# FindMPI

Find a Message Passing Interface (MPI) implementation.

The Message Passing Interface (MPI) is a library used to write high-performance distributed-memory parallel applications, and is typically deployed on a cluster. MPI is a standard interface (defined by the MPI forum) for which many implementations are available.

## Variables for using MPI

The module exposes the components `C`, `CXX`, `MPICXX` and `Fortran`. Each of these controls the various MPI languages to search for. The difference between `CXX` and `MPICXX` is that `CXX` refers to the MPI C API being usable from C++, whereas `MPICXX` refers to the MPI-2 C++ API that was removed again in MPI-3.

Depending on the enabled components the following variables will be set:

`MPI_FOUND`
  Variable indicating that MPI settings for all requested languages have been found. If no components are specified, this is true if MPI settings for all enabled languages were detected. Note that the `MPICXX` component does not affect this variable.
`MPI_VERSION`
  Minimal version of MPI detected among the requested languages, or all enabled languages if no components were specified.

This module will set the following variables per language in your project, where `<lang>` is one of C, CXX, or Fortran:

`MPI_<lang>_FOUND`
  Variable indicating the MPI settings for `<lang>` were found and that simple MPI test programs compile with the provided settings.
`MPI_<lang>_COMPILER`
  MPI compiler for `<lang>` if such a program exists.
`MPI_<lang>_COMPILE_OPTIONS`
  Compilation options for MPI programs in `<lang>`, given as a ;-list.
`MPI_<lang>_COMPILE_DEFINITIONS`
  Compilation definitions for MPI programs in `<lang>`, given as a ;-list.
`MPI_<lang>_INCLUDE_DIRS`
  Include path(s) for MPI header.
`MPI_<lang>_LINK_FLAGS`
  Linker flags for MPI programs.
`MPI_<lang>_LIBRARIES`
  All libraries to link MPI programs against.

Additionally, the following `IMPORTED` targets are defined:

`MPI::MPI_<lang>`
  Target for using MPI from `<lang>`.

The following variables indicating which bindings are present will be defined:

`MPI_MPICXX_FOUND`
  Variable indicating whether the MPI-2 C++ bindings are present (introduced in MPI-2, removed with MPI-3).
`MPI_Fortran_HAVE_F77_HEADER`
  True if the Fortran 77 header `mpif.h` is available.
`MPI_Fortran_HAVE_F90_MODULE`
  True if the Fortran 90 module `mpi` can be used for accessing MPI (MPI-2 and higher only).
`MPI_Fortran_HAVE_F08_MODULE`
  True if the Fortran 2008 `mpi_f08` is available to MPI programs (MPI-3 and higher only).

If possible, the MPI version will be determined by this module. The facilities to detect the MPI version were introduced with MPI-1.2, and therefore cannot be found for older MPI versions.

`MPI_<lang>_VERSION_MAJOR`
  Major version of MPI implemented for `<lang>` by the MPI distribution.
`MPI_<lang>_VERSION_MINOR`
  Minor version of MPI implemented for `<lang>` by the MPI distribution.
`MPI_<lang>_VERSION`
  MPI version implemented for `<lang>` by the MPI distribution.

Note that there’s no variable for the C bindings being accessible through `mpi.h`, since the MPI standards always have required this binding to work in both C and C++ code.

For running MPI programs, the module sets the following variables

`MPIEXEC_EXECUTABLE`
  Executable for running MPI programs, if such exists.
`MPIEXEC_NUMPROC_FLAG`
  Flag to pass to `mpiexec` before giving it the number of processors to run on.
`MPIEXEC_MAX_NUMPROCS`
  Number of MPI processors to utilize. Defaults to the number of processors detected on the host system.
`MPIEXEC_PREFLAGS`
  Flags to pass to `mpiexec` directly before the executable to run.
`MPIEXEC_POSTFLAGS`
  Flags to pass to `mpiexec` after other flags.

## Variables for locating MPI

This module performs a three step search for an MPI implementation:

1. Check if the compiler has MPI support built-in. This is the case if the user passed a compiler wrapper as `CMAKE_<LANG>_COMPILER` or if they’re on a Cray system.
2. Attempt to find an MPI compiler wrapper and determine the compiler information from it.
3. Try to find an MPI implementation that does not ship such a wrapper by guessing settings. Currently, only Microsoft MPI and MPICH2 on Windows are supported.

For controlling the second step, the following variables may be set:

`MPI_<lang>_COMPILER`
  Search for the specified compiler wrapper and use it.
`MPI_<lang>_COMPILER_FLAGS`
  Flags to pass to the MPI compiler wrapper during interrogation. Some compiler wrappers support linking debug or tracing libraries if a specific flag is passed and this variable may be used to obtain them.
`MPI_COMPILER_FLAGS`
  Used to initialize `MPI_<lang>_COMPILER_FLAGS` if no language specific flag has been given. Empty by default.
`MPI_EXECUTABLE_SUFFIX`
  A suffix which is appended to all names that are being looked for. For instance you may set this to `.mpich` or `.openmpi` to prefer the one or the other on Debian and its derivatives.

In order to control the guessing step, the following variable may be set:

`MPI_GUESS_LIBRARY_NAME`
  Valid values are `MSMPI` and `MPICH2`. If set, only the given library will be searched for. By default, `MSMPI` will be preferred over `MPICH2` if both are available. This also sets `MPI_SKIP_COMPILER_WRAPPER` to `true`, which may be overridden.

