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