There is support for rv64 in the patches at https://github.com/lowRISC/riscv-llvm but if you follow the instructions there you’ll be missing six months of other important and useful improvements to RISC-V LLVM.
Hi David. The 64-bit target is not stable, though basically "just
works" with some simple patching due to the use of parameterised
register classes in the design of the backend. Your email is well
timed, as I'm currently cleaning up that support ready for review. Not
much work has been done in evaluating generating code quality though.
In summary: things are about to start moving much more quickly in
regards to RV64, but you're likely to see codegen oddities or bugs for
a little while.