Risc-V 64: unable to lowerGlobalAddress

I am trying to retarget my application to a Risc-V 64-bit core.

I downloaded LLVM 7.0.0 release today, and built LLVM+Clang for both x86_64 and Risc-V.

Alas, I have hit a snag: given a simple test program:

extern int foo;
int *getfoo() { return &foo; }

I try to compile:

/tools/llvm/7.0.0dbg/bin/clang test.c -c -o test.o --target=riscv64-unknown-elf

and I get:

fatal error: error in backend: Unable to lowerGlobalAddress
clang-7: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version 7.0.0 (tags/RELEASE_700/final)
Target: riscv64-unknown–elf
Thread model: posix

Is the 64-bit Risc-V target stable? Or do I require a specific set of flags (e.g. relocation model)?

Upstream LLVM does not yet have support for rv64.

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.