command_buffer/common/command_buffer_shared.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifdef UNSAFE_BUFFERS_BUILD
// TODO(crbug.com/351564777): Remove this and convert code to safer constructs.
#pragma allow_unsafe_buffers
#endif

#ifndef GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
#define GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_

#include <array>
#include <atomic>

#include "base/atomicops.h"
#include "command_buffer.h"

namespace gpu {

// This is a standard 4-slot asynchronous communication mechanism, used to
// ensure that the reader gets a consistent copy of what the writer wrote.
template<typename T>
class SharedState {
  std::array<std::array<T, 2>, 2> states_;
  base::subtle::Atomic32 reading_;
  base::subtle::Atomic32 latest_;
  std::array<base::subtle::Atomic32, 2> slots_;

 public:
  void Initialize() {
    states_ = {};
    base::subtle::NoBarrier_Store(&reading_, 0);
    base::subtle::NoBarrier_Store(&latest_, 0);
    base::subtle::NoBarrier_Store(&slots_[0], 0);
    base::subtle::Release_Store(&slots_[1], 0);
    std::atomic_thread_fence(std::memory_order_seq_cst);
  }

  void Write(const T& state) {
    int towrite = !base::subtle::Acquire_Load(&reading_);
    int index = !base::subtle::Acquire_Load(&slots_[towrite]);
    states_[towrite][index] = state;
    base::subtle::Release_Store(&slots_[towrite], index);
    base::subtle::Release_Store(&latest_, towrite);
    std::atomic_thread_fence(std::memory_order_seq_cst);
  }

  // Attempt to update the state, updating only if the generation counter is
  // newer.
  void Read(T* state) {
    std::atomic_thread_fence(std::memory_order_seq_cst);
    int toread = !!base::subtle::Acquire_Load(&latest_);
    base::subtle::Release_Store(&reading_, toread);
    std::atomic_thread_fence(std::memory_order_seq_cst);
    int index = !!base::subtle::Acquire_Load(&slots_[toread]);
    if (states_[toread][index].generation - state->generation < 0x80000000U)
      *state = states_[toread][index];
  }
};

typedef SharedState<CommandBuffer::State> CommandBufferSharedState;

}  // namespace gpu

#endif  // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_