These rely on the hilarious BSR and BSF instructions on x64. ARMv8 has
an instruction that does something similar: count leading zeros.
Unfortunately, their semantics differ in a few important ways:
- BSR and BSF, when given a zero input, set the zero flag and have
undefined output. CLZ outputs 64.
- BSR and BSF return the index (starting from 0 at the LSB) of the most- or
least-significant 1 bit. CLZ does what it says: count leading zeros.
Bottom line, there has to be some bit trickery somewhere to fit the two
instructions into the same interface. I kept the x64 implementations the
same and put all the nastiness in the ARM implementations to make them
match, since obviously the perf of the ARM implementations doesn't
matter yet.
Owen Yamauchi