Subzero: Reduce copying of Liveness bitvectors.

src/IceLiveness.h

 //===- subzero/src/IceLiveness.h - Liveness analysis ------------*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 67 matching lines @@
   LivenessBV &getLiveIn(const CfgNode *Node) {
     SizeT Index = Node->getIndex();
     resize(Index);
     return Nodes[Index].LiveIn;
   }
   LivenessBV &getLiveOut(const CfgNode *Node) {
     SizeT Index = Node->getIndex();
     resize(Index);
     return Nodes[Index].LiveOut;
   }
+  LivenessBV &getScratchBV() { return ScratchBV; }
   LiveBeginEndMap *getLiveBegin(const CfgNode *Node) {
     SizeT Index = Node->getIndex();
     resize(Index);
     return &Nodes[Index].LiveBegin;
   }
   LiveBeginEndMap *getLiveEnd(const CfgNode *Node) {
     SizeT Index = Node->getIndex();
     resize(Index);
     return &Nodes[Index].LiveEnd;
   }
@@ skipping 14 matching lines @@
   CfgVector<LivenessNode> Nodes;
   /// VarToLiveMap maps a Variable's Variable::Number to its live index within
   /// its basic block.
   CfgVector<SizeT> VarToLiveMap;
   /// LiveToVarMap is analogous to LivenessNode::LiveToVarMap, but for non-local
   /// variables.
   CfgVector<Variable *> LiveToVarMap;
   /// RangeMask[Variable::Number] indicates whether we want to track that
   /// Variable's live range.
   LivenessBV RangeMask;
+  /// ScratchBV is a bitvector that can be reused across CfgNode passes, to
+  /// avoid having to allocate/deallocate memory so frequently.
+  LivenessBV ScratchBV;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICELIVENESS_H