How can I compile with LLVM/Clang to RISC-V target? - compilation

Question

I want to compile a simple program "int main(){return 0;}" to RISC-V processor.
LLVM/Clang version is 9.0 and I want to run the compiled program with a RISC-V simulator like this https://github.com/riscv/riscv-tools
My problem is that I can't list the clang supported targets only the LLC-s whith these commands:
llc --version
llc -march=xxARCHTYPExx -mattr=help
And there is no any type of riscv processor listed.
So I tried to look the triple file: llvm-project\llvm\include\llvm\ADT\Triple.h
and try a command like: clang hello.c -target riscv32 -march=rv32imafd
But I get the following error:
error: unable to create target: 'No available targets are compatible
with triple "riscv32"'
Can somebody help me to how get a valid RISC-V target? I just simple can't compile the program but I know LLVM has a RISC-V support.

Answer 1

LLVM 9 release notes explicitly state that RISC-V support was promoted from experimental to official.
And indeed, on my Fedora 31 machine, the LLVM 9 Fedora package does support RISC-V:
$ llvm-objdump --version | grep riscv
riscv32 - 32-bit RISC-V
riscv64 - 64-bit RISC-V
Also, I can create RISC-V binary code with the LLVM toolchain:
$ clang --target=riscv64 -march=rv64gc rotate.s -c -o rotate.o
$ file rotate.o
rotate.o: ELF 64-bit LSB relocatable, UCB RISC-V, version 1 (SYSV), not stripped
Although it doesn't include a libc for RISC-V targets:
$ clang --target=riscv64 -march=rv64gc hello-world.c -o hello-world
hello-world.c:1:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
^~~~~~~~~
1 error generated.
However, you don't really need one - you could directly call syscalls for your hello world, e.g.:
$ clang --target=riscv64 -march=rv64gc hello.s -c -o hello.o
$ ld.lld hello.o -o hello
$ spike --isa=RV64gc ~/local/riscv/pk/riscv64-unknown-elf/bin/pk ./hello
bbl loader
Hello World
I don't use clang for linking because it seems that I can't convince clang with -fuse-ld to use another linker besides /usr/bin/ld.