I know GCC supports generating multiple versions of a function, compiled for different instruction set extensions. And this can also be done manually when you have hand-optimized implementations: https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Common-Functio... It's based on a GNU extension to the ELF format, not preprocessor directives.

I don't think any of that would conflict with using bitcode for everything you don't have a hand-optimized machine-specific implementation of.