std::atomic does and what it is used for • You’ve heard of lock-free programming and know what a compare_exchange is Daniel Anderson -- danielanderson.net4 Our motivating problem • Required by std::weak_ptr::lock uint64_t read() { return counter.load(); } std::atomic counter{1}; }; compare_exchange(expected&, desired) { if (current_value == expected) { current_value = desired; uint64_t read() { return counter.load(); } std::atomic counter{1}; }; compare_exchange(expected&, desired) { if (current_value == expected) { current_value = desired;

c_queueC++ Atomic Operations / CPU instructions ● std::atomic - provides load / store / compare_exchange ● std::atomic::is_always_lock_free ● Load and Store of aligned 8 bytes ● CAS ( old-expected data_entry{static_cast((wr_index << 1) | 1U), d}; if (_array[wr_index].compare_exchange(e, data_entry)) { // <=== DWCAS atomic instruction if constexpr (!lazy_push) empty_entry{static_cast((rd_index + _array.size()) << 1U)}; if (_array[rd_index].compare_exchange(e, empty_entry)) { // <=== DWCAS atomic instruction d = e.get_data();

collection in C++ Some assumed knowledge • You’ve heard of lock-free programming and know what a compare_exchange is • You know roughly what atomic does and what it is used for • You’ve heard of shared_ptr collection in C++ Some assumed knowledge • You’ve heard of lock-free programming and know what a compare_exchange is • You know roughly what atomic does and what it is used for • You’ve heard of shared_ptr shared_ptr load() • bool compare_exchange_weak(shared_ptr& expected, shared_ptr desired) • … compare_exchange(expected, desired) { if (current_value == expected) current_value = desired else expected

Is += a single/atomic instruction? How about ++? • How many reads/writes are needed for compare_exchange? What if it fails • foo(int volatile n) int volatile foo() are meaningless.Alex Dathskovsky