[ADT] Modernize SparseMultiSet to use llvm::identity_cxx20 (NFC) (llvm#164361)

The legacy llvm::identity is not quite the same as std::identity from
C++20.  llvm::identity is a template struct with an ::argument_type
member.  In contrast, llvm::identity_cxx20 (and std::identity) is a
non-template struct with a templated call operator and no
::argument_type.

This patch modernizes llvm::SparseMultiSet by updating its default
key-extraction functor to llvm::identity_cxx20.  A new template
parameter KeyT takes over the role of ::argument_type.

Existing uses of SparseMultiSet are updated for the new template
signature.

Author: kazutakahirata

llvm/include/llvm/ADT/SparseMultiSet.h

@@ -21,9 +21,9 @@
 #ifndef LLVM_ADT_SPARSEMULTISET_H
 #define LLVM_ADT_SPARSEMULTISET_H
 
+#include "llvm/ADT/STLForwardCompat.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SparseSet.h"
-#include "llvm/ADT/identity.h"
 #include <cassert>
 #include <cstdint>
 #include <cstdlib>
@@ -77,11 +77,12 @@ namespace llvm {
 /// intuitive and fast removal.
 ///
 /// @tparam ValueT      The type of objects in the set.
+/// @tparam KeyT        The type of the key that identifies objects in the set.
 /// @tparam KeyFunctorT A functor that computes an unsigned index from KeyT.
 /// @tparam SparseT     An unsigned integer type. See above.
 ///
-template <typename ValueT, typename KeyFunctorT = identity<unsigned>,
-          typename SparseT = uint8_t>
+template <typename ValueT, typename KeyT = unsigned,
+          typename KeyFunctorT = identity_cxx20, typename SparseT = uint8_t>
 class SparseMultiSet {
   static_assert(std::is_unsigned_v<SparseT>,
                 "SparseT must be an unsigned integer type");
@@ -112,7 +113,6 @@ class SparseMultiSet {
     bool isValid() const { return Prev != INVALID; }
   };
 
-  using KeyT = typename KeyFunctorT::argument_type;
   using DenseT = SmallVector<SMSNode, 8>;
   DenseT Dense;
   SparseT *Sparse = nullptr;

llvm/include/llvm/CodeGen/ScheduleDAGInstrs.h

@@ -90,15 +90,16 @@ namespace llvm {
   /// allocated once for the pass. It can be cleared in constant time and reused
   /// without any frees.
   using RegUnit2SUnitsMap =
-      SparseMultiSet<PhysRegSUOper, identity<unsigned>, uint16_t>;
+      SparseMultiSet<PhysRegSUOper, unsigned, identity_cxx20, uint16_t>;
 
   /// Track local uses of virtual registers. These uses are gathered by the DAG
   /// builder and may be consulted by the scheduler to avoid iterating an entire
   /// vreg use list.
-  using VReg2SUnitMultiMap = SparseMultiSet<VReg2SUnit, VirtReg2IndexFunctor>;
+  using VReg2SUnitMultiMap =
+      SparseMultiSet<VReg2SUnit, Register, VirtReg2IndexFunctor>;
 
   using VReg2SUnitOperIdxMultiMap =
-      SparseMultiSet<VReg2SUnitOperIdx, VirtReg2IndexFunctor>;
+      SparseMultiSet<VReg2SUnitOperIdx, Register, VirtReg2IndexFunctor>;
 
   using ValueType = PointerUnion<const Value *, const PseudoSourceValue *>;