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Fix atomic_ref::is_lock_free() on x64 #4729

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Merged
merged 3 commits into from
Jun 27, 2024
Merged

Fix atomic_ref::is_lock_free() on x64 #4729

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6e81ed6
Fix GH 4728.
StephanTLavavej Jun 16, 2024
5e03fc6
Test GH 4728.
StephanTLavavej Jun 16, 2024
a76faa5
Use `if constexpr (_Is_potentially_lock_free)`.
StephanTLavavej Jun 17, 2024
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4 changes: 3 additions & 1 deletion stl/inc/atomic
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Original file line number Diff line number Diff line change
Expand Up @@ -2370,8 +2370,10 @@ public:
#else // ^^^ _ATOMIC_HAS_DCAS / !_ATOMIC_HAS_DCAS vvv
if constexpr (is_always_lock_free) {
return true;
} else {
} else if constexpr (_Is_potentially_lock_free) {
return __std_atomic_has_cmpxchg16b() != 0;
} else {
return false;
}
#endif // ^^^ !_ATOMIC_HAS_DCAS ^^^
}
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17 changes: 17 additions & 0 deletions tests/std/tests/P0019R8_atomic_ref/test.cpp
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Expand Up @@ -447,6 +447,22 @@ void test_gh_4472() {
assert(ar.is_lock_free());
}

// GH-4728 "<atomic>: On x64, atomic_ref::is_lock_free() incorrectly returns true when it shouldn't"
void test_gh_4728() {
struct Large {
char str[100]{};
};

alignas(std::atomic_ref<Large>::required_alignment) Large lg{};

static_assert(std::atomic_ref<Large>::required_alignment == alignof(Large));

static_assert(!std::atomic_ref<Large>::is_always_lock_free);

std::atomic_ref<Large> ar{lg};
assert(!ar.is_lock_free());
}

int main() {
test_ops<false, char>();
test_ops<false, signed char>();
Expand Down Expand Up @@ -500,6 +516,7 @@ int main() {

test_gh_1497();
test_gh_4472();
test_gh_4728();
}

#endif // ^^^ run test ^^^