Subzero: Reduce copying of Liveness bitvectors.

src/IceLiveness.cpp

 //===- subzero/src/IceLiveness.cpp - Liveness analysis implementation -----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 88 matching lines @@
 
   // Fix up RangeMask for variables before FirstVar.
   for (auto I = Func->getVariables().begin(); I != FirstVar; ++I) {
     Variable *Var = *I;
     SizeT VarIndex = Var->getIndex();
     if (Var->getIgnoreLiveness() ||
         (!IsFullInit && !Var->hasReg() && !Var->mustHaveReg()))
       RangeMask[VarIndex] = false;
   }
 
+  SizeT MaxLocals = 0;
   // Process each node.
   for (auto I = FirstNode, E = Func->getNodes().end(); I != E; ++I) {
     LivenessNode &Node = Nodes[(*I)->getIndex()];
     // NumLocals, LiveToVarMap already initialized
     Node.LiveIn.resize(NumGlobals);
     Node.LiveOut.resize(NumGlobals);
     // LiveBegin and LiveEnd are reinitialized before each pass over the block.
+    MaxLocals = std::max(MaxLocals, Node.NumLocals);
   }
+  ScratchBV.reserve(NumGlobals + MaxLocals);
 }
 
 void Liveness::init() {
   constexpr bool IsFullInit = true;
   NodeList::const_iterator FirstNode = Func->getNodes().begin();
   VarList::const_iterator FirstVar = Func->getVariables().begin();
   initInternal(FirstNode, FirstVar, IsFullInit);
 }
 
 void Liveness::initPhiEdgeSplits(NodeList::const_iterator FirstNode,
                                  VarList::const_iterator FirstVar) {
   constexpr bool IsFullInit = false;
   initInternal(FirstNode, FirstVar, IsFullInit);
 }
 
 Variable *Liveness::getVariable(SizeT LiveIndex, const CfgNode *Node) const {
   if (LiveIndex < NumGlobals)
     return LiveToVarMap[LiveIndex];
   SizeT NodeIndex = Node->getIndex();
   return Nodes[NodeIndex].LiveToVarMap[LiveIndex - NumGlobals];
 }
 
 } // end of namespace Ice