Fix a bug in liveness analysis code and add more comments.

runtime/vm/flow_graph_allocator.h

 // Copyright (c) 2012, 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_ALLOCATOR_H_
 #define VM_FLOW_GRAPH_ALLOCATOR_H_
 
 #include "vm/growable_array.h"
 #include "vm/intermediate_language.h"
 
 namespace dart {
 
 class FlowGraphAllocator : public ValueObject {
  public:
   FlowGraphAllocator(const GrowableArray<BlockEntryInstr*>& postorder,
                      intptr_t max_ssa_temp_index);
 
   void ResolveConstraints();
 
+  // Build live-in and live-out sets for each block.
   void AnalyzeLiveness();
 
  private:
-  void ComputeKillAndGenSets();
+  // Compute initial values for live-out, kill and live-in sets.
+  void ComputeInitialSets();
+
+  // Update live-out set for the given block: live-out should contain
+  // all values that are live-in for block's successors.
+  // Returns true if live-out set was changed.
   bool UpdateLiveOut(BlockEntryInstr* instr);
+
+  // Update live-in set for the given block: live-in should contain
+  // all values taht are live-out from the block and are not defined
+  // by this block.
+  // Returns true if live-in set was changed.
   bool UpdateLiveIn(BlockEntryInstr* instr);
+
+  // Perform fix-point iteration updating live-out and live-in sets
+  // for blocks until they stop changing.
   void ComputeLiveInAndLiveOutSets();
+
+  // Print results of liveness analysis.
   void DumpLiveness();
 
+  // Live-out sets for each block.  They contain indices of SSA values
+  // that are live out from this block: that is values that were either
+  // defined in this block or live into it and that are used in some
+  // successor block.
   GrowableArray<BitVector*> live_out_;
+
+  // Kill sets for each block.  They contain indices of SSA values that
+  // are defined by this block.
   GrowableArray<BitVector*> kill_;
-  GrowableArray<BitVector*> gen_;
+
+  // Live-in sets for each block.  They contain indices of SSA values
+  // that are used by this block or its successors.
   GrowableArray<BitVector*> live_in_;
+
   const GrowableArray<BlockEntryInstr*>& postorder_;
+
+  // Number of virtual registers.  Currently equal to the number of
+  // SSA values.
   const intptr_t vreg_count_;
 
   DISALLOW_COPY_AND_ASSIGN(FlowGraphAllocator);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_ALLOCATOR_H_