Using Clang to compile for RISC-V - compilation

Question

I am trying to build a hello world program using Clang (version 12.0.1) for RISC-V architecture. I have installed it with LLVM (version 12.0.1) with the following setup:
cmake -G "Unix Makefiles" \
-DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;libcxx;libcxxabi;libunwind;lldb;compiler-rt;lld;polly;debuginfo-tests" \
-DCMAKE_BUILD_TYPE=Debug \
-DLLVM_ENABLE_ASSERTIONS=On \
../llvm
According to here, default LLVM_TARGETS_TO_BUILD is LLVM_ALL_TARGETS, which includes RISC-V.
So I try to compile it with clang --target=riscv64 -march=rv64gc hello_world.c -o hello_world and I am getting the error:
hello_world.c:1:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
^~~~~~~~~
1 error generated.
At the same time, /opt/risv/, where I have installed the riscv-gnu-toolchain, is in my path, and I can run riscv64-unknown-linux-gnu-gcc hello_world.c -o hello_world without issues.
I am trying on an Ubuntu machine with kernel 5.8.0-63-generic.
Any idea how can I solve this issue and be able to compile RISC-V programs through Clang?

Answer 1

Foreknowledge:
I had same delusion about this: "If llvm is targeting for all backends including riscv, we should be able to compile our codes just giving a clang flag like --target=riscv32, -target riscv64 etc. without doing extra operation" and I had similar question, but it is not like that. Although LLVM supports riscv target, you need to specify sysroot and gcc toolchain for using riscv headers and libraries, in the other words you need cross compilation. (because you are currently operating a system like x86-64 different from riscv and your default libraries are different) That's why you need to link your riscv-gnu-toolchain paths. Here I assume that you built your riscv-gnu-toolchain from github clone.
Note: Some people have problems with first option (1) and please try to use other options (2) (3) (4) firstly. (Look into comments.)
Solution:
1-) You can add these lines as your cmake configuration flags before building your llvm library:
For 32 bit riscv:
-DDEFAULT_SYSROOT="{your-riscv-gnu-toolchain-install-or-build-path}/riscv32-unknown-elf"
-DGCC_INSTALL_PREFIX="{your-riscv-gnu-toolchain-install-or-build-path}"
For 64 bit riscv:
-DDEFAULT_SYSROOT="{your-riscv-gnu-toolchain-install-or-build-path}/riscv64-unknown-elf"
-DGCC_INSTALL_PREFIX="{your-riscv-gnu-toolchain-install-or-build-path}"
and then build llvm library again. Probably as you know, in your llvm build directory:
cmake --build .
Here is an example in my case according to your cmake configuration for clearance:
cmake -G "Unix Makefiles" \
-DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;libcxx;libcxxabi;libunwind;lldb;compiler-rt;lld;polly;debuginfo-tests" \
-DCMAKE_BUILD_TYPE=Debug \
-DLLVM_ENABLE_ASSERTIONS=On \
-DDEFAULT_SYSROOT="/home/shc/riscv/install/riscv64-unknown-elf" \
-DGCC_INSTALL_PREFIX="/home/shc/riscv/install" \
../llvm
cmake --build .
Also you can set default target triple as riscv with another cmake configuration flag:
For 32 bit riscv:
-DLLVM_DEFAULT_TARGET_TRIPLE="riscv32-unknown-elf"
For 64 bit riscv:
-DLLVM_DEFAULT_TARGET_TRIPLE="riscv64-unknown-elf"
After this you should be able to compile your codes like these examples:
For C:
/home/shc/llvm/llvm-project/build/bin/clang -march=rv64gc hello_world.c -o hello_world
For C++:
/home/shc/llvm/llvm-project/build/bin/clang++ -march=rv64gc hello_world.cpp -o hello_world
If you didn't set default target triple with cmake flag, target option should remain:
For C:
/home/shc/llvm/llvm-project/build/bin/clang --target=riscv64 -march=rv64gc hello_world.c -o hello_world
For C++:
/home/shc/llvm/llvm-project/build/bin/clang++ --target=riscv64 -march=rv64gc hello_world.cpp -o hello_world
2-) You can pass sysroot and gcc toolchain as flags(as in cmake configurations above) while compiling your code without building your library again. However, if you will use this, you need to give these flags in every compilation:
For 32 bit riscv:
--sysroot="{your-riscv-gnu-toolchain-install-or-build-path}/riscv32-unknown-elf"
--gcc-toolchain="{your-riscv-gnu-toolchain-install-or-build-path}"
For 64 bit riscv:
--sysroot="{your-riscv-gnu-toolchain-install-or-build-path}/riscv64-unknown-elf"
--gcc-toolchain="{your-riscv-gnu-toolchain-install-or-build-path}"
Usage of Flags For C:
/home/shc/llvm/llvm-project/build/bin/clang --sysroot=/home/shc/riscv/install/riscv64-unknown-elf --gcc-toolchain=/home/shc/riscv/install --target=riscv64 -march=rv64gc hello_world.c -o hello_world
Usage of Flags For C++:
/home/shc/llvm/llvm-project/build/bin/clang++ --sysroot=/home/shc/riscv/install/riscv64-unknown-elf --gcc-toolchain=/home/shc/riscv/install --target=riscv64 -march=rv64gc hello_world.cpp -o hello_world
3-) You can try to pass these flags while compiling (instead of your --target=riscv64 flag) although it is not more healthy than above options. (Attention: difference is only linux keyword, normally it was -target riscv32-unknown-elf):
For 32 bit riscv:
-target riscv32-unknown-linux-elf
For 64 bit riscv:
-target riscv64-unknown-linux-elf
4-) You can use riscv-llvm repo by following given instructions, although it is outdated.
Note: I adapted my file locations to your examples to better understanding.
Further information you can look here: https://github.com/lowRISC/riscv-llvm#how-can-i-build-upstream-llvmclang-and-use-it-to-cross-compile-for-a-riscv32-target

