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<mutex>: Use if constexpr for lock() and try_lock() #5941

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 1 commit into from
Dec 8, 2025
Merged

<mutex>: Use if constexpr for lock() and try_lock() #5941

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65 changes: 27 additions & 38 deletions stl/inc/mutex
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Original file line number Diff line number Diff line change
Expand Up @@ -343,30 +343,25 @@ int _Try_lock_range(const int _First, const int _Last, _LockN&... _LkN) {
return -1;
}

template <class _Lock0, class _Lock1, class _Lock2, class... _LockN>
int _Try_lock1(_Lock0& _Lk0, _Lock1& _Lk1, _Lock2& _Lk2, _LockN&... _LkN) { // try to lock 3 or more locks
return _Try_lock_range(0, sizeof...(_LockN) + 3, _Lk0, _Lk1, _Lk2, _LkN...);
}
_EXPORT_STD template <class _Lock0, class _Lock1, class... _LockN>
_NODISCARD_TRY_CHANGE_STATE int try_lock(_Lock0& _Lk0, _Lock1& _Lk1, _LockN&... _LkN) { // try to lock multiple locks
if constexpr (sizeof...(_LockN) == 0) {
// try to lock 2 locks, special case for better codegen and reduced metaprogramming for common case
if (!_Lk0.try_lock()) {
return 0;
}

template <class _Lock0, class _Lock1>
int _Try_lock1(_Lock0& _Lk0, _Lock1& _Lk1) {
// try to lock 2 locks, special case for better codegen and reduced metaprogramming for common case
if (!_Lk0.try_lock()) {
return 0;
}
_Unlock_one_guard<_Lock0> _Guard{_Lk0};
if (!_Lk1.try_lock()) {
return 1;
}

_Unlock_one_guard<_Lock0> _Guard{_Lk0};
if (!_Lk1.try_lock()) {
return 1;
_Guard._Lk_ptr = nullptr;
return -1;
} else {
// try to lock 3 or more locks
return _Try_lock_range(0, sizeof...(_LockN) + 2, _Lk0, _Lk1, _LkN...);
}

_Guard._Lk_ptr = nullptr;
return -1;
}

_EXPORT_STD template <class _Lock0, class _Lock1, class... _LockN>
_NODISCARD_TRY_CHANGE_STATE int try_lock(_Lock0& _Lk0, _Lock1& _Lk1, _LockN&... _LkN) { // try to lock multiple locks
return _Try_lock1(_Lk0, _Lk1, _LkN...);
}

template <class... _LockN>
Expand Down Expand Up @@ -399,15 +394,6 @@ int _Lock_attempt(const int _Hard_lock, _LockN&... _LkN) {
return _Failed;
}

template <class _Lock0, class _Lock1, class _Lock2, class... _LockN>
void _Lock_nonmember1(_Lock0& _Lk0, _Lock1& _Lk1, _Lock2& _Lk2, _LockN&... _LkN) {
// lock 3 or more locks, without deadlock
int _Hard_lock = 0;
while (_Hard_lock != -1) {
_Hard_lock = _Lock_attempt(_Hard_lock, _Lk0, _Lk1, _Lk2, _LkN...);
}
}

template <class _Lock0, class _Lock1>
bool _Lock_attempt_small(_Lock0& _Lk0, _Lock1& _Lk1) {
// attempt to lock 2 locks, by first locking _Lk0, and then trying to lock _Lk1 returns whether to try again
Expand All @@ -424,16 +410,19 @@ bool _Lock_attempt_small(_Lock0& _Lk0, _Lock1& _Lk1) {
return true;
}

template <class _Lock0, class _Lock1>
void _Lock_nonmember1(_Lock0& _Lk0, _Lock1& _Lk1) {
// lock 2 locks, without deadlock, special case for better codegen and reduced metaprogramming for common case
while (_Lock_attempt_small(_Lk0, _Lk1) && _Lock_attempt_small(_Lk1, _Lk0)) { // keep trying
}
}

_EXPORT_STD template <class _Lock0, class _Lock1, class... _LockN>
void lock(_Lock0& _Lk0, _Lock1& _Lk1, _LockN&... _LkN) { // lock multiple locks, without deadlock
_Lock_nonmember1(_Lk0, _Lk1, _LkN...);
if constexpr (sizeof...(_LockN) == 0) {
// lock 2 locks, without deadlock, special case for better codegen and reduced metaprogramming for common case
while (_Lock_attempt_small(_Lk0, _Lk1) && _Lock_attempt_small(_Lk1, _Lk0)) { // keep trying
}
} else {
// lock 3 or more locks, without deadlock
int _Hard_lock = 0;
while (_Hard_lock != -1) {
_Hard_lock = _Lock_attempt(_Hard_lock, _Lk0, _Lk1, _LkN...);
}
}
}

_EXPORT_STD template <class _Mutex>
Expand Down