[CostModel] Templatize EntryCost::Cost to allow custom cost metrics

[RKSimon]

Summary

We currently use an unsigned value for our CostTblEntry and TypeConversionCostTblEntry cost tables which is limiting depending on how the target wishes to handle various CostKinds etc.

For instance, targets might wish to store separate instruction count, latency or throughput values etc. On D46276 we have been investigating storing a code snippet to improve latency/throughput cost calculations.

There is a slight problem in that template argument deduction was struggling to match the now templatized Costs[] tables in a ArrayRef constructor - I've added helper wrappers for CostTableLookup/ConvertCostTableLookup which avoids us having to update all existing calls with a template hint.

[RKSimon]

llvm/include/llvm/CodeGen/CostTable.h

24–25

oops - I forgot to update all of these to CostType to avoid confusion with TargetCostKind - will fix shortly

[RKSimon]

Consistently using CostType for template param

[dmgreen]

Sounds useful. My C++ templates are not super strong (for someone who works on compilers) but could it use:

template <typename CostType = unsigned> struct CostTblEntry

It looks like that might be a C++17 thing, to make it clean though.

llvm/include/llvm/CodeGen/CostTable.h

32–36

CostTblEntry -> CostTblEntryT<CostType>?

47

CostTblEntry -> CostTblEntryT<CostType>?

[ABataev]

llvm/include/llvm/CodeGen/CostTable.h

48–53

Drop llvm::?

52

makeArrayRef(Table) instead of llvm::ArrayRef<llvm::CostTblEntryT<CostType>>(Table)?

79–88

Drop llvm::?

85

makeArrayRef(Table)

[RKSimon]

Thanks for the reviews - tidied up the code as suggested (it'd taken so long to get the template code to work I'd rather over complicated it all.....)

[ABataev]

LGTM with a nit, unless there are no others comments

llvm/include/llvm/CodeGen/CostTable.h

81

llvm::?

[dmgreen]

Yeah, I'm happy with this.

[RKSimon]

llvm/include/llvm/CodeGen/CostTable.h

79–88

Sorry - I missed that one!

llvm/include/llvm/CodeGen/CostTable.h

 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/MachineValueType.h"
 
 namespace llvm {
 
 /// Cost Table Entry
-struct CostTblEntry {
+template <typename CostType>
+struct CostTblEntryT {
   int ISD;
   MVT::SimpleValueType Type;
-  unsigned Cost;
+  CostType Cost;
 };
+using CostTblEntry = CostTblEntryT<unsigned>;
 
-/// Find in cost table, TypeTy must be comparable to CompareTy by ==
-inline const CostTblEntry *CostTableLookup(ArrayRef<CostTblEntry> Tbl,
-                                           int ISD, MVT Ty) {
-  auto I = find_if(Tbl, [=](const CostTblEntry &Entry) {
+/// Find in cost table.
+template <class CostType>
+inline const CostTblEntryT<CostType> *
+CostTableLookup(ArrayRef<CostTblEntryT<CostType>> Tbl, int ISD, MVT Ty) {
+  auto I = find_if(Tbl, [=](const CostTblEntryT<CostType> &Entry) {
     return ISD == Entry.ISD && Ty == Entry.Type;
   });
   if (I != Tbl.end())
     return I;
 
   // Could not find an entry.
   return nullptr;
 }
 
+template <size_t N, class CostType>
+inline const CostTblEntryT<CostType> *
+CostTableLookup(const CostTblEntryT<CostType> (&Table)[N], int ISD, MVT Ty) {
+  // Wrapper to fix template argument deduction failures.
+  return CostTableLookup<CostType>(makeArrayRef(Table), ISD, Ty);
+}
+
 /// Type Conversion Cost Table
-struct TypeConversionCostTblEntry {
+template <typename CostType>
+struct TypeConversionCostTblEntryT {
   int ISD;
   MVT::SimpleValueType Dst;
   MVT::SimpleValueType Src;
-  unsigned Cost;
+  CostType Cost;
 };
+using TypeConversionCostTblEntry = TypeConversionCostTblEntryT<unsigned>;
 
-/// Find in type conversion cost table, TypeTy must be comparable to CompareTy
-/// by ==
-inline const TypeConversionCostTblEntry *
-ConvertCostTableLookup(ArrayRef<TypeConversionCostTblEntry> Tbl,
+/// Find in type conversion cost table.
+template <class CostType>
+inline const TypeConversionCostTblEntryT<CostType> *
+ConvertCostTableLookup(ArrayRef<TypeConversionCostTblEntryT<CostType>> Tbl,
                        int ISD, MVT Dst, MVT Src) {
-  auto I = find_if(Tbl, [=](const TypeConversionCostTblEntry &Entry) {
+  auto I =
+      find_if(Tbl, [=](const TypeConversionCostTblEntryT<CostType> &Entry) {
         return ISD == Entry.ISD && Src == Entry.Src && Dst == Entry.Dst;
       });
   if (I != Tbl.end())
     return I;
 
   // Could not find an entry.
   return nullptr;
 }
 
+template <size_t N, class CostType>
+inline const TypeConversionCostTblEntryT<CostType> *
+ConvertCostTableLookup(const TypeConversionCostTblEntryT<CostType> (&Table)[N],
+                       int ISD, MVT Dst, MVT Src) {
+  // Wrapper to fix template argument deduction failures.
+  return ConvertCostTableLookup<CostType>(makeArrayRef(Table), ISD, Dst, Src);
+}
+
 } // namespace llvm
 
 #endif /* LLVM_CODEGEN_COSTTABLE_H_ */