For now multiple set functions call helpers that returns one of three possible values (0, 1, and -1 for error) and then analyze return code in the loop.

    while (...) {
        rv = some_set_operation();
        if (rv < 0) {
            // clean up
            return NULL;
        }
        if (rv) {
            other_set_operation();
        }
        // if (rv == 0) do nothing
    }

If rv is 0 or 1, it is tested two times. If rv is -1 (unlikely), it is tested only once.

It is possible to rewrite testing in other way:

    while (...) {
        rv = some_set_operation();
        if (rv != 0) { // 1 or -1
            if (rv < 0) {
                // clean up
                return NULL;
            }
            other_set_operation();
        }
    }

Now if rv is 0 (common case), it is tested only once, and likely the test jump will be merged with unconditional loop jump.

I have no benchmarking results, but I suppose that such rewritting will generate better machine code.