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Question 1624

Flag Without A Fence

int payload = 0;                       // plain int
std::atomic<bool> ready{false};

void producer() {
    payload = 42;
    ready.store(true, std::memory_order_relaxed);
}

void consumer() {
    while (!ready.load(std::memory_order_relaxed)) { }
    assert(payload == 42);             // fires on ARM; on x86 the compiler may still sink the payload store
}
Relaxed store and relaxed load leave the payload unordered producer consumer payload = 42 ready.store(true, relaxed) ready.load(relaxed) == true reads payload: 42 or 0? no ordering edge

Which single change makes the assertion hold on every platform?