[ADT] Modernize SparseSet to use llvm::identity_cxx20 (NFC) (llvm#164362)

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::SparseSet 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 SparseSet are updated for the new template signature.
Most use sites are of the form SparseSet<T>, requiring no update.

Author: kazutakahirata

llvm/include/llvm/ADT/SparseSet.h

@@ -20,8 +20,8 @@
 #ifndef LLVM_ADT_SPARSESET_H
 #define LLVM_ADT_SPARSESET_H
 
+#include "llvm/ADT/STLForwardCompat.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/identity.h"
 #include "llvm/Support/AllocatorBase.h"
 #include <cassert>
 #include <cstdint>
@@ -112,16 +112,16 @@ struct SparseSetValFunctor<KeyT, KeyT, KeyFunctorT> {
 /// uint16_t or uint32_t.
 ///
 /// @tparam ValueT      The type of objects in the set.
+/// @tparam KeyT        The type of the key, which is passed to the key functor.
 /// @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 SparseSet {
   static_assert(std::is_unsigned_v<SparseT>,
                 "SparseT must be an unsigned integer type");
 
-  using KeyT = typename KeyFunctorT::argument_type;
   using DenseT = SmallVector<ValueT, 8>;
   using size_type = unsigned;
   DenseT Dense;

llvm/include/llvm/CodeGen/LivePhysRegs.h

@@ -51,7 +51,7 @@ class raw_ostream;
 /// when walking backward/forward through a basic block.
 class LivePhysRegs {
   const TargetRegisterInfo *TRI = nullptr;
-  using RegisterSet = SparseSet<MCPhysReg, identity<MCPhysReg>>;
+  using RegisterSet = SparseSet<MCPhysReg, MCPhysReg>;
   RegisterSet LiveRegs;
 
 public:

llvm/include/llvm/CodeGen/RegisterPressure.h

@@ -393,7 +393,7 @@ class RegPressureTracker {
   LiveRegSet LiveRegs;
 
   /// Set of vreg defs that start a live range.
-  SparseSet<Register, VirtReg2IndexFunctor> UntiedDefs;
+  SparseSet<Register, Register, VirtReg2IndexFunctor> UntiedDefs;
   /// Live-through pressure.
   std::vector<unsigned> LiveThruPressure;
 

llvm/lib/CodeGen/IfConversion.cpp

@@ -1498,7 +1498,7 @@ static void UpdatePredRedefs(MachineInstr &MI, LivePhysRegs &Redefs) {
   // Before stepping forward past MI, remember which regs were live
   // before MI. This is needed to set the Undef flag only when reg is
   // dead.
-  SparseSet<MCPhysReg, identity<MCPhysReg>> LiveBeforeMI;
+  SparseSet<MCPhysReg, MCPhysReg> LiveBeforeMI;
   LiveBeforeMI.setUniverse(TRI->getNumRegs());
   for (unsigned Reg : Redefs)
     LiveBeforeMI.insert(Reg);

llvm/lib/CodeGen/RegAllocFast.cpp

@@ -211,7 +211,7 @@ class RegAllocFastImpl {
     unsigned getSparseSetIndex() const { return VirtReg.virtRegIndex(); }
   };
 
-  using LiveRegMap = SparseSet<LiveReg, identity<unsigned>, uint16_t>;
+  using LiveRegMap = SparseSet<LiveReg, unsigned, identity_cxx20, uint16_t>;
   /// This map contains entries for each virtual register that is currently
   /// available in a physical register.
   LiveRegMap LiveVirtRegs;