In practice, at least C compilers don't automatically generate code for multiple extensions, so you get the performance of the base ISA by default. This does in

In practice, at least C compilers don't automatically generate code for multiple extensions, so you get the performance of the base ISA by default. This does indeed mean that every C program for x86-64 that isn't compiled with a specific target architecture just won't ever use AVX/AVX2/AVX-512/any other extension. There is the idea of profiles, i.e. a single nice name for a group of extensions so that there's something one could provide a precompiled binary for, but I don't know what, if any, are RISC-V's plans for that.