   GrowableArray<BitVector*> available_at_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_H_