[ncmncm]

> "...C++20 finally added a way of accessing this instruction, using std::countl_zero. I don’t have C++20 though so I had to do it the old platform specific ways ..."

You don't need C++20. Even C++98 had std::bitset<>::count(), which has a nice conversion from unsigned long, and which, when compiled for SSE3 or better (e.g. Core2), uses a POPCNT instruction. It is pretty simple to produce various other results, including countl_zero, from a popcount, with just a couple of additional bitwise ops.

Modern compilers are happy to keep a bitset<32> in the same register as an unsigned long, so the conversion takes exactly zero cycles. POPCNT takes three cycles, and the extra bitwise ops another couple.