> For what it's worth, this is largely unnecessary nowadays.

Not true, compilers remain extremely conservative in their default codegen, we’re talking pre-nehalem (SSE2 at the top end).

I just checked on godbolt, by default u32::count_ones compiled with rustc 1.62 with -O generates 15 lines of assembly hand-counting the bits in the word.

Adding `-C target-cpu=x86-64-v2` reduces that to a single popcnt.

Even if you don’t care for SSE3+, there’s a handful of really useful instructions which are not part of the baseline x86 profile, especially for bit-twiddling and endianness manipulation: popcnt, lzcnt (v3), movbe (v3), BMI1 and BMI2 (v3)