[mlir] Optimize ThreadLocalCache by removing atomic bottleneck

The ThreadLocalCache implementation is used by the MLIRContext (among
other things) to try to manage thread contention in the StorageUniquers.
There is a bunch of fancy shared pointer/weak pointer setups that
basically keeps everything alive across threads at the right time, but a
huge bottleneck is the `weak_ptr::lock` call inside the `::get` method.

This is because the `lock` method has to hit the atomic refcount several
times, and this is bottlenecking performance across many threads.
However, all this is doing is checking whether the storage is
initialized. Importantly, when the `PerThreadInstance` goes out of
scope, it does not remove all of its associated entries from the
thread-local hash map (it contains dangling `PerThreadInstance *` keys).
The `weak_ptr` also allows the thread local cache to synchronize with
the `PerThreadInstance`'s destruction:

1. if `ThreadLocalCache` destructs, the `weak_ptr`s that reference its
   contained values are immediately invalidated
2. if `CacheType` destructs within a thread, any entries still live are
   removed from the owning `PerThreadInstance`, and it locks the
   `weak_ptr` first to ensure it's kept alive long enough for the
   removal.

This PR changes the TLC entries to contain a `shared_ptr<ValueT*>` and a
`weak_ptr<PerInstanceState>`. It gives the `PerInstanceState` entries a
`weak_ptr<ValueT*>` on top of the `unique_ptr<ValueT>`. This enables
`ThreadLocalCache::get` to check if the value is initialized by
dereferencing the `shared_ptr<ValueT*>` and check if the contained
pointer is null. When `PerInstanceState` destructs, the values inside
the TLC are written to nullptr. The TLC uses the
`weak_ptr<PerInstanceState>` to satisfy (2).

(1) is no longer the case. When `ThreadLocalCache` begins destruction,
the `weak_ptr<PerInstanceState>` are invalidated, but not the
`shared_ptr<ValueT*>`. This is OK: because the overall object is being
destroyed, `::get` cannot get called and because the
`shared_ptr<PerInstanceState>` finishes destruction before freeing the
pointer, it cannot get reallocated to another `ThreadLocalCache` during
destruction. I.e. the values inside the TLC associated with a
`PerInstanceState` cannot be read during destruction. The most important
thing is to make sure destruction of the TLC doesn't race with the
destructor of `PerInstanceState`. Because `PerInstanceState` carries
`weak_ptr` references into the TLC, we guarantee to not have any
use-after-frees.

mlir/include/mlir/Support/ThreadLocalCache.h

@@ -25,28 +25,80 @@ namespace mlir {
 /// cache has very large lock contention.
 template <typename ValueT>
 class ThreadLocalCache {
+  struct PerInstanceState;
+
+  /// The "observer" is owned by a thread-local cache instance. It is
+  /// constructed the first time a `ThreadLocalCache` instance is accessed by a
+  /// thread, unless `perInstanceState` happens to get re-allocated to the same
+  /// address as a previous one. This class is destructed the thread in which
+  /// the `thread_local` cache lives is destroyed.
+  ///
+  /// This class is called the "observer" because while values cached in
+  /// thread-local caches are owned by `PerInstanceState`, a reference is stored
+  /// via this class in the TLC. With a double pointer, it knows when the
+  /// referenced value has been destroyed.
+  struct Observer {
+    /// This is the double pointer, explicitly allocated because we need to keep
+    /// the address stable if the TLC map re-allocates. It is owned by the
+    /// observer and shared with the value owner.
+    std::shared_ptr<ValueT *> ptr = std::make_shared<ValueT *>(nullptr);
+    /// Because `Owner` living inside `PerInstanceState` contains a reference to
+    /// the double pointer, and livkewise this class contains a reference to the
+    /// value, we need to synchronize destruction of the TLC and the
+    /// `PerInstanceState` to avoid racing. This weak pointer is acquired during
+    /// TLC destruction if the `PerInstanceState` hasn't entered its destructor
+    /// yet, and prevents it from happening.
+    std::weak_ptr<PerInstanceState> keepalive;
+  };
+
+  /// This struct owns the cache entries. It contains a reference back to the
+  /// reference inside the cache so that it can be written to null to indicate
+  /// that the cache entry is invalidated. It needs to do this because
+  /// `perInstanceState` could get re-allocated to the same pointer and we don't
+  /// remove entries from the TLC when it is deallocated. Thus, we have to reset
+  /// the TLC entries to a starting state in case the `ThreadLocalCache` lives
+  /// shorter than the threads.
+  struct Owner {
+    /// Save a pointer to the reference and write it to the newly created entry.
+    Owner(Observer &observer)
+        : value(std::make_unique<ValueT>()), ptrRef(observer.ptr) {
+      *observer.ptr = value.get();
+    }
+    ~Owner() {
+      if (std::shared_ptr<ValueT *> ptr = ptrRef.lock())
+        *ptr = nullptr;
+    }
+
+    Owner(Owner &&) = default;
+    Owner &operator=(Owner &&) = default;
+
+    std::unique_ptr<ValueT> value;
+    std::weak_ptr<ValueT *> ptrRef;
+  };
+
   // Keep a separate shared_ptr protected state that can be acquired atomically
   // instead of using shared_ptr's for each value. This avoids a problem
   // where the instance shared_ptr is locked() successfully, and then the
   // ThreadLocalCache gets destroyed before remove() can be called successfully.
   struct PerInstanceState {
-    /// Remove the given value entry. This is generally called when a thread
-    /// local cache is destructing.
+    /// Remove the given value entry. This is called when a thread local cache
+    /// is destructing but still contains references to values owned by the
+    /// `PerInstanceState`. Removal is required because it prevents writeback to
+    /// a pointer that was deallocated.
     void remove(ValueT *value) {
       // Erase the found value directly, because it is guaranteed to be in the
       // list.
       llvm::sys::SmartScopedLock<true> threadInstanceLock(instanceMutex);
-      auto it =
-          llvm::find_if(instances, [&](std::unique_ptr<ValueT> &instance) {
-            return instance.get() == value;
-          });
+      auto it = llvm::find_if(instances, [&](Owner &instance) {
+        return instance.value.get() == value;
+      });
       assert(it != instances.end() && "expected value to exist in cache");
       instances.erase(it);
     }
 
