Subzero: Add a peephole to fuse cmpxchg w/ later cmp+branch.

src/IceTargetLoweringX8632.h

 //===- subzero/src/IceTargetLoweringX8632.h - x86-32 lowering ---*- 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 TargetLoweringX8632 class, which
@@ skipping 79 matching lines @@
   virtual void lowerLoad(const InstLoad *Inst);
   virtual void lowerPhi(const InstPhi *Inst);
   virtual void lowerRet(const InstRet *Inst);
   virtual void lowerSelect(const InstSelect *Inst);
   virtual void lowerStore(const InstStore *Inst);
   virtual void lowerSwitch(const InstSwitch *Inst);
   virtual void lowerUnreachable(const InstUnreachable *Inst);
   virtual void doAddressOptLoad();
   virtual void doAddressOptStore();
 
+  // Naive lowering of cmpxchg.
   void lowerAtomicCmpxchg(Variable *DestPrev, Operand *Ptr, Operand *Expected,
                           Operand *Desired);
+  // Attempt a more optimized lowering of cmpxchg. Returns true if optimized.
+  bool tryOptimizedCmpxchgCmpBr(Variable *DestPrev, Operand *Ptr,
+                                Operand *Expected, Operand *Desired);
   void lowerAtomicRMW(Variable *Dest, uint32_t Operation, Operand *Ptr,
                       Operand *Val);
   void lowerCountZeros(bool Cttz, Type Ty, Variable *Dest, Operand *FirstVal,
                        Operand *SecondVal);
 
   typedef void (TargetX8632::*LowerBinOp)(Variable *, Operand *);
   void expandAtomicRMWAsCmpxchg(LowerBinOp op_lo, LowerBinOp op_hi,
                                 Variable *Dest, Operand *Ptr, Operand *Val);
 
   void eliminateNextVectorSextInstruction(Variable *SignExtendedResult);
@@ skipping 349 matching lines @@
   virtual ~TargetGlobalInitX8632() {}
 };
 
 template <> void ConstantInteger::emit(GlobalContext *Ctx) const;
 template <> void ConstantFloat::emit(GlobalContext *Ctx) const;
 template <> void ConstantDouble::emit(GlobalContext *Ctx) const;
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX8632_H