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 class __barrier_algorithm_base {
  15 public:
  16   struct alignas(64) /* naturally-align the heap state */ __state_t {
  17     struct {
  18       atomic<__barrier_phase_t> __phase{0};
  19     } __tickets[64];
  20   };
  21
  22   ptrdiff_t& __expected_;
  23   unique_ptr<__state_t[]> __state_;
  24
  25   _LIBCPP_HIDDEN __barrier_algorithm_base(ptrdiff_t& __expected) : __expected_(__expected) {
  26     size_t const __count = (__expected + 1) >> 1;
  27     __state_             = unique_ptr<__state_t[]>(new __state_t[__count]);
  28   }
  29   _LIBCPP_HIDDEN bool __arrive(__barrier_phase_t __old_phase) {
  30     __barrier_phase_t const __half_step = __old_phase + 1, __full_step = __old_phase + 2;
  31     size_t __current_expected = __expected_,
  32            __current          = hash<thread::id>()(this_thread::get_id()) % ((__expected_ + 1) >> 1);
  33     for (int __round = 0;; ++__round) {
  34       if (__current_expected <= 1)
  35         return true;
  36       size_t const __end_node = ((__current_expected + 1) >> 1), __last_node = __end_node - 1;
  37       for (;; ++__current) {
  38         if (__current == __end_node)
  39           __current = 0;
  40         __barrier_phase_t expect = __old_phase;
  41         if (__current == __last_node && (__current_expected & 1)) {
  42           if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
  43                   expect, __full_step, memory_order_acq_rel))
  44             break; // I'm 1 in 1, go to next __round
  45         } else if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
  46                        expect, __half_step, memory_order_acq_rel)) {
  47           return false; // I'm 1 in 2, done with arrival
  48         } else if (expect == __half_step) {
  49           if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
  50                   expect, __full_step, memory_order_acq_rel))
  51             break; // I'm 2 in 2, go to next __round
  52         }
  53       }
  54       __current_expected = __last_node + 1;
  55       __current >>= 1;
  56     }
  57   }
  58 };
  59
  60 _LIBCPP_EXPORTED_FROM_ABI __barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected) {
  61   return new __barrier_algorithm_base(__expected);
  62 }
  63 _LIBCPP_EXPORTED_FROM_ABI bool
  64 __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier, __barrier_phase_t __old_phase) noexcept {
  65   return __barrier->__arrive(__old_phase);
  66 }
  67 _LIBCPP_EXPORTED_FROM_ABI void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier) noexcept {
  68   delete __barrier;
  69 }
  70
  71 _LIBCPP_END_NAMESPACE_STD