gpu/command_buffer/common/command_buffer_shared.h

   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_