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1 // -*- C++ -*-
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2 //===--------------------------- barrier ----------------------------------===//
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3 //
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4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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5 // See https://llvm.org/LICENSE.txt for license information.
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6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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7 //
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8 //===----------------------------------------------------------------------===//
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9
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10 #ifndef _LIBCPP_BARRIER
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11 #define _LIBCPP_BARRIER
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12
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13 /*
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14 barrier synopsis
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15
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16 namespace std
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17 {
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18
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19 template<class CompletionFunction = see below>
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20 class barrier
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21 {
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22 public:
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23 using arrival_token = see below;
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24
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25 constexpr explicit barrier(ptrdiff_t phase_count,
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26 CompletionFunction f = CompletionFunction());
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27 ~barrier();
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28
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29 barrier(const barrier&) = delete;
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30 barrier& operator=(const barrier&) = delete;
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31
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32 [[nodiscard]] arrival_token arrive(ptrdiff_t update = 1);
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33 void wait(arrival_token&& arrival) const;
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34
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35 void arrive_and_wait();
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36 void arrive_and_drop();
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37
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38 private:
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39 CompletionFunction completion; // exposition only
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40 };
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41
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42 }
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43
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44 */
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45
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46 #include <__config>
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47 #include <atomic>
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48 #ifndef _LIBCPP_HAS_NO_TREE_BARRIER
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49 # include <memory>
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50 #endif
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51
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52 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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53 #pragma GCC system_header
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54 #endif
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55
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56 #ifdef _LIBCPP_HAS_NO_THREADS
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57 # error <barrier> is not supported on this single threaded system
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58 #endif
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59
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60 #if _LIBCPP_STD_VER >= 14
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61
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62 _LIBCPP_BEGIN_NAMESPACE_STD
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63
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64 struct __empty_completion
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65 {
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66 inline _LIBCPP_INLINE_VISIBILITY
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67 void operator()() noexcept
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68 {
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69 }
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70 };
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71
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72 #ifndef _LIBCPP_HAS_NO_TREE_BARRIER
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73
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74 /*
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75
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76 The default implementation of __barrier_base is a classic tree barrier.
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77
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78 It looks different from literature pseudocode for two main reasons:
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79 1. Threads that call into std::barrier functions do not provide indices,
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80 so a numbering step is added before the actual barrier algorithm,
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81 appearing as an N+1 round to the N rounds of the tree barrier.
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82 2. A great deal of attention has been paid to avoid cache line thrashing
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83 by flattening the tree structure into cache-line sized arrays, that
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84 are indexed in an efficient way.
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85
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86 */
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87
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88 using __barrier_phase_t = uint8_t;
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89
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90 class __barrier_algorithm_base;
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91
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92 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
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93 __barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected);
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94
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95 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
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96 bool __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier,
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97 __barrier_phase_t __old_phase);
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98
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99 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
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100 void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier);
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101
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102 template<class _CompletionF>
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103 class __barrier_base {
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104
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105 ptrdiff_t __expected;
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106 unique_ptr<__barrier_algorithm_base,
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107 void (*)(__barrier_algorithm_base*)> __base;
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108 __atomic_base<ptrdiff_t> __expected_adjustment;
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109 _CompletionF __completion;
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110 __atomic_base<__barrier_phase_t> __phase;
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111
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112 public:
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113 using arrival_token = __barrier_phase_t;
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114
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115 static constexpr ptrdiff_t max() noexcept {
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116 return numeric_limits<ptrdiff_t>::max();
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117 }
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118
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119 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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120 __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
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121 : __expected(__expected), __base(__construct_barrier_algorithm_base(this->__expected),
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122 &__destroy_barrier_algorithm_base),
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123 __expected_adjustment(0), __completion(move(__completion)), __phase(0)
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124 {
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125 }
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126 [[nodiscard]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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127 arrival_token arrive(ptrdiff_t update)
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128 {
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129 auto const __old_phase = __phase.load(memory_order_relaxed);
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130 for(; update; --update)
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131 if(__arrive_barrier_algorithm_base(__base.get(), __old_phase)) {
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132 __completion();
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133 __expected += __expected_adjustment.load(memory_order_relaxed);
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134 __expected_adjustment.store(0, memory_order_relaxed);
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135 __phase.store(__old_phase + 2, memory_order_release);
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136 __phase.notify_all();
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137 }
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138 return __old_phase;
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139 }
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140 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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141 void wait(arrival_token&& __old_phase) const
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142 {
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143 auto const __test_fn = [=]() -> bool {
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144 return __phase.load(memory_order_acquire) != __old_phase;
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145 };
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146 __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
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147 }
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148 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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149 void arrive_and_drop()
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150 {
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151 __expected_adjustment.fetch_sub(1, memory_order_relaxed);
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152 (void)arrive(1);
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153 }
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154 };
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155
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156 #else
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157
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158 /*
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159
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160 The alternative implementation of __barrier_base is a central barrier.
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161
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162 Two versions of this algorithm are provided:
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163 1. A fairly straightforward implementation of the litterature for the
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164 general case where the completion function is not empty.
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165 2. An optimized implementation that exploits 2's complement arithmetic
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166 and well-defined overflow in atomic arithmetic, to handle the phase
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167 roll-over for free.
