avoid cost of std::mutex when not multi-threading?

Question

Suppose I have an application that may or may not have spawned multiple threads. Is it worth it to protect operations that need synchronization conditionally with a std::mutex a

Answer 1

I disagree with wide-spread idea that locking mutext is cheap. If you really are after performance, you wouldn't want to do this.

Mutexes (even uncontested) hit you with three hummers: they penalize compiler optimizations (mutexes are optimization barriers), they incure memory fences (on un-pessimized platforms) and they are kernel calls. So if you are after nanoseconds performance in tight loops, it is something worth considering.

Branching is not great, either - for multiple reasons. The real solution is to avoid operatations requiring synchronization in multithread environment. As simple as that.