[RISC-V] OpenMP portion of the build tries to use system linker

[ppenzin]

On x86_64 Ubuntu 24.04, ultimately trying to build flang-compiler/flang#1462

$ git branch
* release_19x
$ ./build-llvm-project.sh -n 30 -t 'X86;RISCV' -p $PREFIX -c -i -X "riscv64-unknown-linux-gnu"
... LLVM build finishes, but then openmp builds start ...
CMake Error at /usr/share/cmake-3.28/Modules/CMakeTestCCompiler.cmake:67 (message):
  The C compiler

    <path here>

  is not able to compile a simple test program.

  It fails with the following output:

    ...
 
    /usr/bin/ld: unrecognised emulation mode: elf64lriscv
    Supported emulations: elf_x86_64 elf32_x86_64 elf_i386 elf_iamcu i386pep i386pe
    clang: error: linker command failed with exit code 1 (use -v to see invocation)

I tried setting CMAKE_REQUIRED_FLAGS and CMAKE_REQUIRED_LINK_OPTIONS to override system linker, but that didn't work, I think because openmp CMake actually messes around with required flags.

A related question - to build without the script I just need to set -DLLVM_ENABLE_CLASSIC_FLANG=ON, right?

[bryanpkc]

Try adding -DCMAKE_EXE_LINKER_FLAGS=-fuse-ld=lld to the CMAKE_OPTIONS variable on line 106 of build-llvm-project.sh and see if that helps.

If you build without the script, you'll still have to do it in two steps, i.e. build Clang and LLD first, then cross-compile OpenMP for RISC-V. -DLLVM_ENABLE_CLASSIC_FLANG=ON only needs to be added to the cmake comamnd for building Clang/LLD.

[ppenzin]

I tried REQUIRED_FLAGS and that did't catch, but EXE_LINKER_FLAGS worked, however it fails to find link libraries:

    ld.lld: error: unable to find library -lgcc
    ld.lld: error: unable to find library -lgcc_s
    ld.lld: error: unable to find library -lc
    ld.lld: error: unable to find library -lgcc
    ld.lld: error: unable to find library -lgcc_s

This is indicative of missing riscv sysroot.

[vielmetti]

@ppenzin if there's a sysroot available, it looks like you can point to it in CMake with this

https://cmake.org/cmake/help/latest/variable/CMAKE_SYSROOT.html

with the note "This variable may only be set in a toolchain file specified by the CMAKE_TOOLCHAIN_FILE variable."

[ppenzin]

I was thinking to just set riscv-gnu-toolchain wrapper on the path, will report back how that worked out.

[bryanpkc]

On Ubuntu 22.04.3 LTS, with CMake 3.22.1, I was able to build using the instructions I wrote. I did have to install the cross-compiling toolchain and sysroot first:

sudo apt install gcc-aarch64-linux-gnu binutils-aarch64-linux-gnu gcc-riscv64-linux-gnu binutils-riscv64-linux-gnu

Clang was able to pick up the sysroot automatically, and proceeded to build the OpenMP runtime for RISC-V successfully.

Would it help if I update the build scripts to allow a native build of Classic Flang on RISC-V Linux?

[ppenzin]

I was missing the cross toolchain part, I am sure it would work with one or the other variety of that. Can you add that to the notes?

Native RISCV build is interesting interesting in theory, though in practice a lot of available hardware is slow to the point builds like that being painful (maybe flang-compiler build natively, with LLVM cross-build).

[bryanpkc]

I was missing the cross toolchain part, I am sure it would work with one or the other variety of that. Can you add that to the notes?

I have updated the wiki page. Sorry for the confusion!

[ppenzin]

Thank you!