[libc++] Remove unused _LIBCPP_HAS_NO_TREE_BARRIER macro and associated dead code

That macro was present in the original implementation of the synchronization
library, but it was never defined and so it's effectively unused.

Author: ldionne

libcxx/include/barrier

@@ -79,8 +79,6 @@ struct __empty_completion {
   inline _LIBCPP_HIDE_FROM_ABI void operator()() noexcept {}
 };
 
-#      ifndef _LIBCPP_HAS_NO_TREE_BARRIER
-
 /*
 
 The default implementation of __barrier_base is a classic tree barrier.
@@ -153,109 +151,6 @@ public:
   }
 };
 
-#      else
-
-/*
-
-The alternative implementation of __barrier_base is a central barrier.
-
-Two versions of this algorithm are provided:
- 1. A fairly straightforward implementation of the litterature for the
-    general case where the completion function is not empty.
- 2. An optimized implementation that exploits 2's complement arithmetic
-    and well-defined overflow in atomic arithmetic, to handle the phase
-    roll-over for free.
-
-*/
-
-template <class _CompletionF>
-class __barrier_base {
-  atomic<ptrdiff_t> __expected;
-  atomic<ptrdiff_t> __arrived;
-  _CompletionF __completion;
-  atomic<bool> __phase;
-
-public:
-  using arrival_token = bool;
-
-  static constexpr ptrdiff_t max() noexcept { return numeric_limits<ptrdiff_t>::max(); }
-
-  _LIBCPP_HIDE_FROM_ABI __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
-      : __expected(__expected), __arrived(__expected), __completion(std::move(__completion)), __phase(false) {}
-  [[nodiscard]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI arrival_token arrive(ptrdiff_t update) {
-    auto const __old_phase  = __phase.load(memory_order_relaxed);
-    auto const __result     = __arrived.fetch_sub(update, memory_order_acq_rel) - update;
-    auto const new_expected = __expected.load(memory_order_relaxed);
-
-    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
-        update <= new_expected, "update is greater than the expected count for the current barrier phase");
-
-    if (0 == __result) {
-      __completion();
-      __arrived.store(new_expected, memory_order_relaxed);
-      __phase.store(!__old_phase, memory_order_release);
-      __phase.notify_all();
-    }
-    return __old_phase;
-  }
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void wait(arrival_token&& __old_phase) const {
-    __phase.wait(__old_phase, memory_order_acquire);
-  }
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void arrive_and_drop() {
-    __expected.fetch_sub(1, memory_order_relaxed);
-    (void)arrive(1);
-  }
-};
-
-template <>
-class __barrier_base<__empty_completion> {
-  static constexpr uint64_t __expected_unit = 1ull;
-  static constexpr uint64_t __arrived_unit  = 1ull << 32;
-  static constexpr uint64_t __expected_mask = __arrived_unit - 1;
-  static constexpr uint64_t __phase_bit     = 1ull << 63;
-  static constexpr uint64_t __arrived_mask  = (__phase_bit - 1) & ~__expected_mask;
-
-  atomic<uint64_t> __phase_arrived_expected;
-
-  static _LIBCPP_HIDE_FROM_ABI constexpr uint64_t __init(ptrdiff_t __count) _NOEXCEPT {
-    return ((uint64_t(1u << 31) - __count) << 32) | (uint64_t(1u << 31) - __count);
-  }
-
-public:
-  using arrival_token = uint64_t;
-
-  static constexpr ptrdiff_t max() noexcept { return ptrdiff_t(1u << 31) - 1; }
-
-  _LIBCPP_HIDE_FROM_ABI explicit inline __barrier_base(ptrdiff_t __count, __empty_completion = __empty_completion())
-      : __phase_arrived_expected(__init(__count)) {}
-  [[nodiscard]] inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI arrival_token arrive(ptrdiff_t update) {
-    auto const __inc = __arrived_unit * update;
-    auto const __old = __phase_arrived_expected.fetch_add(__inc, memory_order_acq_rel);
-
-    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
-        update <= __old, "update is greater than the expected count for the current barrier phase");
-
-    if ((__old ^ (__old + __inc)) & __phase_bit) {
-      __phase_arrived_expected.fetch_add((__old & __expected_mask) << 32, memory_order_relaxed);
-      __phase_arrived_expected.notify_all();
-    }
-    return __old & __phase_bit;
-  }
-  inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void wait(arrival_token&& __phase) const {
-    auto const __test_fn = [=]() -> bool {
-      uint64_t const __current = __phase_arrived_expected.load(memory_order_acquire);
-      return ((__current & __phase_bit) != __phase);
-    };
-    __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
-  }
-  inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void arrive_and_drop() {
-    __phase_arrived_expected.fetch_add(__expected_unit, memory_order_relaxed);
-    (void)arrive(1);
-  }
-};
-
-#      endif // !_LIBCPP_HAS_NO_TREE_BARRIER
-
 template <class _CompletionF = __empty_completion>
 class barrier {
   __barrier_base<_CompletionF> __b_;

libcxx/src/barrier.cpp

@@ -11,8 +11,6 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if !defined(_LIBCPP_HAS_NO_TREE_BARRIER)
-
 class __barrier_algorithm_base {
 public:
   struct alignas(64) /* naturally-align the heap state */ __state_t {
@@ -70,6 +68,4 @@ _LIBCPP_EXPORTED_FROM_ABI void __destroy_barrier_algorithm_base(__barrier_algori
   delete __barrier;
 }
 
-#endif // !defined(_LIBCPP_HAS_NO_TREE_BARRIER)
-
 _LIBCPP_END_NAMESPACE_STD

libcxx/test/tools/clang_tidy_checks/internal_ftm_use.cpp

@@ -26,9 +26,6 @@ std::array valid_macros{
     // Testing macros
     "_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER",
 
-    // TODO: Why does this macro even exist?
-    "_LIBCPP_HAS_NO_TREE_BARRIER",
-
     // Experimental features
     "_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB",
     "_LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM",