Subzero: Improve performance by using llvm::SmallVector<>.

src/IceOperand.h

 //===- subzero/src/IceOperand.h - High-level operands -----------*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares the Operand class and its target-independent
@@ skipping 344 matching lines @@
   void untrim() { TrimmedBegin = Range.begin(); }
   void trim(InstNumberT Lower);
 
   RegWeight getWeight() const { return Weight; }
   void setWeight(const RegWeight &NewWeight) { Weight = NewWeight; }
   void addWeight(uint32_t Delta) { Weight.addWeight(Delta); }
   void dump(Ostream &Str) const;
 
 private:
   typedef std::pair<InstNumberT, InstNumberT> RangeElementType;
-  typedef std::vector<RangeElementType> RangeType;
+  // Assume a common case of 2 or fewer segments per live range.
+  typedef llvm::SmallVector<RangeElementType, 2> RangeType;
   RangeType Range;
   RegWeight Weight;
   // TrimmedBegin is an optimization for the overlaps() computation.
   // Since the linear-scan algorithm always calls it as overlaps(Cur)
   // and Cur advances monotonically according to live range start, we
   // can optimize overlaps() by ignoring all segments that end before
   // the start of Cur's range.  The linear-scan code enables this by
   // calling trim() on the ranges of interest as Cur advances.  Note
   // that linear-scan also has to initialize TrimmedBegin at the
   // beginning by calling untrim().
@@ skipping 256 matching lines @@
 private:
   const Cfg *Func;
   MetadataKind Kind;
   std::vector<VariableTracking> Metadata;
   const static InstDefList NoDefinitions;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEOPERAND_H