Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 420 insertions, 0 deletions

diff --git a/mfbt/SPSCQueue.h b/mfbt/SPSCQueue.h
new file mode 100644
index 0000000000..bd4223d70a
--- /dev/null
+++ b/mfbt/SPSCQueue.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Single producer single consumer lock-free and wait-free queue. */
+
+#ifndef mozilla_LockFreeQueue_h
+#define mozilla_LockFreeQueue_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/PodOperations.h"
+#include <algorithm>
+#include <atomic>
+#include <cstddef>
+#include <limits>
+#include <memory>
+#include <thread>
+#include <type_traits>
+
+namespace mozilla {
+
+namespace detail {
+template <typename T, bool IsPod = std::is_trivial<T>::value>
+struct MemoryOperations {
+  /**
+   * This allows zeroing (using memset) or default-constructing a number of
+   * elements calling the constructors if necessary.
+   */
+  static void ConstructDefault(T* aDestination, size_t aCount);
+  /**
+   * This allows either moving (if T supports it) or copying a number of
+   * elements from a `aSource` pointer to a `aDestination` pointer.
+   * If it is safe to do so and this call copies, this uses PodCopy. Otherwise,
+   * constructors and destructors are called in a loop.
+   */
+  static void MoveOrCopy(T* aDestination, T* aSource, size_t aCount);
+};
+
+template <typename T>
+struct MemoryOperations<T, true> {
+  static void ConstructDefault(T* aDestination, size_t aCount) {
+    PodZero(aDestination, aCount);
+  }
+  static void MoveOrCopy(T* aDestination, T* aSource, size_t aCount) {
+    PodCopy(aDestination, aSource, aCount);
+  }
+};
+
+template <typename T>
+struct MemoryOperations<T, false> {
+  static void ConstructDefault(T* aDestination, size_t aCount) {
+    for (size_t i = 0; i < aCount; i++) {
+      aDestination[i] = T();
+    }
+  }
+  static void MoveOrCopy(T* aDestination, T* aSource, size_t aCount) {
+    std::move(aSource, aSource + aCount, aDestination);
+  }
+};
+}  // namespace detail
+
+/**
+ * This data structure allows producing data from one thread, and consuming it
+ * on another thread, safely and without explicit synchronization.
+ *
+ * The role for the producer and the consumer must be constant, i.e., the
+ * producer should always be on one thread and the consumer should always be on
+ * another thread.
+ *
+ * Some words about the inner workings of this class:
+ * - Capacity is fixed. Only one allocation is performed, in the constructor.
+ *   When reading and writing, the return value of the method allows checking if
+ *   the ring buffer is empty or full.
+ * - We always keep the read index at least one element ahead of the write
+ *   index, so we can distinguish between an empty and a full ring buffer: an
+ *   empty ring buffer is when the write index is at the same position as the
+ *   read index. A full buffer is when the write index is exactly one position
+ *   before the read index.
+ * - We synchronize updates to the read index after having read the data, and
+ *   the write index after having written the data. This means that the each
+ *   thread can only touch a portion of the buffer that is not touched by the
+ *   other thread.
+ * - Callers are expected to provide buffers. When writing to the queue,
+ *   elements are copied into the internal storage from the buffer passed in.
+ *   When reading from the queue, the user is expected to provide a buffer.
+ *   Because this is a ring buffer, data might not be contiguous in memory;
+ *   providing an external buffer to copy into is an easy way to have linear
+ *   data for further processing.
+ */
+template <typename T>
+class SPSCRingBufferBase {
+ public:
+  /**
+   * Constructor for a ring buffer.
+   *
+   * This performs an allocation on the heap, but is the only allocation that
+   * will happen for the life time of a `SPSCRingBufferBase`.
+   *
+   * @param Capacity The maximum number of element this ring buffer will hold.
