When you configure RISC-V gcc you tell it what default set of extensions you want it to use, but you can always tell it a different set with -march. I think tha

When you configure RISC-V gcc you tell it what default set of extensions you want it to use, but you can always tell it a different set with -march. I think that can't even be disabled. It may be that the toolchains you've seen were built with only one set of libraries. If you specify --enable-multilib when you build the toolchain then libraries for a dozen or so common combinations of extensions are built. If you're doing something embedded then you can build precisely the core and toolchain you want. If you're doing an OS for packaged software then you'll specify one (or a very small number) of combinations of extensions. Linux distributions such as Debian and Fedora decided on RV64GC a few years ago already. By the end of this year the B and V extensions (and a few smaller ones) should be ratified and the Linux distros will make a second set of supported extensions.