I like the new <bit> header. In C++/20 they added support for BSR and BSF instructions from 1985, introduced in Intel i386. In C++/23 they even added BSWAP instruction, much newer one, introduced in 1989, in Intel i486. Most other instruction sets (ARM, Power, etc) have equivalents of these instructions.

Currently I use C++/17, have to rely on compiler-specific intrinsics. They work fine in practice, but standard library functions are IMO better.