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

Yes, it is worth it.

Underneath your question, David Schwarz commented:

An uncontested mutex is nearly free. The cost of the if is probably comparable.

This is blatantly wrong (but a common misconception).
Try running this:

#include 

#include 
#include 

static std::atomic single_threaded(true);

int main(int argc, char *argv[])
{
    (void)argv;
    if (argc == 100001) { single_threaded = !single_threaded; /* to prevent compiler optimization later */ }
    int n = argc == 100000 ? -1 : 10000000;
    {
        std::mutex mutex;
        clock_t const begin = clock();
        unsigned int total = 0;
        for (int i = 0; i < n; ++i)
        {
            if (single_threaded)
            {
                total = ((total << 1) ^ i) + ((total >> 1) & i);
            }
            else
            {
                std::lock_guard lock(mutex);
                total = ((total << 1) ^ i) + ((total >> 1) & i);
            }
        }
        clock_t const end = clock();
        printf("Conditional: %u ms, total = %u\n", (unsigned int)((end - begin) * 1000U / CLOCKS_PER_SEC), total);
    }
    {
        std::mutex mutex;
        clock_t const begin = clock();
        unsigned int total = 0;
        for (int i = 0; i < n; ++i)
        {
            std::lock_guard lock(mutex);
            total = ((total << 1) ^ i) + ((total >> 1) & i);
        }
        clock_t const end = clock();
        printf("Unconditional: %u ms, total = %u\n", (unsigned int)((end - begin) * 1000U / CLOCKS_PER_SEC), total);
    }
}

My output? (Visual C++)

Conditional: 24 ms, total = 3684292139
Unconditional: 845 ms, total = 3684292139