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How to Build the LLVM* OpenMP* Libraries

This repository requires CMake v2.8.0 or later. LLVM and Clang need a more recent version which also applies for in-tree builds. For more information than available in this document please see LLVM's CMake documentation and the official documentation.

How to Call CMake Initially, then Repeatedly

  • When calling CMake for the first time, all needed compiler options must be specified on the command line. After this initial call to CMake, the compiler definitions must not be included for further calls to CMake. Other options can be specified on the command line multiple times including all definitions in the build options section below.

  • Example of configuring, building, reconfiguring, rebuilding:

    $ mkdir build
$ cd build
$ cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ..  # Initial configuration
$ make
...
$ make clean
$ cmake -DCMAKE_BUILD_TYPE=Debug ..                               # Second configuration
$ make
...
$ rm -rf *
$ cmake -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ ..        # Third configuration
$ make

  • Notice in the example how the compiler definitions are only specified for an empty build directory, but other build options are used at any time.

  • The file CMakeCache.txt which is created after the first call to CMake is a configuration file which holds all values for the build options. These values can be changed using a text editor to modify CMakeCache.txt as opposed to using definitions on the command line.

  • To have CMake create a particular type of build generator file simply include the -G <Generator name> option:

    $ cmake -G "Unix Makefiles" ...

    You can see a list of generators CMake supports by executing the cmake command with no arguments.

Instructions to Build

$ cd openmp_top_level/ [ this directory with libomptarget/, runtime/, etc. ]
$ mkdir build
$ cd build

[ Unix* Libraries ]
$ cmake -DCMAKE_C_COMPILER=<C Compiler> -DCMAKE_CXX_COMPILER=<C++ Compiler> ..

[ Windows* Libraries ]
$ cmake -G <Generator Type> -DCMAKE_C_COMPILER=<C Compiler> -DCMAKE_CXX_COMPILER=<C++ Compiler> -DCMAKE_ASM_MASM_COMPILER=[ml | ml64] -DCMAKE_BUILD_TYPE=Release ..

$ make
$ make install

CMake Options

Builds with CMake can be customized by means of options as already seen above. One possibility is to pass them via the command line:

$ cmake -DOPTION=<value> path/to/source

Note

The first value listed is the respective default for that option.

Generic Options

For full documentation consult the CMake manual or execute cmake --help-variable VARIABLE_NAME to get information about a specific variable.

CMAKE_BUILD_TYPE = Release|Debug|RelWithDebInfo
Build type can be Release, Debug, or RelWithDebInfo which chooses the optimization level and presence of debugging symbols.
CMAKE_C_COMPILER = <C compiler name>
Specify the C compiler.
CMAKE_CXX_COMPILER = <C++ compiler name>
Specify the C++ compiler.
CMAKE_Fortran_COMPILER = <Fortran compiler name>
Specify the Fortran compiler. This option is only needed when LIBOMP_FORTRAN_MODULES is ON (see below). So typically, a Fortran compiler is not needed during the build.
CMAKE_ASM_MASM_COMPILER = ml|ml64
This option is only relevant for Windows*.

Options for all Libraries

OPENMP_ENABLE_WERROR = OFF|ON
Treat warnings as errors and fail, if a compiler warning is triggered.
OPENMP_LIBDIR_SUFFIX = ""
Extra suffix to append to the directory where libraries are to be installed.
OPENMP_TEST_C_COMPILER = ${CMAKE_C_COMPILER}
Compiler to use for testing. Defaults to the compiler that was also used for building.
OPENMP_TEST_CXX_COMPILER = ${CMAKE_CXX_COMPILER}
Compiler to use for testing. Defaults to the compiler that was also used for building.
OPENMP_TEST_Fortran_COMPILER = ${CMAKE_Fortran_COMPILER}
Compiler to use for testing. Defaults to the compiler that was also used for building. Will default to flang if build is in-tree.
OPENMP_LLVM_TOOLS_DIR = /path/to/built/llvm/tools
Additional path to search for LLVM tools needed by tests.
OPENMP_LLVM_LIT_EXECUTABLE = /path/to/llvm-lit
Specify full path to llvm-lit executable for running tests. The default is to search the PATH and the directory in OPENMP_LLVM_TOOLS_DIR.
OPENMP_FILECHECK_EXECUTABLE = /path/to/FileCheck
Specify full path to FileCheck executable for running tests. The default is to search the PATH and the directory in OPENMP_LLVM_TOOLS_DIR.
OPENMP_NOT_EXECUTABLE = /path/to/not
Specify full path to not executable for running tests. The default is to search the PATH and the directory in OPENMP_LLVM_TOOLS_DIR.

