Add the Optimize Compares pass (disabled by default).

This pass tries to remove comparison instructions when possible. For instance,
if you have this code:

   sub r1, 1
   cmp r1, 0
   bz  L1

and "sub" either sets the same flag as the "cmp" instruction or could be
converted to set the same flag, then we can eliminate the "cmp" instruction all
together. This is a important for ARM where the ALU instructions could set the
CPSR flag, but need a special suffix ('s') to do so.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110423 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -180,6 +180,10 @@ namespace llvm {
   /// to take advantage of opportunities created during DAG legalization.
   FunctionPass *createOptimizePHIsPass();
 
+  /// createOptimizeCmpsPass - This pass performs redundant comparison removal
+  /// optimization.
+  FunctionPass *createOptimizeCmpsPass();
+
   /// createStackSlotColoringPass - This pass performs stack slot coloring.
   FunctionPass *createStackSlotColoringPass(bool);
 

include/llvm/Target/TargetInstrInfo.h

@@ -576,6 +576,21 @@ public:
   /// register allocation.
   virtual ScheduleHazardRecognizer*
   CreateTargetPostRAHazardRecognizer(const InstrItineraryData&) const = 0;
+
+  /// isCompareInstr - If the machine instruction is a comparison instruction,
+  /// then return true. Also return the source register in SrcReg and the value
+  /// it compares against in CmpValue.
+  virtual bool isCompareInstr(const MachineInstr *MI,
+                              unsigned &SrcReg, int &CmpValue) const {
+    return false;
+  }
+
+  /// convertToSetZeroFlag - Convert the instruction to set the zero flag so
+  /// that we can remove a "comparison with zero".
+  virtual bool convertToSetZeroFlag(MachineInstr *Instr,
+                                    MachineInstr *CmpInstr) const {
+    return false;
+  }
 };
 
 /// TargetInstrInfoImpl - This is the default implementation of