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[libc][CPP] correct cmpxchg failure ordering inference #133127

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Merged
merged 5 commits into from
Mar 26, 2025
Merged

[libc][CPP] correct cmpxchg failure ordering inference #133127

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[libc][CPP] correct cmpxchg failure ordering inference
SchrodingerZhu Mar 26, 2025
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address comments
SchrodingerZhu Mar 26, 2025
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address comments
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23 changes: 20 additions & 3 deletions libc/src/__support/CPP/atomic.h
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,14 @@ template <typename T> struct Atomic {
return static_cast<int>(mem_scope);
}

LIBC_INLINE static constexpr int infer_failure_order(MemoryOrder mem_ord) {
if (mem_ord == MemoryOrder::RELEASE)
return order(MemoryOrder::RELAXED);
if (mem_ord == MemoryOrder::ACQ_REL)
return order(MemoryOrder::ACQUIRE);
return order(mem_ord);
}

LIBC_INLINE static T *addressof(T &ref) { return __builtin_addressof(ref); }

// Require types that are 1, 2, 4, 8, or 16 bytes in length to be aligned to
Expand Down Expand Up @@ -129,7 +137,7 @@ template <typename T> struct Atomic {
[[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
return __atomic_compare_exchange(addressof(val), addressof(expected),
addressof(desired), false, order(mem_ord),
order(mem_ord));
infer_failure_order(mem_ord));
}

// Atomic compare exchange (separate success and failure memory orders)
Expand All @@ -148,7 +156,7 @@ template <typename T> struct Atomic {
[[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
return __atomic_compare_exchange(addressof(val), addressof(expected),
addressof(desired), true, order(mem_ord),
order(mem_ord));
infer_failure_order(mem_ord));
}

// Atomic compare exchange (weak version with separate success and failure
Expand Down Expand Up @@ -252,6 +260,14 @@ template <typename T> struct AtomicRef {
return static_cast<int>(mem_scope);
}

LIBC_INLINE static constexpr int infer_failure_order(MemoryOrder mem_ord) {
if (mem_ord == MemoryOrder::RELEASE)
return order(MemoryOrder::RELAXED);
if (mem_ord == MemoryOrder::ACQ_REL)
return order(MemoryOrder::ACQUIRE);
return order(mem_ord);
}

public:
// Constructor from T reference
LIBC_INLINE explicit constexpr AtomicRef(T &obj) : ptr(&obj) {}
Expand Down Expand Up @@ -298,7 +314,8 @@ template <typename T> struct AtomicRef {
T &expected, T desired, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
[[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) const {
return __atomic_compare_exchange(ptr, &expected, &desired, false,
order(mem_ord), order(mem_ord));
order(mem_ord),
infer_failure_order(mem_ord));
}

// Atomic compare exchange (strong, separate success/failure memory orders)
Expand Down
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