docs: LLVM OpenMP build may require clang project even when system clang is installed

[rintaro-okahara]

Description

I am trying to build Codon's LLVM fork by following the build-from-source documentation by following official docs (link)

The documentation currently suggests the LLVMdependency with:

git clone --depth 1 -b codon https://github.com/exaloop/llvm-project
cmake -S llvm-project/llvm -B llvm-project/build \
    -DCMAKE_BUILD_TYPE=Release \
    -DLLVM_INCLUDE_TESTS=OFF \
    -DLLVM_ENABLE_RTTI=ON \
    -DLLVM_ENABLE_ZLIB=OFF \
    -DLLVM_ENABLE_ZSTD=OFF \
    -DLLVM_ENABLE_PROJECTS="openmp" \
    -DLLVM_TARGETS_TO_BUILD=all

It also says:

You can use -DLLVM_ENABLE_PROJECTS="clang;openmp" if you do not have clang installed on your system.

However, on my machine, clang is already installed, but the CMake generation step still falls because the OpenMP test targets depend on a CMake target named clang , not. just a system-installed clang

The error log is below

Make Error at cmake/modules/AddLLVM.cmake:2100 (add_dependencies):
  The dependency target "clang" of target "check-openmp" does not exist.
Call Stack (most recent call first):
  cmake/modules/AddLLVM.cmake:2167 (add_lit_target)
  /Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:225 (add_lit_testsuite)
  /Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:248 (add_openmp_testsuite)
  /Users/rintaro/work/dev/private/llvm-project/openmp/CMakeLists.txt:156 (construct_check_openmp_target)


CMake Error at cmake/modules/AddLLVM.cmake:2100 (add_dependencies):
  The dependency target "clang" of target "check-libomp" does not exist.
Call Stack (most recent call first):
  cmake/modules/AddLLVM.cmake:2167 (add_lit_target)
  /Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:233 (add_lit_testsuite)
  /Users/rintaro/work/dev/private/llvm-project/openmp/runtime/test/CMakeLists.txt:44 (add_openmp_testsuite)


CMake Error at cmake/modules/AddLLVM.cmake:2100 (add_dependencies):
  The dependency target "clang" of target "check-ompt" does not exist.
Call Stack (most recent call first):
  cmake/modules/AddLLVM.cmake:2167 (add_lit_target)
  /Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:225 (add_lit_testsuite)
  /Users/rintaro/work/dev/private/llvm-project/openmp/runtime/test/CMakeLists.txt:46 (add_openmp_testsuite)

After checking the LLVM/OpenMP CMake files, this appears to be a CMake target dependency rather than a lookup for a clang executable on PATH.

Relevant reference is shown below.

• https://github.com/exaloop/llvm-project/blob/codon/openmp/cmake/OpenMPTesting.cmake#L223-L237
• https://github.com/exaloop/llvm-project/blob/codon/llvm/cmake/modules/AddLLVM.cmake#L2099-L2101

My Environment

• OS : macOS
• Architecture : arm64