The hardest part is probably <setjmp.h>. We tried once on s390x (after it introduced vector registers), and it didn't work: https://lwn.net/Articles/605607/ And was eventually reverted: https://lwn.net/Articles/606377/

Not every function boundary is an ABI boundary, and compilers can and will automatically use custom ABIs for static functions (and, with LTO, for unexported global functions, too).