[izabera]

atomics aren't free even without contention. the slogan of the language is "you don't pay for what you don't use", and it's really not great that there's no non atomic refcount in the standard. the fact that it is default atomic has also lead people to assume guarantees that it doesn't provide, which was trivially predictable when the standard first introduced it.

[loeg]

OP specifically mentioned contention, though -- not marginally higher cost of atomic inc/dec vs plain inc/dec.

> For our use case, we in fact do not use std::shared_ptr in our implementation, but instead a single-threaded shared_ptr-like class that has no atomics (to avoid cross-core contention).

A single-threaded program will not have cross-core contention whether it uses std::atomic<> refcounts or plain integer refcounts, period. You're right that non-atomic refcounts can be anywhere from somewhat cheaper to a lot cheaper than atomic refcounts, depending on that platform. But that is orthogonal to cross-core contention.

[SkiFire13]

> not marginally higher cost of atomic inc/dec vs plain inc/dec.

Note that the difference is not so marginal, and the difference is not just in hardware instructions as the non-atomic operations generally allow for more optimizations by the compiler.

[vlovich123]

Can’t you have cross core contention just purely because of other processes doing atomics that happen to have a cache line address collision in the lock broadcast?

[eMSF]

Related to this, GNU's libstdc++ shared_ptr implementation actually opts not to use atomic arithmetic when it infers that the program is not using threads.

[menaerus]

I never heard of this and went to check in the source and it really does exist: https://codebrowser.dev/llvm/include/c++/11/ext/concurrence....

[grogers]

People assume non-existent guarantees such as?

[izabera]

"is shared_ptr thread safe?" is a classic question asked thousands of times. the answer by the way is "it's as thread safe as a regular pointer"