Question

I want to save memory by converting an existing 32 bit counter to a 16 bit counter. This counter is atomically incremented/decremented. If I do this:

1. What instructions do I use for atomic_inc(uint16_t x) on x86/x86_64?
2. Is this reliable in multi-processor x86/x86_64 machines?
3. Is there a performance penalty to pay on any of these architectures for doing this?
4. If yes for (3), what’s the expected performance penalty?

Thanks for your comments!

Answer 1

Here’s one that uses GCC assembly extensions, as an alternative to Steve’s Delphi answer:

uint16_t atomic_inc(uint16_t volatile* ptr)
{
    uint16_t value(1);
    __asm__("lock xadd %w0, %w1" : "+r" (value) : "m" (*ptr));
    return ++value;
}

Change the 1 with -1, and the ++ with --, for decrement.