libcxx/src/barrier.cpp

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include <barrier>
  10 #include <thread>
  11
  12 _LIBCPP_BEGIN_NAMESPACE_STD
  13
  14 #if !defined(_LIBCPP_HAS_NO_TREE_BARRIER)
  15
  16 class __barrier_algorithm_base {
  17 public:
  18   struct alignas(64) /* naturally-align the heap state */ __state_t {
  19     struct {
  20       atomic<__barrier_phase_t> __phase{0};
  21     } __tickets[64];
  22   };
  23
  24   ptrdiff_t& __expected_;
  25   unique_ptr<__state_t[]> __state_;
  26
  27   _LIBCPP_HIDDEN __barrier_algorithm_base(ptrdiff_t& __expected) : __expected_(__expected) {
  28     size_t const __count = (__expected + 1) >> 1;
  29     __state_             = unique_ptr<__state_t[]>(new __state_t[__count]);
  30   }
  31   _LIBCPP_HIDDEN bool __arrive(__barrier_phase_t __old_phase) {
  32     __barrier_phase_t const __half_step = __old_phase + 1, __full_step = __old_phase + 2;
  33     size_t __current_expected = __expected_,
  34            __current          = hash<thread::id>()(this_thread::get_id()) % ((__expected_ + 1) >> 1);
  35     for (int __round = 0;; ++__round) {
  36       if (__current_expected <= 1)
  37         return true;
  38       size_t const __end_node = ((__current_expected + 1) >> 1), __last_node = __end_node - 1;
  39       for (;; ++__current) {
  40         if (__current == __end_node)
  41           __current = 0;
  42         __barrier_phase_t expect = __old_phase;
  43         if (__current == __last_node && (__current_expected & 1)) {
  44           if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
  45                   expect, __full_step, memory_order_acq_rel))
  46             break; // I'm 1 in 1, go to next __round
  47         } else if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
  48                        expect, __half_step, memory_order_acq_rel)) {
  49           return false; // I'm 1 in 2, done with arrival
  50         } else if (expect == __half_step) {
  51           if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
  52                   expect, __full_step, memory_order_acq_rel))
  53             break; // I'm 2 in 2, go to next __round
  54         }
  55       }
  56       __current_expected = __last_node + 1;
  57       __current >>= 1;
  58     }
  59   }
  60 };
  61
  62 _LIBCPP_EXPORTED_FROM_ABI __barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected) {
  63   return new __barrier_algorithm_base(__expected);
  64 }
  65 _LIBCPP_EXPORTED_FROM_ABI bool
  66 __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier, __barrier_phase_t __old_phase) noexcept {
  67   return __barrier->__arrive(__old_phase);
  68 }
  69 _LIBCPP_EXPORTED_FROM_ABI void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier) noexcept {
  70   delete __barrier;
  71 }
  72
  73 #endif // !defined(_LIBCPP_HAS_NO_TREE_BARRIER)
  74
  75 _LIBCPP_END_NAMESPACE_STD