Each of the search steps may be skipped with the following control variables:

`MPI_ASSUME_NO_BUILTIN_MPI`
  If true, the module assumes that the compiler itself does not provide an MPI implementation and skips to step 2.
`MPI_SKIP_COMPILER_WRAPPER`
  If true, no compiler wrapper will be searched for.
`MPI_SKIP_GUESSING`
  If true, the guessing step will be skipped.

Additionally, the following control variable is available to change search behavior:

`MPI_CXX_SKIP_MPICXX`
  Add some definitions that will disable the MPI-2 C++ bindings. Currently supported are MPICH, Open MPI, Platform MPI and derivatives thereof, for example MVAPICH or Intel MPI.

If the find procedure fails for a variable `MPI_<lang>_WORKS`, then the settings detected by or passed to the module did not work and even a simple MPI test program failed to compile.

If all of these parameters were not sufficient to find the right MPI implementation, a user may disable the entire autodetection process by specifying both a list of libraries in `MPI_<lang>_LIBRARIES` and a list of include directories in `MPI_<lang>_ADDITIONAL_INCLUDE_DIRS`. Any other variable may be set in addition to these two. The module will then validate the MPI settings and store the settings in the cache.

## Cache variables for MPI

The variable `MPI_<lang>_INCLUDE_DIRS` will be assembled from the following variables. For C and CXX:

`MPI_<lang>_HEADER_DIR`
  Location of the `mpi.h` header on disk.

For Fortran:

`MPI_Fortran_F77_HEADER_DIR`
  Location of the Fortran 77 header `mpif.h`, if it exists.
`MPI_Fortran_MODULE_DIR`
  Location of the `mpi` or `mpi_f08` modules, if available.

For all languages the following variables are additionally considered:

`MPI_<lang>_ADDITIONAL_INCLUDE_DIRS`
  A ;-list of paths needed in addition to the normal include directories.
`MPI_<include_name>_INCLUDE_DIR`
  Path variables for include folders referred to by `<include_name>`.
`MPI_<lang>_ADDITIONAL_INCLUDE_VARS`
  A ;-list of `<include_name>` that will be added to the include locations of `<lang>`.

The variable `MPI_<lang>_LIBRARIES` will be assembled from the following variables:

`MPI_<lib_name>_LIBRARY`
  The location of a library called `<lib_name>` for use with MPI.
`MPI_<lang>_LIB_NAMES`
  A ;-list of `<lib_name>` that will be added to the include locations of `<lang>`.

## Usage of mpiexec

When using `MPIEXEC_EXECUTABLE` to execute MPI applications, you should typically use all of the `MPIEXEC_EXECUTABLE` flags as follows:

    ${MPIEXEC_EXECUTABLE} ${MPIEXEC_NUMPROC_FLAG} ${MPIEXEC_MAX_NUMPROCS}
      ${MPIEXEC_PREFLAGS} EXECUTABLE ${MPIEXEC_POSTFLAGS} ARGS

where `EXECUTABLE` is the MPI program, and `ARGS` are the arguments to pass to the MPI program.

## Advanced variables for using MPI

The module can perform some advanced feature detections upon explicit request.

Important notice: The following checks cannot be performed without executing an MPI test program. Consider the special considerations for the behavior of `try_run()` during cross compilation. Moreover, running an MPI program can cause additional issues, like a firewall notification on some systems. You should only enable these detections if you absolutely need the information.

If the following variables are set to true, the respective search will be performed:

`MPI_DETERMINE_Fortran_CAPABILITIES`
  Determine for all available Fortran bindings what the values of `MPI_SUBARRAYS_SUPPORTED` and `MPI_ASYNC_PROTECTS_NONBLOCKING` are and make their values available as `MPI_Fortran_<binding>_SUBARRAYS` and `MPI_Fortran_<binding>_ASYNCPROT`, where `<binding>` is one of `F77_HEADER`, `F90_MODULE` and `F08_MODULE`.
`MPI_DETERMINE_LIBRARY_VERSION`
  For each language, find the output of `MPI_Get_library_version` and make it available as `MPI_<lang>_LIBRARY_VERSION`. This information is usually tied to the runtime component of an MPI implementation and might differ depending on `<lang>`. Note that the return value is entirely implementation defined. This information might be used to identify the MPI vendor and for example pick the correct one of multiple third party binaries that matches the MPI vendor.

## Backward Compatibility

For backward compatibility with older versions of FindMPI, these variables are set, but deprecated:

    MPI_COMPILER        MPI_LIBRARY        MPI_EXTRA_LIBRARY
    MPI_COMPILE_FLAGS   MPI_INCLUDE_PATH   MPI_LINK_FLAGS
    MPI_LIBRARIES

In new projects, please use the `MPI_<lang>_XXX` equivalents. Additionally, the following variables are deprecated:

`MPI_<lang>_COMPILE_FLAGS`
  Use `MPI_<lang>_COMPILE_OPTIONS` and `MPI_<lang>_COMPILE_DEFINITIONS` instead.
`MPI_<lang>_INCLUDE_PATH`
  For consumption use `MPI_<lang>_INCLUDE_DIRS` and for specifying folders use `MPI_<lang>_ADDITIONAL_INCLUDE_DIRS` instead.
`MPIEXEC`
  Use `MPIEXEC_EXECUTABLE` instead.