[Conformance cache] Add allocated extended-storage for entries to the free list

This memory is part of the conformance cache concurrent hash map, so
when we clear the conformance cache, record each of the allocated
pointers within the concurrent map's free list. This way, it'll be
freed with the rest of the concurrent map when it's safe to do so.

Author: DougGregor

include/swift/Runtime/Concurrent.h

@@ -750,7 +750,12 @@ struct ConcurrentReadableHashMap {
 
   /// Clear the hash table, freeing (when safe) all memory currently used for
   /// indices and elements.
-  void clear() {
+  ///
+  /// - addFreedNodes: if nonempty, this function will be called with the head
+  /// of the free list. The function may add additional nodes to the free
+  /// list, which will be freed when it is safe to do so.
+  void clear(std::function<void(ConcurrentFreeListNode *&)> addFreedNodes
+                 = nullptr) {
     typename MutexTy::ScopedLock guard(WriterLock);
 
     IndexStorage indices = Indices.load(std::memory_order_relaxed);
@@ -766,6 +771,9 @@ struct ConcurrentReadableHashMap {
       ConcurrentFreeListNode::add(&FreeList, ptr);
     ConcurrentFreeListNode::add(&FreeList, elements);
 
+    if (addFreedNodes)
+      addFreedNodes(FreeList);
+
     deallocateFreeListIfSafe();
   }
 };

stdlib/public/runtime/ProtocolConformance.cpp

@@ -536,7 +536,7 @@ namespace {
   };
 
   struct ConformanceCacheEntry {
-  private:
+  public:
     /// Storage used when we have global actor isolation on the conformance.
     struct ExtendedStorage {
       /// The protocol to which the type conforms.
@@ -549,18 +549,22 @@ namespace {
       /// When the conformance is global-actor-isolated, this is the conformance
       /// of globalActorIsolationType to GlobalActor.
       const WitnessTable *globalActorIsolationWitnessTable = nullptr;
+
+      /// The next pointer in the list of extended storage allocations.
+      ExtendedStorage *next = nullptr;
     };
 
     const Metadata *Type;
-    llvm::PointerUnion<const ProtocolDescriptor *, const ExtendedStorage *>
+    llvm::PointerUnion<const ProtocolDescriptor *, ExtendedStorage *>
         ProtoOrStorage;
 
     /// The witness table.
     const WitnessTable *Witness;
 
   public:
     ConformanceCacheEntry(ConformanceCacheKey key,
-                          ConformanceLookupResult result)
+                          ConformanceLookupResult result,
+                          std::atomic<ExtendedStorage *> &storageHead)
       : Type(key.Type), Witness(result.witnessTable)
     {
       if (!result.globalActorIsolationType) {
@@ -576,6 +580,17 @@ namespace {
       };
 
       ProtoOrStorage = storage;
+
+      // Add the storage pointer to the list of extended storage allocations
+      // so that we can free them later.
+      auto head = storageHead.load(std::memory_order_relaxed);
+      while (true) {
+        storage->next = head;
+        if (storageHead.compare_exchange_weak(
+                head, storage, std::memory_order_release,
+                std::memory_order_relaxed))
+          break;
+      };
     }
 
     bool matchesKey(const ConformanceCacheKey &key) const {
@@ -591,7 +606,7 @@ namespace {
       if (auto proto = ProtoOrStorage.dyn_cast<const ProtocolDescriptor *>())
         return proto;
 
-      if (auto storage = ProtoOrStorage.dyn_cast<const ExtendedStorage *>())
+      if (auto storage = ProtoOrStorage.dyn_cast<ExtendedStorage *>())
         return storage->Proto;
 
       return nullptr;
@@ -611,7 +626,7 @@ namespace {
       if (ProtoOrStorage.is<const ProtocolDescriptor *>())
         return ConformanceLookupResult { Witness, nullptr, nullptr };
 
-      if (auto storage = ProtoOrStorage.dyn_cast<const ExtendedStorage *>()) {
+      if (auto storage = ProtoOrStorage.dyn_cast<ExtendedStorage *>()) {
         return ConformanceLookupResult(
             Witness, storage->globalActorIsolationType,
             storage->globalActorIsolationWitnessTable);
@@ -626,6 +641,11 @@ namespace {
 struct ConformanceState {
   ConcurrentReadableHashMap<ConformanceCacheEntry> Cache;
   ConcurrentReadableArray<ConformanceSection> SectionsToScan;
+
+  /// The head of an intrusive linked list that keeps track of all of the
+  /// conformance cache entries that require extended storage.
+  std::atomic<ConformanceCacheEntry::ExtendedStorage *> ExtendedStorageHead{nullptr};
+
   bool scanSectionsBackwards;
 
 #if USE_DYLD_SHARED_CACHE_CONFORMANCE_TABLES
@@ -714,7 +734,8 @@ struct ConformanceState {
                           return false; // abandon the new entry
 
                         ::new (entry) ConformanceCacheEntry(
-                            ConformanceCacheKey(type, proto), result);
+                            ConformanceCacheKey(type, proto), result,
+                            ExtendedStorageHead);
                         return true; // keep the new entry
                       });
   }
@@ -748,7 +769,20 @@ static void _registerProtocolConformances(ConformanceState &C,
 
   // Blow away the conformances cache to get rid of any negative entries that
   // may now be obsolete.
-  C.Cache.clear();
+  C.Cache.clear([&](ConcurrentFreeListNode *&freeListHead) {
+    // The extended storage for conformance entries will need to be freed
+    // eventually. Put it on the concurrent free list so the cache will do so.
+    auto storageHead = C.ExtendedStorageHead.load(std::memory_order_relaxed);
+    while (storageHead) {
+      auto current = storageHead;
+      auto newHead = current->next;
+      if (C.ExtendedStorageHead.compare_exchange_weak(
+              storageHead, newHead, std::memory_order_release,
+              std::memory_order_relaxed)) {
+        ConcurrentFreeListNode::add(&freeListHead, current);
+      }
+    }
+  });
 }
 
 void swift::addImageProtocolConformanceBlockCallbackUnsafe(