Enable partial and runtime loop unrolling for NVPTX

[meheff]

Summary

Enable partial and runtime loop unrolling for NVPTX backend via
TTI::UnrollingPreferences with a small threshold. This partially unrolls
small loops which are often unrolled by the PTX to SASS compiler
and unrolling earlier can be beneficial.

[jholewinski]

Looks reasonable to me

[Eugene.Zelenko]

Committed in r242049.

lib/Target/NVPTX/NVPTXTargetTransformInfo.h

public:
   bool isSourceOfDivergence(const Value *V);
 
   unsigned getArithmeticInstrCost(
       unsigned Opcode, Type *Ty,
       TTI::OperandValueKind Opd1Info = TTI::OK_AnyValue,
       TTI::OperandValueKind Opd2Info = TTI::OK_AnyValue,
       TTI::OperandValueProperties Opd1PropInfo = TTI::OP_None,
       TTI::OperandValueProperties Opd2PropInfo = TTI::OP_None);
+
+  void getUnrollingPreferences(Loop *L, TTI::UnrollingPreferences &UP);
 };
 
 } // end namespace llvm
 
 #endif

lib/Target/NVPTX/NVPTXTargetTransformInfo.cpp

case ISD::AND:
     // those on types that can fit into one machine register.
     if (LT.second.SimpleTy == MVT::i64)
       return 2 * LT.first;
     // Delegate other cases to the basic TTI.
     return BaseT::getArithmeticInstrCost(Opcode, Ty, Opd1Info, Opd2Info,
                                          Opd1PropInfo, Opd2PropInfo);
   }
 }
+
+void NVPTXTTIImpl::getUnrollingPreferences(Loop *L,
+                                           TTI::UnrollingPreferences &UP) {
+  BaseT::getUnrollingPreferences(L, UP);
+
+  // Enable partial unrolling and runtime unrolling, but reduce the
+  // threshold.  This partially unrolls small loops which are often
+  // unrolled by the PTX to SASS compiler and unrolling earlier can be
+  // beneficial.
+  UP.Partial = UP.Runtime = true;
+  UP.PartialThreshold = UP.Threshold / 4;
+}