Repo created

TMessagesProj/jni/voip/webrtc/base/lazy_instance_helpers.h

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+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_LAZY_INSTANCE_INTERNAL_H_
+#define BASE_LAZY_INSTANCE_INTERNAL_H_
+
+#include "base/atomicops.h"
+#include "base/base_export.h"
+#include "base/logging.h"
+
+// Helper methods used by LazyInstance and a few other base APIs for thread-safe
+// lazy construction.
+
+namespace base {
+namespace internal {
+
+// Our AtomicWord doubles as a spinlock, where a value of
+// kLazyInstanceStateCreating means the spinlock is being held for creation.
+constexpr subtle::AtomicWord kLazyInstanceStateCreating = 1;
+
+// Helper for GetOrCreateLazyPointer(). Checks if instance needs to be created.
+// If so returns true otherwise if another thread has beat us, waits for
+// instance to be created and returns false.
+BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
+
+// Helper for GetOrCreateLazyPointer(). After creating an instance, this is
+// called to register the dtor to be called at program exit and to update the
+// atomic state to hold the |new_instance|
+BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
+                                      subtle::AtomicWord new_instance,
+                                      void (*destructor)(void*),
+                                      void* destructor_arg);
+
+}  // namespace internal
+
+namespace subtle {
+
+// If |state| is uninitialized (zero), constructs a value using
+// |creator_func(creator_arg)|, stores it into |state| and registers
+// |destructor(destructor_arg)| to be called when the current AtExitManager goes
+// out of scope. Then, returns the value stored in |state|. It is safe to have
+// concurrent calls to this function with the same |state|. |creator_func| may
+// return nullptr if it doesn't want to create an instance anymore (e.g. on
+// shutdown), it is from then on required to return nullptr to all callers (ref.
+// StaticMemorySingletonTraits). In that case, callers need to synchronize
+// before |creator_func| may return a non-null instance again (ref.
+// StaticMemorySingletonTraits::ResurectForTesting()).
+// Implementation note on |creator_func/creator_arg|. It makes for ugly adapters
+// but it avoids redundant template instantiations (e.g. saves 27KB in
+// chrome.dll) because linker is able to fold these for multiple Types but
+// couldn't with the more advanced CreatorFunc template type which in turn
+// improves code locality (and application startup) -- ref.
+// https://chromium-review.googlesource.com/c/chromium/src/+/530984/5/base/lazy_instance.h#140,
+// worsened by https://chromium-review.googlesource.com/c/chromium/src/+/868013
+// and caught then as https://crbug.com/804034.
+template <typename Type>
+Type* GetOrCreateLazyPointer(subtle::AtomicWord* state,
+                             Type* (*creator_func)(void*),
+                             void* creator_arg,
+                             void (*destructor)(void*),
+                             void* destructor_arg) {
+  DCHECK(state);
+  DCHECK(creator_func);
+
+  // If any bit in the created mask is true, the instance has already been
+  // fully constructed.
+  constexpr subtle::AtomicWord kLazyInstanceCreatedMask =
+      ~internal::kLazyInstanceStateCreating;
+
+  // We will hopefully have fast access when the instance is already created.
+  // Since a thread sees |state| == 0 or kLazyInstanceStateCreating at most
+  // once, the load is taken out of NeedsLazyInstance() as a fast-path. The load
+  // has acquire memory ordering as a thread which sees |state| > creating needs
+  // to acquire visibility over the associated data. Pairing Release_Store is in
+  // CompleteLazyInstance().
+  subtle::AtomicWord instance = subtle::Acquire_Load(state);
+  if (!(instance & kLazyInstanceCreatedMask)) {
+    if (internal::NeedsLazyInstance(state)) {
+      // This thread won the race and is now responsible for creating the
+      // instance and storing it back into |state|.
+      instance =
+          reinterpret_cast<subtle::AtomicWord>((*creator_func)(creator_arg));
+      internal::CompleteLazyInstance(state, instance, destructor,
+                                     destructor_arg);
+    } else {
+      // This thread lost the race but now has visibility over the constructed
+      // instance (NeedsLazyInstance() doesn't return until the constructing
+      // thread releases the instance via CompleteLazyInstance()).
+      instance = subtle::Acquire_Load(state);
+      DCHECK(instance & kLazyInstanceCreatedMask);
+    }
+  }
+  return reinterpret_cast<Type*>(instance);
+}
+
+}  // namespace subtle
+
+}  // namespace base
+
+#endif  // BASE_LAZY_INSTANCE_INTERNAL_H_