Options for libomp

LIBOMP_ARCH = aarch64|arm|i386|loongarch64|mic|mips|mips64|ppc64|ppc64le|x86_64|riscv64|s390x
The default value for this option is chosen based on probing the compiler for architecture macros (e.g., is __x86_64__ predefined by compiler?).
LIBOMP_MIC_ARCH = knc|knf
Intel(R) Many Integrated Core Architecture (Intel(R) MIC Architecture) to build for. This value is ignored if LIBOMP_ARCH does not equal mic.
LIBOMP_LIB_TYPE = normal|profile|stubs
Library type can be normal, profile, or stubs.
LIBOMP_USE_VERSION_SYMBOLS = ON|OFF
Use versioned symbols for building the library. This option only makes sense for ELF based libraries where version symbols are supported (Linux*, some BSD* variants). It is OFF by default for Windows* and macOS*, but ON for other Unix based operating systems.
LIBOMP_ENABLE_SHARED = ON|OFF

Build a shared library. If this option is OFF, static OpenMP libraries will be built instead of dynamic ones.

Note

Static libraries are not supported on Windows*.

LIBOMP_FORTRAN_MODULES = OFF|ON
Create the Fortran modules (requires Fortran compiler).

macOS* Fat Libraries

On macOS* machines, it is possible to build universal (or fat) libraries which include both i386 and x86_64 architecture objects in a single archive.

$ cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_OSX_ARCHITECTURES='i386;x86_64' ..
$ make

There is also an option LIBOMP_OSX_ARCHITECTURES which can be set in case this is an LLVM source tree build. It will only apply for the libomp library avoids having the entire LLVM/Clang build produce universal binaries.

Optional Features

LIBOMP_USE_ADAPTIVE_LOCKS = ON|OFF
Include adaptive locks, based on Intel(R) Transactional Synchronization Extensions (Intel(R) TSX). This feature is x86 specific and turned ON by default for IA-32 architecture and Intel(R) 64 architecture.
LIBOMP_USE_INTERNODE_ALIGNMENT = OFF|ON
Align certain data structures on 4096-byte. This option is useful on multi-node systems where a small CACHE_LINE setting leads to false sharing.
LIBOMP_OMPT_SUPPORT = ON|OFF
Include support for the OpenMP Tools Interface (OMPT). This option is supported and ON by default for x86, x86_64, AArch64, PPC64, RISCV64, LoongArch64, and s390x on Linux* and macOS*. This option is OFF if this feature is not supported for the platform.
LIBOMP_OMPT_OPTIONAL = ON|OFF
Include support for optional OMPT functionality. This option is ignored if LIBOMP_OMPT_SUPPORT is OFF.
LIBOMP_STATS = OFF|ON
Include stats-gathering code.
LIBOMP_USE_DEBUGGER = OFF|ON
Include the friendly debugger interface.
LIBOMP_USE_HWLOC = OFF|ON
Use OpenMPI's hwloc library for topology detection and affinity.
LIBOMP_HWLOC_INSTALL_DIR = /path/to/hwloc/install/dir
Specify install location of hwloc. The configuration system will look for hwloc.h in ${LIBOMP_HWLOC_INSTALL_DIR}/include and the library in ${LIBOMP_HWLOC_INSTALL_DIR}/lib. The default is /usr/local. This option is only used if LIBOMP_USE_HWLOC is ON.

Additional Compiler Flags

These flags are appended, they do not overwrite any of the preset flags.

LIBOMP_CPPFLAGS = <space-separated flags>
Additional C preprocessor flags.
LIBOMP_CXXFLAGS = <space-separated flags>
Additional C++ compiler flags.
LIBOMP_ASMFLAGS = <space-separated flags>
Additional assembler flags.
LIBOMP_LDFLAGS = <space-separated flags>
Additional linker flags.
LIBOMP_LIBFLAGS = <space-separated flags>
Additional libraries to link.
LIBOMP_FFLAGS = <space-separated flags>
Additional Fortran compiler flags.

Options for libomptarget

An installed LLVM package is a prerequisite for building libomptarget library. So libomptarget may only be built in two cases:

  • As a project of a regular LLVM build via LLVM_ENABLE_PROJECTS, LLVM_EXTERNAL_PROJECTS, or LLVM_ENABLE_RUNTIMES or
  • as a standalone project build that uses a pre-installed LLVM package. In this mode one has to make sure that the default CMake find_package(LLVM) call succeeds.
LIBOMPTARGET_OPENMP_HEADER_FOLDER = ""
Path of the folder that contains omp.h. This is required for testing out-of-tree builds.
LIBOMPTARGET_OPENMP_HOST_RTL_FOLDER = ""
Path of the folder that contains libomp.so, and libLLVMSupport.so when profiling is enabled. This is required for testing.