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168
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169 */
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170
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171 template<class _CompletionF>
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172 class __barrier_base {
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173
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174 __atomic_base<ptrdiff_t> __expected;
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175 __atomic_base<ptrdiff_t> __arrived;
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176 _CompletionF __completion;
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177 __atomic_base<bool> __phase;
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178 public:
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179 using arrival_token = bool;
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180
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181 static constexpr ptrdiff_t max() noexcept {
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182 return numeric_limits<ptrdiff_t>::max();
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183 }
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184
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185 _LIBCPP_INLINE_VISIBILITY
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186 __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
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187 : __expected(__expected), __arrived(__expected), __completion(move(__completion)), __phase(false)
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188 {
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189 }
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190 [[nodiscard]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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191 arrival_token arrive(ptrdiff_t update)
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192 {
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193 auto const __old_phase = __phase.load(memory_order_relaxed);
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194 auto const __result = __arrived.fetch_sub(update, memory_order_acq_rel) - update;
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195 auto const new_expected = __expected.load(memory_order_relaxed);
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196 if(0 == __result) {
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197 __completion();
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198 __arrived.store(new_expected, memory_order_relaxed);
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199 __phase.store(!__old_phase, memory_order_release);
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200 __phase.notify_all();
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201 }
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202 return __old_phase;
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203 }
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204 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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205 void wait(arrival_token&& __old_phase) const
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206 {
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207 __phase.wait(__old_phase, memory_order_acquire);
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208 }
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209 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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210 void arrive_and_drop()
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211 {
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212 __expected.fetch_sub(1, memory_order_relaxed);
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213 (void)arrive(1);
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214 }
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215 };
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216
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217 template<>
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218 class __barrier_base<__empty_completion> {
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219
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220 static constexpr uint64_t __expected_unit = 1ull;
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221 static constexpr uint64_t __arrived_unit = 1ull << 32;
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222 static constexpr uint64_t __expected_mask = __arrived_unit - 1;
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223 static constexpr uint64_t __phase_bit = 1ull << 63;
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224 static constexpr uint64_t __arrived_mask = (__phase_bit - 1) & ~__expected_mask;
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225
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226 __atomic_base<uint64_t> __phase_arrived_expected;
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227
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228 static _LIBCPP_INLINE_VISIBILITY
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229 constexpr uint64_t __init(ptrdiff_t __count) _NOEXCEPT
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230 {
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231 return ((uint64_t(1u << 31) - __count) << 32)
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232 | (uint64_t(1u << 31) - __count);
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233 }
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234
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235 public:
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236 using arrival_token = uint64_t;
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237
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238 static constexpr ptrdiff_t max() noexcept {
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239 return ptrdiff_t(1u << 31) - 1;
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240 }
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241
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242 _LIBCPP_INLINE_VISIBILITY
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243 explicit inline __barrier_base(ptrdiff_t __count, __empty_completion = __empty_completion())
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244 : __phase_arrived_expected(__init(__count))
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245 {
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246 }
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247 [[nodiscard]] inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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248 arrival_token arrive(ptrdiff_t update)
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249 {
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250 auto const __inc = __arrived_unit * update;
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251 auto const __old = __phase_arrived_expected.fetch_add(__inc, memory_order_acq_rel);
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252 if((__old ^ (__old + __inc)) & __phase_bit) {
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253 __phase_arrived_expected.fetch_add((__old & __expected_mask) << 32, memory_order_relaxed);
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254 __phase_arrived_expected.notify_all();
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255 }
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256 return __old & __phase_bit;
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257 }
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258 inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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259 void wait(arrival_token&& __phase) const
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260 {
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261 auto const __test_fn = [=]() -> bool {
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262 uint64_t const __current = __phase_arrived_expected.load(memory_order_acquire);
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263 return ((__current & __phase_bit) != __phase);
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264 };
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265 __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
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266 }
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267 inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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268 void arrive_and_drop()
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269 {
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270 __phase_arrived_expected.fetch_add(__expected_unit, memory_order_relaxed);
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271 (void)arrive(1);
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272 }
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273 };
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274
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275 #endif //_LIBCPP_HAS_NO_TREE_BARRIER
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276
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277 template<class _CompletionF = __empty_completion>
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278 class barrier {
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279
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280 __barrier_base<_CompletionF> __b;
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281 public:
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282 using arrival_token = typename __barrier_base<_CompletionF>::arrival_token;
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283
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284 static constexpr ptrdiff_t max() noexcept {
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285 return __barrier_base<_CompletionF>::max();
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286 }
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287
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288 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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289 barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
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290 : __b(__count, std::move(__completion)) {
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291 }
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292
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293 barrier(barrier const&) = delete;
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294 barrier& operator=(barrier const&) = delete;
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295
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296 [[nodiscard]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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297 arrival_token arrive(ptrdiff_t update = 1)
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298 {
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299 return __b.arrive(update);
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300 }
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301 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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302 void wait(arrival_token&& __phase) const
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303 {
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304 __b.wait(std::move(__phase));
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305 }
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306 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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307 void arrive_and_wait()
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308 {
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309 wait(arrive());
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310 }
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311 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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312 void arrive_and_drop()
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313 {
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314 __b.arrive_and_drop();
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315 }
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316 };
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317
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318 _LIBCPP_END_NAMESPACE_STD
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319
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320 #endif // _LIBCPP_STD_VER >= 14
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321
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322 #endif //_LIBCPP_BARRIER
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