+   */
+  explicit SPSCRingBufferBase(int aCapacity)
+      : mReadIndex(0),
+        mWriteIndex(0),
+        /* One more element to distinguish from empty and full buffer. */
+        mCapacity(aCapacity + 1) {
+    MOZ_RELEASE_ASSERT(aCapacity != std::numeric_limits<int>::max());
+    MOZ_RELEASE_ASSERT(mCapacity > 0);
+
+    mData = std::make_unique<T[]>(StorageCapacity());
+
+    std::atomic_thread_fence(std::memory_order_seq_cst);
+  }
+  /**
+   * Push `aCount` zero or default constructed elements in the array.
+   *
+   * Only safely called on the producer thread.
+   *
+   * @param count The number of elements to enqueue.
+   * @return The number of element enqueued.
+   */
+  [[nodiscard]] int EnqueueDefault(int aCount) {
+    return Enqueue(nullptr, aCount);
+  }
+  /**
+   * @brief Put an element in the queue.
+   *
+   * Only safely called on the producer thread.
+   *
+   * @param element The element to put in the queue.
+   *
+   * @return 1 if the element was inserted, 0 otherwise.
+   */
+  [[nodiscard]] int Enqueue(T& aElement) { return Enqueue(&aElement, 1); }
+  /**
+   * Push `aCount` elements in the ring buffer.
+   *
+   * Only safely called on the producer thread.
+   *
+   * @param elements a pointer to a buffer containing at least `count` elements.
+   * If `elements` is nullptr, zero or default constructed elements are enqueud.
+   * @param count The number of elements to read from `elements`
+   * @return The number of elements successfully coped from `elements` and
+   * inserted into the ring buffer.
+   */
+  [[nodiscard]] int Enqueue(T* aElements, int aCount) {
+#ifdef DEBUG
+    AssertCorrectThread(mProducerId);
+#endif
+
+    int rdIdx = mReadIndex.load(std::memory_order_acquire);
+    int wrIdx = mWriteIndex.load(std::memory_order_relaxed);
+
+    if (IsFull(rdIdx, wrIdx)) {
+      return 0;
+    }
+
+    int toWrite = std::min(AvailableWriteInternal(rdIdx, wrIdx), aCount);
+
+    /* First part, from the write index to the end of the array. */
+    int firstPart = std::min(StorageCapacity() - wrIdx, toWrite);
+    /* Second part, from the beginning of the array */
+    int secondPart = toWrite - firstPart;
+
+    if (aElements) {
+      detail::MemoryOperations<T>::MoveOrCopy(mData.get() + wrIdx, aElements,
+                                              firstPart);
+      detail::MemoryOperations<T>::MoveOrCopy(
+          mData.get(), aElements + firstPart, secondPart);
+    } else {
+      detail::MemoryOperations<T>::ConstructDefault(mData.get() + wrIdx,
+                                                    firstPart);
+      detail::MemoryOperations<T>::ConstructDefault(mData.get(), secondPart);
+    }
+
+    mWriteIndex.store(IncrementIndex(wrIdx, toWrite),
+                      std::memory_order_release);
+
+    return toWrite;
+  }
+  /**
+   * Retrieve at most `count` elements from the ring buffer, and copy them to
+   * `elements`, if non-null.
+   *
+   * Only safely called on the consumer side.
+   *
+   * @param elements A pointer to a buffer with space for at least `count`
+   * elements. If `elements` is `nullptr`, `count` element will be discarded.
+   * @param count The maximum number of elements to Dequeue.
+   * @return The number of elements written to `elements`.
+   */
+  [[nodiscard]] int Dequeue(T* elements, int count) {
+#ifdef DEBUG
+    AssertCorrectThread(mConsumerId);
+#endif
+
+    int wrIdx = mWriteIndex.load(std::memory_order_acquire);
+    int rdIdx = mReadIndex.load(std::memory_order_relaxed);
+
+    if (IsEmpty(rdIdx, wrIdx)) {
+      return 0;
+    }
+
+    int toRead = std::min(AvailableReadInternal(rdIdx, wrIdx), count);
+
+    int firstPart = std::min(StorageCapacity() - rdIdx, toRead);
+    int secondPart = toRead - firstPart;
+
+    if (elements) {
+      detail::MemoryOperations<T>::MoveOrCopy(elements, mData.get() + rdIdx,
+                                              firstPart);
+      detail::MemoryOperations<T>::MoveOrCopy(elements + firstPart, mData.get(),
+                                              secondPart);
+    }
+
+    mReadIndex.store(IncrementIndex(rdIdx, toRead), std::memory_order_release);
+
+    return toRead;
+  }
+  /**
+   * Get the number of available elements for consuming.
