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

src/IceTargetLowering.h

 //===- subzero/src/IceTargetLowering.h - Lowering interface -----*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 47 matching lines @@
   InstList::iterator getNext() const { return Next; }
   InstList::iterator getEnd() const { return End; }
   void insert(Inst *Inst);
   Inst *getLastInserted() const;
   void advanceCur() { Cur = Next; }
   void advanceNext() { advanceForward(Next); }
   void setCur(InstList::iterator C) { Cur = C; }
   void setNext(InstList::iterator N) { Next = N; }
   void rewind();
   void setInsertPoint(const InstList::iterator &Position) { Next = Position; }
+  void availabilityReset();
+  void availabilityUpdate();
+  Variable *availabilityGet(Operand *Src) const;
 
 private:
   /// Node is the argument to Inst::updateVars().
   CfgNode *Node = nullptr;
   Inst *LastInserted = nullptr;
   /// Cur points to the current instruction being considered. It is guaranteed
   /// to point to a non-deleted instruction, or to be End.
   InstList::iterator Cur;
   /// Next doubles as a pointer to the next valid instruction (if any), and the
   /// new-instruction insertion point. It is also updated for the caller in case
   /// the lowering consumes more than one high-level instruction. It is
   /// guaranteed to point to a non-deleted instruction after Cur, or to be End.
   // TODO: Consider separating the notion of "next valid instruction" and "new
   // instruction insertion point", to avoid confusion when previously-deleted
   // instructions come between the two points.
   InstList::iterator Next;
   /// Begin is a copy of Insts.begin(), used if iterators are moved backward.
   InstList::iterator Begin;
   /// End is a copy of Insts.end(), used if Next needs to be advanced.
   InstList::iterator End;
+  /// LastDest and LastSrc capture the parameters of the last "Dest=Src" simple
+  /// assignment inserted (provided Src is a variable).  This is used for simple
+  /// availability analysis.
+  Variable *LastDest = nullptr;
+  Variable *LastSrc = nullptr;
 
   void skipDeleted(InstList::iterator &I) const;
   void advanceForward(InstList::iterator &I) const;
 };
 
 /// A helper class to advance the LoweringContext at each loop iteration.
 class PostIncrLoweringContext {
   PostIncrLoweringContext() = delete;
   PostIncrLoweringContext(const PostIncrLoweringContext &) = delete;
   PostIncrLoweringContext &operator=(const PostIncrLoweringContext &) = delete;
@@ skipping 355 matching lines @@
   virtual void lower() {}
 
 protected:
   explicit TargetHeaderLowering(GlobalContext *Ctx) : Ctx(Ctx) {}
   GlobalContext *Ctx;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERING_H