[ADT] Use a dedicated empty type for StringSet (NFC) (#165967)

This patch introduces StringSetTag, a dedicated empty struct to serve
as the "value type" for llvm::StringSet.  This change is part of an
effort to reduce the use of std::nullopt_t outside the context of
std::optional.

Author: kazutakahirata

bolt/lib/Profile/DataAggregator.cpp

@@ -2215,7 +2215,7 @@ DataAggregator::writeAggregatedFile(StringRef OutputFilename) const {
     OutFile << "boltedcollection\n";
   if (opts::BasicAggregation) {
     OutFile << "no_lbr";
-    for (const StringMapEntry<std::nullopt_t> &Entry : EventNames)
+    for (const StringMapEntry<EmptyStringSetTag> &Entry : EventNames)
       OutFile << " " << Entry.getKey();
     OutFile << "\n";
 
@@ -2291,7 +2291,7 @@ std::error_code DataAggregator::writeBATYAML(BinaryContext &BC,
 
   ListSeparator LS(",");
   raw_string_ostream EventNamesOS(BP.Header.EventNames);
-  for (const StringMapEntry<std::nullopt_t> &EventEntry : EventNames)
+  for (const StringMapEntry<EmptyStringSetTag> &EventEntry : EventNames)
     EventNamesOS << LS << EventEntry.first().str();
 
   BP.Header.Flags = opts::BasicAggregation ? BinaryFunction::PF_BASIC

bolt/lib/Profile/YAMLProfileWriter.cpp

@@ -382,7 +382,7 @@ std::error_code YAMLProfileWriter::writeProfile(const RewriteInstance &RI) {
   StringSet<> EventNames = RI.getProfileReader()->getEventNames();
   if (!EventNames.empty()) {
     std::string Sep;
-    for (const StringMapEntry<std::nullopt_t> &EventEntry : EventNames) {
+    for (const StringMapEntry<EmptyStringSetTag> &EventEntry : EventNames) {
       BP.Header.EventNames += Sep + EventEntry.first().str();
       Sep = ",";
     }

llvm/include/llvm/ADT/StringMap.h

@@ -302,7 +302,7 @@ class LLVM_ALLOCATORHOLDER_EMPTYBASE StringMap
       if (FindInRHS == RHS.end())
         return false;
 
-      if constexpr (!std::is_same_v<ValueTy, std::nullopt_t>) {
+      if constexpr (!std::is_same_v<ValueTy, EmptyStringSetTag>) {
         if (!(KeyValue.getValue() == FindInRHS->getValue()))
           return false;
       }

llvm/include/llvm/ADT/StringMapEntry.h

@@ -21,6 +21,9 @@
 
 namespace llvm {
 
+/// The "value type" of StringSet represented as an empty struct.
+struct EmptyStringSetTag {};
+
 /// StringMapEntryBase - Shared base class of StringMapEntry instances.
 class StringMapEntryBase {
   size_t keyLength;
@@ -85,14 +88,13 @@ class StringMapEntryStorage : public StringMapEntryBase {
 };
 
 template <>
-class StringMapEntryStorage<std::nullopt_t> : public StringMapEntryBase {
+class StringMapEntryStorage<EmptyStringSetTag> : public StringMapEntryBase {
 public:
-  explicit StringMapEntryStorage(size_t keyLength,
-                                 std::nullopt_t = std::nullopt)
+  explicit StringMapEntryStorage(size_t keyLength, EmptyStringSetTag = {})
       : StringMapEntryBase(keyLength) {}
   StringMapEntryStorage(StringMapEntryStorage &entry) = delete;
 
-  std::nullopt_t getValue() const { return std::nullopt; }
+  EmptyStringSetTag getValue() const { return {}; }
 };
 
 /// StringMapEntry - This is used to represent one value that is inserted into

llvm/include/llvm/ADT/StringSet.h

@@ -22,8 +22,8 @@ namespace llvm {
 
 /// StringSet - A wrapper for StringMap that provides set-like functionality.
 template <class AllocatorTy = MallocAllocator>
-class StringSet : public StringMap<std::nullopt_t, AllocatorTy> {
-  using Base = StringMap<std::nullopt_t, AllocatorTy>;
+class StringSet : public StringMap<EmptyStringSetTag, AllocatorTy> {
+  using Base = StringMap<EmptyStringSetTag, AllocatorTy>;
 
 public:
   StringSet() = default;

llvm/include/llvm/DWARFLinker/StringPool.h

@@ -20,7 +20,7 @@ namespace dwarf_linker {
 
 /// StringEntry keeps data of the string: the length, external offset
 /// and a string body which is placed right after StringEntry.
-using StringEntry = StringMapEntry<std::nullopt_t>;
+using StringEntry = StringMapEntry<EmptyStringSetTag>;
 
 class StringPoolEntryInfo {
 public:

mlir/include/mlir/Support/Timing.h

@@ -44,7 +44,7 @@ class DefaultTimingManagerImpl;
 /// This is a POD type with pointer size, so it should be passed around by
 /// value. The underlying data is owned by the `TimingManager`.
 class TimingIdentifier {
-  using EntryType = llvm::StringMapEntry<std::nullopt_t>;
+  using EntryType = llvm::StringMapEntry<llvm::EmptyStringSetTag>;
 
 public:
   TimingIdentifier(const TimingIdentifier &) = default;

mlir/lib/Support/Timing.cpp

@@ -50,7 +50,8 @@ class TimingManagerImpl {
   llvm::sys::SmartRWMutex<true> identifierMutex;
 
   /// A thread local cache of identifiers to reduce lock contention.
-  ThreadLocalCache<llvm::StringMap<llvm::StringMapEntry<std::nullopt_t> *>>
+  ThreadLocalCache<
+      llvm::StringMap<llvm::StringMapEntry<llvm::EmptyStringSetTag> *>>
       localIdentifierCache;
 
   TimingManagerImpl() : identifiers(identifierAllocator) {}