+   *
+   * This can be less than the actual number of elements in the queue, since the
+   * mWriteIndex is updated at the very end of the Enqueue method on the
+   * producer thread, but consequently always returns a number of elements such
+   * that a call to Dequeue return this number of elements.
+   *
+   * @return The number of available elements for reading.
+   */
+  int AvailableRead() const {
+    return AvailableReadInternal(mReadIndex.load(std::memory_order_relaxed),
+                                 mWriteIndex.load(std::memory_order_relaxed));
+  }
+  /**
+   * Get the number of available elements for writing.
+   *
+   * This can be less than than the actual number of slots that are available,
+   * because mReadIndex is updated at the very end of the Deque method. It
+   * always returns a number such that a call to Enqueue with this number will
+   * succeed in enqueuing this number of elements.
+   *
+   * @return The number of empty slots in the buffer, available for writing.
+   */
+  int AvailableWrite() const {
+    return AvailableWriteInternal(mReadIndex.load(std::memory_order_relaxed),
+                                  mWriteIndex.load(std::memory_order_relaxed));
+  }
+  /**
+   * Get the total Capacity, for this ring buffer.
+   *
+   * Can be called safely on any thread.
+   *
+   * @return The maximum Capacity of this ring buffer.
+   */
+  int Capacity() const { return StorageCapacity() - 1; }
+
+  /**
+   * Reset the consumer thread id to the current thread. The caller must
+   * guarantee that the last call to Dequeue() on the previous consumer thread
+   * has completed, and subsequent calls to Dequeue() will only happen on the
+   * current thread.
+   */
+  void ResetConsumerThreadId() {
+#ifdef DEBUG
+    mConsumerId = std::this_thread::get_id();
+#endif
+
+    // When changing consumer from thread A to B, the last Dequeue on A (synced
+    // by mReadIndex.store with memory_order_release) must be picked up by B
+    // through an acquire operation.
+    std::ignore = mReadIndex.load(std::memory_order_acquire);
+  }
+
+  /**
+   * Reset the producer thread id to the current thread. The caller must
+   * guarantee that the last call to Enqueue() on the previous consumer thread
+   * has completed, and subsequent calls to Dequeue() will only happen on the
+   * current thread.
+   */
+  void ResetProducerThreadId() {
+#ifdef DEBUG
+    mProducerId = std::this_thread::get_id();
+#endif
+
+    // When changing producer from thread A to B, the last Enqueue on A (synced
+    // by mWriteIndex.store with memory_order_release) must be picked up by B
+    // through an acquire operation.
+    std::ignore = mWriteIndex.load(std::memory_order_acquire);
+  }
+
+ private:
+  /** Return true if the ring buffer is empty.
+   *
+   * This can be called from the consumer or the producer thread.
+   *
+   * @param aReadIndex the read index to consider
+   * @param writeIndex the write index to consider
+   * @return true if the ring buffer is empty, false otherwise.
+   **/
+  bool IsEmpty(int aReadIndex, int aWriteIndex) const {
+    return aWriteIndex == aReadIndex;
+  }
+  /** Return true if the ring buffer is full.
+   *
+   * This happens if the write index is exactly one element behind the read
+   * index.
+   *
+   * This can be called from the consummer or the producer thread.
+   *
+   * @param aReadIndex the read index to consider
+   * @param writeIndex the write index to consider
+   * @return true if the ring buffer is full, false otherwise.
