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
Instructions to Build
CMake Options
Example Usages of CMake
- Typical Invocations
- Advanced Builds with Various Options

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_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.

Options for `libomp`

LIBOMP_ARCH = aarch64|arm|i386|mic|mips|mips64|ppc64|ppc64le|x86_64|riscv64: 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 and RISCV64 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`

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. This is required for testing out-of-tree builds.

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 and the CMake C compiler is Clang, then Clang is used.

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 and the CMake C compiler is Clang and there exists a llvm-link binary in the directory containing Clang, then this llvm-link binary is used.

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.

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.
System Message: ERROR/3 (<stdin>, line 324)

Unknown directive type "code-blocks".
```
.. code-blocks:: console

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

Build the stubs library

System Message: ERROR/3 (<stdin>, line 330)

Unknown directive type "code-blocks".

.. 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.