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

[kazutakahirata]

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.

[llvmbot]

@llvm/pr-subscribers-mlir
@llvm/pr-subscribers-mlir-core
@llvm/pr-subscribers-bolt

@llvm/pr-subscribers-llvm-adt

Author: Kazu Hirata (kazutakahirata)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/165967.diff

8 Files Affected:

• (modified) bolt/lib/Profile/DataAggregator.cpp (+2-2)
• (modified) bolt/lib/Profile/YAMLProfileWriter.cpp (+1-1)
• (modified) llvm/include/llvm/ADT/StringMap.h (+1-1)
• (modified) llvm/include/llvm/ADT/StringMapEntry.h (+6-4)
• (modified) llvm/include/llvm/ADT/StringSet.h (+2-2)
• (modified) llvm/include/llvm/DWARFLinker/StringPool.h (+1-1)
• (modified) mlir/include/mlir/Support/Timing.h (+1-1)
• (modified) mlir/lib/Support/Timing.cpp (+1-1)

diff --git a/bolt/lib/Profile/DataAggregator.cpp b/bolt/lib/Profile/DataAggregator.cpp
index dc3d918d14bd6..c4d06d53c6659 100644
--- a/bolt/lib/Profile/DataAggregator.cpp
+++ b/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 auto &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 auto &EventEntry : EventNames)
     EventNamesOS << LS << EventEntry.first().str();
 
   BP.Header.Flags = opts::BasicAggregation ? BinaryFunction::PF_BASIC
diff --git a/bolt/lib/Profile/YAMLProfileWriter.cpp b/bolt/lib/Profile/YAMLProfileWriter.cpp
index 1632aa1c6bfe2..5027c0dbb036a 100644
--- a/bolt/lib/Profile/YAMLProfileWriter.cpp
+++ b/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 auto &EventEntry : EventNames) {
       BP.Header.EventNames += Sep + EventEntry.first().str();
       Sep = ",";
     }
diff --git a/llvm/include/llvm/ADT/StringMap.h b/llvm/include/llvm/ADT/StringMap.h
index 01cbf2d3fff71..8e4b78e4488b9 100644
--- a/llvm/include/llvm/ADT/StringMap.h
+++ b/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, StringSetTag>) {
         if (!(KeyValue.getValue() == FindInRHS->getValue()))
           return false;
       }
diff --git a/llvm/include/llvm/ADT/StringMapEntry.h b/llvm/include/llvm/ADT/StringMapEntry.h
index 21be5ec343059..d9bf13aa9bda3 100644
--- a/llvm/include/llvm/ADT/StringMapEntry.h
+++ b/llvm/include/llvm/ADT/StringMapEntry.h
@@ -21,6 +21,9 @@
 
 namespace llvm {
 
+/// The "value type" of StringSet represented as an empty struct.
+struct StringSetTag {};
+
 /// 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<StringSetTag> : public StringMapEntryBase {
 public:
-  explicit StringMapEntryStorage(size_t keyLength,
-                                 std::nullopt_t = std::nullopt)
+  explicit StringMapEntryStorage(size_t keyLength, StringSetTag = {})
       : StringMapEntryBase(keyLength) {}
   StringMapEntryStorage(StringMapEntryStorage &entry) = delete;
 
-  std::nullopt_t getValue() const { return std::nullopt; }
+  StringSetTag getValue() const { return {}; }
 };
 
 /// StringMapEntry - This is used to represent one value that is inserted into
diff --git a/llvm/include/llvm/ADT/StringSet.h b/llvm/include/llvm/ADT/StringSet.h
index c8be3f2a503e4..55de843e1a107 100644
--- a/llvm/include/llvm/ADT/StringSet.h
+++ b/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<StringSetTag, AllocatorTy> {
+  using Base = StringMap<StringSetTag, AllocatorTy>;
 
 public:
   StringSet() = default;
diff --git a/llvm/include/llvm/DWARFLinker/StringPool.h b/llvm/include/llvm/DWARFLinker/StringPool.h
index d0f4e211fac3e..c7afef9cc1bac 100644
--- a/llvm/include/llvm/DWARFLinker/StringPool.h
+++ b/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<StringSetTag>;
 
 class StringPoolEntryInfo {
 public:
diff --git a/mlir/include/mlir/Support/Timing.h b/mlir/include/mlir/Support/Timing.h
index a8a4bfd1c6cf1..540a6fe65299e 100644
--- a/mlir/include/mlir/Support/Timing.h
+++ b/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::StringSetTag>;
 
 public:
   TimingIdentifier(const TimingIdentifier &) = default;
diff --git a/mlir/lib/Support/Timing.cpp b/mlir/lib/Support/Timing.cpp
index 16306d72815f7..c5d8902b14c59 100644
--- a/mlir/lib/Support/Timing.cpp
+++ b/mlir/lib/Support/Timing.cpp
@@ -50,7 +50,7 @@ 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::StringSetTag> *>>
       localIdentifierCache;
 
   TimingManagerImpl() : identifiers(identifierAllocator) {}