Thanks. Did you see an actual difference for any compiler that you tried? Did you look at the generated assembly, for example?

I haven't looked at the assembly output for Py_EnterRecursiveCall specifically, but I did see assembly differences when I removed __builtin_expect from the following very similar test program:

#include <stdio.h>

int main(int argc, char *argv)
{
    if (__builtin_expect(argc < 2, 0)) return -1;
    printf("%s\n", argv[1]);
    return 0;
}

That's ok. I just wanted to know if you saw an actual improvement in any way, because C compilers should be able to guess correctly here even without the unlikely(). The program you showed might look similar, but is actually very different when it comes to C compiler heuristics.

It's not wrong to add the unikely(), just in case.

Thanks!