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// 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.
#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_
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