+   **/
+  bool IsFull(int aReadIndex, int aWriteIndex) const {
+    return (aWriteIndex + 1) % StorageCapacity() == aReadIndex;
+  }
+  /**
+   * Return the size of the storage. It is one more than the number of elements
+   * that can be stored in the buffer.
+   *
+   * This can be called from any thread.
+   *
+   * @return the number of elements that can be stored in the buffer.
+   */
+  int StorageCapacity() const { return mCapacity; }
+  /**
+   * Returns the number of elements available for reading.
+   *
+   * This can be called from the consummer or producer thread, but see the
+   * comment in `AvailableRead`.
+   *
+   * @return the number of available elements for reading.
+   */
+  int AvailableReadInternal(int aReadIndex, int aWriteIndex) const {
+    if (aWriteIndex >= aReadIndex) {
+      return aWriteIndex - aReadIndex;
+    } else {
+      return aWriteIndex + StorageCapacity() - aReadIndex;
+    }
+  }
+  /**
+   * Returns the number of empty elements, available for writing.
+   *
+   * This can be called from the consummer or producer thread, but see the
+   * comment in `AvailableWrite`.
+   *
+   * @return the number of elements that can be written into the array.
+   */
+  int AvailableWriteInternal(int aReadIndex, int aWriteIndex) const {
+    /* We subtract one element here to always keep at least one sample
+     * free in the buffer, to distinguish between full and empty array. */
+    int rv = aReadIndex - aWriteIndex - 1;
+    if (aWriteIndex >= aReadIndex) {
+      rv += StorageCapacity();
+    }
+    return rv;
+  }
+  /**
+   * Increments an index, wrapping it around the storage.
+   *
+   * Incrementing `mWriteIndex` can be done on the producer thread.
+   * Incrementing `mReadIndex` can be done on the consummer thread.
+   *
+   * @param index a reference to the index to increment.
+   * @param increment the number by which `index` is incremented.
+   * @return the new index.
+   */
+  int IncrementIndex(int aIndex, int aIncrement) const {
+    MOZ_ASSERT(aIncrement >= 0 && aIncrement < StorageCapacity() &&
+               aIndex < StorageCapacity());
+    return (aIndex + aIncrement) % StorageCapacity();
+  }
+  /**
+   * @brief This allows checking that Enqueue (resp. Dequeue) are always
+   * called by the right thread.
+   *
+   * The role of the thread are assigned the first time they call Enqueue or
+   * Dequeue, and cannot change, except by a ResetThreadId method.
+   *
+   * @param id the id of the thread that has called the calling method first.
+   */
+#ifdef DEBUG
+  static void AssertCorrectThread(std::thread::id& aId) {
+    if (aId == std::thread::id()) {
+      aId = std::this_thread::get_id();
+      return;
+    }
+    MOZ_ASSERT(aId == std::this_thread::get_id());
+  }
+#endif
+  /** Index at which the oldest element is. */
+  std::atomic<int> mReadIndex;
+  /** Index at which to write new elements. `mWriteIndex` is always at
+   * least one element ahead of `mReadIndex`. */
+  std::atomic<int> mWriteIndex;
+  /** Maximum number of elements that can be stored in the ring buffer. */
+  const int mCapacity;
+  /** Data storage, of size `mCapacity + 1` */
+  std::unique_ptr<T[]> mData;
+#ifdef DEBUG
+  /** The id of the only thread that is allowed to read from the queue. */
+  mutable std::thread::id mConsumerId;
+  /** The id of the only thread that is allowed to write from the queue. */
+  mutable std::thread::id mProducerId;
+#endif
+};
+
+/**
+ * Instantiation of the `SPSCRingBufferBase` type. This is safe to use
+ * from two threads, one producer, one consumer (that never change role),
+ * without explicit synchronization.
+ */
+template <typename T>
+using SPSCQueue = SPSCRingBufferBase<T>;
+
+}  // namespace mozilla
+
+#endif  // mozilla_LockFreeQueue_h