Subzero: Use register availability during lowering to improve the code.

src/IceCfgNode.cpp

 //===- subzero/src/IceCfgNode.cpp - Basic block (node) implementation -----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 552 matching lines @@
   Context.init(this);
   Target->initNodeForLowering(this);
   while (!Context.atEnd()) {
     InstList::iterator Orig = Context.getCur();
     if (llvm::isa<InstRet>(*Orig))
       setHasReturn();
     Target->lower();
     // Ensure target lowering actually moved the cursor.
     assert(Context.getCur() != Orig);
   }
+  Context.availabilityReset();
   // Do preliminary lowering of the Phi instructions.
   Target->prelowerPhis();
 }
 
 void CfgNode::livenessLightweight() {
   SizeT NumVars = Func->getNumVariables();
   LivenessBV Live(NumVars);
   // Process regular instructions in reverse order.
   for (Inst &I : reverse_range(Insts)) {
     if (I.isDeleted())
@@ skipping 93 matching lines @@
   bool LiveInChanged = (Live != LiveIn);
   Changed = (NumNonDeadPhis != PrevNumNonDeadPhis || LiveInChanged);
   if (LiveInChanged)
     LiveIn = Live;
   PrevNumNonDeadPhis = NumNonDeadPhis;
   return Changed;
 }
 
 // Validate the integrity of the live ranges in this block.  If there are any
 // errors, it prints details and returns false.  On success, it returns true.
-bool CfgNode::livenessValidateIntervals(Liveness *Liveness) {
+bool CfgNode::livenessValidateIntervals(Liveness *Liveness) const {
   if (!BuildDefs::asserts())
     return true;
 
   // Verify there are no duplicates.
   auto ComparePair =
       [](const LiveBeginEndMapEntry &A, const LiveBeginEndMapEntry &B) {
         return A.first == B.first;
       };
   LiveBeginEndMap &MapBegin = *Liveness->getLiveBegin(this);
   LiveBeginEndMap &MapEnd = *Liveness->getLiveEnd(this);
@@ skipping 656 matching lines @@
   InstIntrinsicCall *Inst = InstIntrinsicCall::create(
       Func, 5, Func->makeVariable(IceType_i64), RMWI64Name, Info->Info);
   Inst->addArg(AtomicRMWOp);
   Inst->addArg(Counter);
   Inst->addArg(One);
   Inst->addArg(OrderAcquireRelease);
   Insts.push_front(Inst);
 }
 
 } // end of namespace Ice