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<semaphore>: Add missed mandates for counting_semaphore #3747

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merged 3 commits into from
Jun 15, 2023
Merged

<semaphore>: Add missed mandates for counting_semaphore #3747

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bce57a0
Add missed mandates for `counting_semaphore`
frederick-vs-ja Jun 5, 2023
e7220a8
Address @achabense's review comments
frederick-vs-ja Jun 8, 2023
a835b1a
`_Prev` can be zero
frederick-vs-ja Jun 8, 2023
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27 changes: 15 additions & 12 deletions stl/inc/semaphore
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Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,9 @@ inline constexpr ptrdiff_t _Semaphore_max = (1ULL << (sizeof(ptrdiff_t) * CHAR_B
_EXPORT_STD template <ptrdiff_t _Least_max_value = _Semaphore_max>
class counting_semaphore {
public:
static_assert(_Least_max_value >= 0,
"The least maximum value for a counting_semaphore must be nonnegative (N4950 [thread.sema.cnt]/2).");
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_NODISCARD static constexpr ptrdiff_t(max)() noexcept {
return _Least_max_value;
}
Expand Down Expand Up @@ -90,8 +93,8 @@ public:
// memory_order_seq_cst might be superfluous for some hardware mappings of the C++ memory model,
// but from the point of view of the C++ memory model itself it is needed; weaker orders don't work.

const ptrdiff_t _Prev = _Counter.fetch_add(static_cast<ptrdiff_t>(_Update));
_STL_VERIFY(_Prev + _Update > 0 && _Prev + _Update <= _Least_max_value,
const ptrdiff_t _Prev = _Counter.fetch_add(_Update);
_STL_VERIFY(_Prev >= 0 && _Update <= _Least_max_value - _Prev,
"Precondition: update <= max() - counter (N4950 [thread.sema.cnt]/8)");

const ptrdiff_t _Waiting_upper_bound = _Waiting.load();
Expand All @@ -111,16 +114,6 @@ public:
}
}

void _Wait(const unsigned long _Remaining_timeout) noexcept {
// See the comment in release()
_Waiting.fetch_add(1);
ptrdiff_t _Current = _Counter.load();
if (_Current == 0) {
__std_atomic_wait_direct(&_Counter, &_Current, sizeof(_Current), _Remaining_timeout);
}
_Waiting.fetch_sub(1, memory_order_relaxed);
}

void acquire() noexcept /* strengthened */ {
ptrdiff_t _Current = _Counter.load(memory_order_relaxed);
for (;;) {
Expand Down Expand Up @@ -200,6 +193,16 @@ public:
}

private:
void _Wait(const unsigned long _Remaining_timeout) noexcept {
// See the comment in release()
_Waiting.fetch_add(1);
ptrdiff_t _Current = _Counter.load();
if (_Current == 0) {
__std_atomic_wait_direct(&_Counter, &_Current, sizeof(_Current), _Remaining_timeout);
}
_Waiting.fetch_sub(1, memory_order_relaxed);
}

atomic<ptrdiff_t> _Counter;
atomic<ptrdiff_t> _Waiting;
};
Expand Down