> Also, from a strictly prose point of view, isn't it strange that the `clz` instruction

It's using the `bsr` instruction which is similar (but worse). The `lzcnt` instruction in x86_64 is a part of the BMI feature introduced in Intel Haswell. The compiler does not generate these instructions by default so it runs on any x86_64.

If you add `-mbmi` or `-march=haswell` or newer to the compiler command line, you should get `clz`/`lzcnt` instead.