C is honestly a pretty bad language for writing modern high performance code. Between C99 and C21, there was a ~20 year gap where the language just didn't add features needed to idiomatically target lots of the new instructions added (without inline asm). Just getting good abstract machine instructions for clz/popcnt/clmul/pdep etc helps a lot for writing this kind of code.
Popcount, clz, and ctz are provided as nonstandard functions in GCC (and clang might also support them in GNU mode, but I don't know for sure). PDEP and PEXT do not seem to be, but I think they should be (and PEXT is something that INTERCAL already had, anyways) (although PDEP and PEXP can be used with -mbmi2 on x86, but are not available for general use). The MOR and MXOR of MMIX are also something that I would want to be available as built-in functions.
