Subzero: Slight improvement to phi lowering.

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 82 matching lines @@
 
   // 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.
   }
+
+  // Initialize the bitmask of which variables to track.
+  RangeMask.resize(NumVars);
+  RangeMask.set(0, NumVars);
+  if (!IsFullInit) {
+    // Reset initial variables that are not pre-colored or infinite-weight.
+    for (auto I = Func->getVariables().begin(); I != FirstVar; ++I) {
+      Variable *Var = *I;
+      RangeMask[Var->getIndex()] = (Var->hasReg() || Var->getWeight().isInf());
+    }
+  }
 }
 
 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