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      1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #ifndef GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
      6 #define GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
      7 
      8 #include "command_buffer.h"
      9 #include "base/atomicops.h"
     10 
     11 namespace gpu {
     12 
     13 // This is a standard 4-slot asynchronous communication mechanism, used to
     14 // ensure that the reader gets a consistent copy of what the writer wrote.
     15 template<typename T>
     16 class SharedState {
     17   T states_[2][2];
     18   base::subtle::Atomic32 reading_;
     19   base::subtle::Atomic32 latest_;
     20   base::subtle::Atomic32 slots_[2];
     21 
     22 public:
     23 
     24   void Initialize() {
     25     for (int i = 0; i < 2; ++i) {
     26       for (int j = 0; j < 2; ++j) {
     27         states_[i][j] = T();
     28       }
     29     }
     30     base::subtle::NoBarrier_Store(&reading_, 0);
     31     base::subtle::NoBarrier_Store(&latest_, 0);
     32     base::subtle::NoBarrier_Store(&slots_[0], 0);
     33     base::subtle::Release_Store(&slots_[1], 0);
     34     base::subtle::MemoryBarrier();
     35   }
     36 
     37   void Write(const T& state) {
     38     int towrite = !base::subtle::Acquire_Load(&reading_);
     39     int index = !base::subtle::Acquire_Load(&slots_[towrite]);
     40     states_[towrite][index] = state;
     41     base::subtle::Release_Store(&slots_[towrite], index);
     42     base::subtle::Release_Store(&latest_, towrite);
     43     base::subtle::MemoryBarrier();
     44   }
     45 
     46   // Attempt to update the state, updating only if the generation counter is
     47   // newer.
     48   void Read(T* state) {
     49     base::subtle::MemoryBarrier();
     50     int toread = !!base::subtle::Acquire_Load(&latest_);
     51     base::subtle::Release_Store(&reading_, toread);
     52     base::subtle::MemoryBarrier();
     53     int index = !!base::subtle::Acquire_Load(&slots_[toread]);
     54     if (states_[toread][index].generation - state->generation < 0x80000000U)
     55       *state = states_[toread][index];
     56   }
     57 };
     58 
     59 typedef SharedState<CommandBuffer::State> CommandBufferSharedState;
     60 
     61 }  // namespace gpu
     62 
     63 #endif  // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
     64