Answer 2

Finally i deal with that! i`m used next keys:
cmake -G "Unix Makefiles" \
-DCMAKE_BUILD_TYPE="Release" \
-DBUILD_SHARED_LIBS=True \
-DCMAKE_INSTALL_PREFIX="/home/username/llvm-install" \
-DLLVM_OPTIMIZED_TABLEGEN=ON -DLLVM_BUILD_TESTS=False \
-DDEFAULT_SYSROOT="/home/username/esp-toolchain-install/riscv64-unknown-elf" \
-DGCC_INSTALL_PREFIX="/home/username/esp-toolchain-install" \
-DLLVM_ENABLE_PROJECTS="clang;lld" \
-DLLVM_DEFAULT_TARGET_TRIPLE="riscv64-unknown-elf" \
-DLLVM_TARGETS_TO_BUILD="RISCV" ../llvm
and i copied content of esp-toolchain-install/* into llvm-install/*. And clang finally saw all of GCC files:
clang -v -print-search-dirs
clang version 12.0.1 (https://github.com/llvm/llvm-project.git fed41342a82f5a3a9201819a82bf7a48313e296b)
Target: riscv64-unknown-unknown-elf
Thread model: posix
InstalledDir: /home/username/llvm-install/bin
Found candidate GCC installation: /home/username/llvm-install/bin/../lib/gcc/riscv64-unknown-elf/9.2.0
Selected GCC installation: /home/username/llvm-install/bin/../lib/gcc/riscv64-unknown-elf/9.2.0
Candidate multilib: rv32i/ilp32;#march=rv32i#mabi=ilp32
Candidate multilib: rv32im/ilp32;#march=rv32im#mabi=ilp32
Candidate multilib: rv32iac/ilp32;#march=rv32iac#mabi=ilp32
Candidate multilib: rv32imac/ilp32;#march=rv32imac#mabi=ilp32
Candidate multilib: rv32imafc/ilp32f;#march=rv32imafc#mabi=ilp32f
Candidate multilib: rv64imac/lp64;#march=rv64imac#mabi=lp64
Candidate multilib: rv64imafdc/lp64d;#march=rv64imafdc#mabi=lp64d
Selected multilib: rv64imac/lp64;#march=rv64imac#mabi=lp64
programs: =/home/username/llvm-install/bin:/home/username/llvm-install/bin/../lib/gcc/riscv64-unknown-elf/9.2.0/../../../../riscv64-unknown-elf/bin:/home/username/llvm-install/bin/../lib/gcc/riscv64-unknown-elf/9.2.0/../../../../bin
libraries: =/home/username/llvm-install/lib/clang/12.0.1:/home/username/llvm-install/bin/../lib/gcc/riscv64-unknown-elf/9.2.0/rv64imac/lp64:/home/username/llvm-install/bin/../lib/gcc/riscv64-unknown-elf/9.2.0/../../../../riscv64-unknown-elf/lib/rv64imac/lp64:/home/username/llvm-install/bin/../lib/gcc/riscv64-unknown-elf/9.2.0:/home/username/esp-toolchain-install/riscv64-unknown-elf/lib