TL;DR, clang in XCode toolchain (I call it Apple Clang, run xcrun -f clang to locate) doesn’t support -fopenmp option. In order to use OpenMP, follow this post and this PR.

    CACHE BOOL "build on x86 architecture")

    CACHE STRING "build in place")

# build openmp from openmp source tree

    CACHE STRING "build time generated omp.h path")

target_include_directories(omp INTERFACE ${TURI_OMP_H_PATH})

In short, Apple Clang doesn’t support ARM architecture, which is discussed in my previous posts, and ON_X86 is set explicitly here. LIBOMP_COPY_EXPORTS is set to FALSE to prevent OpenMP from moving the architecture dependent and on-the-fly generated omp.h to somewhere else but to put it in place. The last line is to explicitly add an include path to the CMake target omp set in OpenMP source, in order to provide OpenMP progma directive location to the compiler to use -fopenmp, as it is mentioned by post.

To compile with -fopenmp with Apple clang,

# equivalent to -fopenmp -I/path/to/openmp/include ...
target_compile_options(dummy PRIVATE -fopenmp)
target_link_libraries(dummy PRIVATE omp)

Build structure and process

Recently, I’ve been working on integrating OpenMP to Turicreate codebase. Let me elaborate through the approaches I have tried.

Before digging into details, there’s one point I want to clarify. TuriCreate has only one src tree, rooted at src, and 2 build trees, rooted at debug and release respectively. The layout is quite simple:

├── CMakeLists.txt
├── debug
├── deps
├── release
├── src

External Project, out-of-tree build

deps contains the source of libraries that are built out-of-source-tree, and those libraries are called as external dependencies.

The overall build process is that we build the external dependencies, and install them into a known location outside of build tree, located at turicreate/deps/local/{lib,lib64,include}, where turicreate refers to the root of the project and I will use this convention from now on.

After all external dependencies are built, those external libraries are known at compile and link time when we compile the source tree and put all build into the build tree. ExternalProject_add is a great fit for this task!

With everything set up, I get an unexpected error:

clang: error: unsupported option '-fopenmp'

I haven’t realized this is a the problem from the clang I’m using. So I proceed to use in-tree build because I suspect there are some extra hidden flags set by Apple Clang used by xcodebuild.

In-source build with xcodebuild

This is the approach actually that gets adopted. For details, check this PR.

The in-tree build is to put the source file under turicreate/src/external/. During the build, the OpenMP source file will be treated as part of TuriCreate source and xcodebuild will pick up these source to compile with flags that are potentially missing in out-tree build.

├── CMakeLists.txt
├── crt_externs.h
└── openmp-src
    ├── CMakeLists.txt
    ├── CREDITS.txt
    ├── LICENSE.txt
    ├── README.rst
    ├── cmake
    ├── libomptarget
    ├── runtime
    └── www

I copy the source of OpenMP to turicreate/src/external/openmp/openmp-src and put a CMakeLists.txt file, with customized build configurations, outside of the openmp-src at openmp directory.

set(_openmp_dir openmp-src)

# custom settings
    CACHE STRING "build in place")

# if <crt_extern.h> is missing
# add <crt_extern.h> manually

# build openmp

The full CMakeLists.txt can be found here. To pick up target openmp into build process, one-liner add_subdirectory(openmp) is added into turicreate/src/external/CMakeLists.txt.

And, I get the same error in the end.

clang: error: unsupported option '-fopenmp'

Both in- and out-tree build doesn’t work. The -fopenmp option probably is not supported by the clang I’m using.

final call of faith

Let’s get to the final call.