[tyfighter]

The BSF/BSR quirk is annoying, but I think there is a reason for it is that they were only thinking about it being used in a loop (or maybe an if) with something like:

int mask = something; ... for (int index; _bit_scan_forward(&index, mask); mask ^= 1<<index) { ... }

Since it sets the ZF on a zero input, they thought that must be all you need. But there are many other uses for (trailing|leading) zero count operations, and it would have been much better for them to just write the register anyway.