Understanding std::atomic::compare_exchange_weak() in C++11

Question

bool compare_exchange_weak (T& expected, T val, ..);

compare_exchange_weak() is one of compare-exchange primitives provided in C++

Answer 1

I think most of the answers above address "spurious failure" as some kind of problem, performance VS correctness tradeoff.

It can be seen as the weak version is faster most of the times, but in case of spurious failure, it becomes slower. And the strong version is a version that has no possibility of spurious failure, but it is almost always slower.

For me, the main difference is how these two version handle the ABA problem:

weak version will succeed only if noone has touched the cache line between load and store, so it will 100% detect ABA problem.

strong version will fail only if the comparison fails, so it will not detect ABA problem without extra measures.

So, in theory, if you use weak version on weak-ordered architecture, you don't need ABA detection mechanism and the implementation will be much simpler, giving better performance.

But, on x86 (strong-ordered architecture), weak version and strong version are the same, and they both suffer from ABA problem.

So if you write a completely cross-platform algorithm, you need to address ABA problem anyway, so there is no performance benefit from using the weak version, but there is a performance penalty for handling spurious failures.

In conclusion - for portability and performance reasons, the strong version is always a better-or-equal option.

Weak version can only be a better option if it lets you skip ABA countermeasures completely or your algorithm doesn't care about ABA.