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

2 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index f80e1e8..3ac6d2a 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/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);
diff --git a/llvm/lib/CodeGen/RegAllocFast.cpp b/llvm/lib/CodeGen/RegAllocFast.cpp
index 804480c..72b364c 100644
--- a/llvm/lib/CodeGen/RegAllocFast.cpp
+++ b/llvm/lib/CodeGen/RegAllocFast.cpp
@@ -211,7 +211,7 @@ private:
     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;