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

runtime/vm/flow_graph_allocator.h

Index: runtime/vm/flow_graph_allocator.h
diff --git a/runtime/vm/flow_graph_allocator.h b/runtime/vm/flow_graph_allocator.h
index 8ca9216a0f5953338c683e81e5d6a9eb27673198..7848d2b11822e7b5d6a25d69c61d9430cd34ba02 100644
--- a/runtime/vm/flow_graph_allocator.h
+++ b/runtime/vm/flow_graph_allocator.h
@@ -17,20 +17,49 @@ class FlowGraphAllocator : public ValueObject {
 
   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);