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[libc++] Remove unused _LIBCPP_HAS_NO_TREE_BARRIER macro and associated dead code #122769

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Merged
merged 1 commit into from
Jan 14, 2025
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[libc++] Remove unused _LIBCPP_HAS_NO_TREE_BARRIER macro and associated dead code #122769

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105 changes: 0 additions & 105 deletions libcxx/include/barrier
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand Down Expand Up @@ -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_;
Expand Down
4 changes: 0 additions & 4 deletions libcxx/src/barrier.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down Expand Up @@ -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
3 changes: 0 additions & 3 deletions libcxx/test/tools/clang_tidy_checks/internal_ftm_use.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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",
Expand Down
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