Options for NVPTX device RTL

LIBOMPTARGET_NVPTX_ENABLE_BCLIB = ON|OFF
Enable CUDA LLVM bitcode offloading device RTL. This is used for link time optimization of the OMP runtime and application code. This option is enabled by default if the build system determines that CMAKE_C_COMPILER is able to compile and link the library.
LIBOMPTARGET_NVPTX_CUDA_COMPILER = ""
Location of a CUDA compiler capable of emitting LLVM bitcode. Currently only the Clang compiler is supported. This is only used when building the CUDA LLVM bitcode offloading device RTL. If unspecified, either the Clang from the build itself is used (i.e. an in-tree build with LLVM_ENABLE_PROJECTS including clang), or the Clang compiler that the build uses as C compiler (CMAKE_C_COMPILER; only if it is Clang). The latter is common for a stage2-build or when using -DLLVM_ENABLE_RUNTIMES=openmp.
LIBOMPTARGET_NVPTX_BC_LINKER = ""
Location of a linker capable of linking LLVM bitcode objects. This is only used when building the CUDA LLVM bitcode offloading device RTL. If unspecified, either the llvm-link in that same directory as LIBOMPTARGET_NVPTX_CUDA_COMPILER is used, or the llvm-link from the same build (available in an in-tree build).
LIBOMPTARGET_NVPTX_ALTERNATE_HOST_COMPILER = ""
Host compiler to use with NVCC. This compiler is not going to be used to produce any binary. Instead, this is used to overcome the input compiler checks done by NVCC. E.g. if using a default host compiler that is not compatible with NVCC, this option can be use to pass to NVCC a valid compiler to avoid the error.
LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES = 35
List of CUDA compute capabilities that should be supported by the NVPTX device RTL. E.g. for compute capabilities 6.0 and 7.0, the option "60;70" should be used. Compute capability 3.5 is the minimum required.
LIBOMPTARGET_NVPTX_DEBUG = OFF|ON
Enable printing of debug messages from the NVPTX device RTL.
LIBOMPTARGET_LIT_ARGS = ""
Arguments given to lit. make check-libomptarget and make check-libomptarget-* are affected. For example, use LIBOMPTARGET_LIT_ARGS="-j4" to force lit to start only four parallel jobs instead of by default the number of threads in the system.

Example Usages of CMake

Typical Invocations

$ cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ..
$ cmake -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ ..
$ cmake -DCMAKE_C_COMPILER=icc -DCMAKE_CXX_COMPILER=icpc ..

Advanced Builds with Various Options

  • Build the i386 Linux* library using GCC*

    $ cmake -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -DLIBOMP_ARCH=i386 ..

  • Build the x86_64 debug Mac library using Clang*

    $ cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLIBOMP_ARCH=x86_64 -DCMAKE_BUILD_TYPE=Debug ..

  • Build the library (architecture determined by probing compiler) using the Intel(R) C Compiler and the Intel(R) C++ Compiler. Also, create Fortran modules with the Intel(R) Fortran Compiler.

    $ cmake -DCMAKE_C_COMPILER=icc -DCMAKE_CXX_COMPILER=icpc -DCMAKE_Fortran_COMPILER=ifort -DLIBOMP_FORTRAN_MODULES=on ..

  • Have CMake find the C/C++ compiler and specify additional flags for the preprocessor and C++ compiler.

    .. code-blocks:: console

  $ cmake -DLIBOMP_CPPFLAGS='-DNEW_FEATURE=1 -DOLD_FEATURE=0' -DLIBOMP_CXXFLAGS='--one-specific-flag --two-specific-flag' ..

  • Build the stubs library

    .. code-blocks:: console

  $ cmake -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -DLIBOMP_LIB_TYPE=stubs ..

Footnotes

[*]Other names and brands may be claimed as the property of others.

How to Run Tests

There are following check-* make targets for tests.

  • check-ompt (ompt tests under runtime/test/ompt)
  • check-ompt-multiplex (ompt multiplex tests under tools/multiplex/tests)
  • check-libarcher (libarcher tests under tools/archer/tests)
  • check-libomp (libomp tests under runtime/test. This includes check-ompt tests too)
  • check-libomptarget-* (libomptarget tests for specific target under libomptarget/test)
  • check-libomptarget (all check-libomptarget-* tests)
  • check-openmp (combination of all above tests excluding duplicates)

For example, to run all available tests, use make check-openmp.

Options for Tests

Tests use lit framework. See lit documentation for lit options.

CHECK_OPENMP_ENV = ""
Default environment variables which test process uses for check-openmp separated by space. This can be used for individual targets (check-ompt, check-ompt-multiplex, check-libarcher, check-libomp and check-libomptarget-*) too. Note that each test still overrides environment variables if needed. For example, to change barrier pattern to be used from default hyper barrier to hierarchical barrier, run:
$ CHECK_OPENMP_ENV="KMP_PLAIN_BARRIER_PATTERN=hier,hier KMP_FORKJOIN_BARRIER_PATTERN=hier,hier KMP_REDUCTION_BARRIER_PATTERN=hier,hier" make check-openmp