     /// Owning pointers to all of the values that have been constructed for this
     /// object in the static cache.
-    SmallVector<std::unique_ptr<ValueT>, 1> instances;
+    SmallVector<Owner, 1> instances;
 
     /// A mutex used when a new thread instance has been added to the cache for
     /// this object.
@@ -57,21 +109,22 @@ class ThreadLocalCache {
   /// instance of the non-static cache and a weak reference to an instance of
   /// ValueT. We use a weak reference here so that the object can be destroyed
   /// without needing to lock access to the cache itself.
-  struct CacheType
-      : public llvm::SmallDenseMap<PerInstanceState *, std::weak_ptr<ValueT>> {
+  struct CacheType : public llvm::SmallDenseMap<PerInstanceState *, Observer> {
     ~CacheType() {
-      // Remove the values of this cache that haven't already expired.
-      for (auto &it : *this)
-        if (std::shared_ptr<ValueT> value = it.second.lock())
-          it.first->remove(value.get());
+      // Remove the values of this cache that haven't already expired. This is
+      // required because if we don't remove them, they will contain a reference
+      // back to the data here that is being destroyed.
+      for (auto &[instance, observer] : *this)
+        if (std::shared_ptr<PerInstanceState> state = observer.keepalive.lock())
+          state->remove(*observer.ptr);
     }
 
     /// Clear out any unused entries within the map. This method is not
     /// thread-safe, and should only be called by the same thread as the cache.
     void clearExpiredEntries() {
       for (auto it = this->begin(), e = this->end(); it != e;) {
         auto curIt = it++;
-        if (curIt->second.expired())
+        if (!*curIt->second.ptr)
           this->erase(curIt);
       }
     }
@@ -88,22 +141,23 @@ class ThreadLocalCache {
   ValueT &get() {
     // Check for an already existing instance for this thread.
     CacheType &staticCache = getStaticCache();
-    std::weak_ptr<ValueT> &threadInstance = staticCache[perInstanceState.get()];
-    if (std::shared_ptr<ValueT> value = threadInstance.lock())
+    Observer &threadInstance = staticCache[perInstanceState.get()];
+    if (ValueT *value = *threadInstance.ptr)
       return *value;
 
     // Otherwise, create a new instance for this thread.
-    llvm::sys::SmartScopedLock<true> threadInstanceLock(
-        perInstanceState->instanceMutex);
-    perInstanceState->instances.push_back(std::make_unique<ValueT>());
-    ValueT *instance = perInstanceState->instances.back().get();
-    threadInstance = std::shared_ptr<ValueT>(perInstanceState, instance);
+    {
+      llvm::sys::SmartScopedLock<true> threadInstanceLock(
+          perInstanceState->instanceMutex);
+      perInstanceState->instances.emplace_back(threadInstance);
+    }
+    threadInstance.keepalive = perInstanceState;
 
     // Before returning the new instance, take the chance to clear out any used
     // entries in the static map. The cache is only cleared within the same
     // thread to remove the need to lock the cache itself.
     staticCache.clearExpiredEntries();
-    return *instance;
+    return **threadInstance.ptr;
   }
   ValueT &operator*() { return get(); }
   ValueT *operator->() { return &get(); }