9.4. ABI compatibility to previous versions of Open MPI

Note

Open MPI supports two ABIs:

  • The Open MPI ABI: this is the ABI that Open MPI has supported for multiple releases over many years. It is provided by the libmpi library.

  • The MPI Forum ABI: this is the official ABI as defined by the MPI standard. It was not supported in Open MPI until v6.0.0. It is provided by the libmpi_abi library.

These two ABIs are different and not interchangeable.

Open MPI builds support for both ABIs by default.

This section describes the Open MPI ABI compatibility between Open MPI releases.

For information about the MPI standard ABI defined by MPI-5.0, see Building MPI applications using the MPI Forum ABI.

9.4.1. What is the Open MPI ABI?

The Open MPI v6.1.x series maintains Application Binary Interface (ABI) compatibility for the C MPI bindings to the last major Open MPI release, as described in the Version numbers and compatibility section.

Specifically, an application compiled with Open MPI v4.x can be executed with Open MPI v6.1.x without having to recompile the application.

Important

ABI is maintained for most of the Fortran MPI bindings, too — see below for additional information.

9.4.2. Open MPI ABI exceptions

There are however a few scenarios where an application compiled with Open MPI v4.0.x or v4.1.x might not execute correctly with Open MPI v6.1.x.

  • Fortran compilers provide varying degrees of ABI guarantees between their releases. As such, Open MPI can only provide ABI guarantees with MPI applications that use the Fortran MPI bindings when all three entities — Open MPI v4.x, Open MPI v6.1.x, and the MPI application — were built with exactly the same Fortran compiler and environment.

    If these conditions are met, Open MPI’s ABI guarantees between the Open MPI v4.x and v6.1.x series are in effect, with the exception of the following cases:

    • When using the Fortran mpi module bindings with GCC compiler versions prior to v4.8, non-compliant Fortran interfaces for MPI_Comm_spawn_multiple, MPI_Testall, MPI_Testsome, MPI_Waitall, and MPI_Waitsome were removed starting with Open MPI v5.0.0.

    • When using the Fortran mpi module with modern Fortran compilers (e.g., GNU Fortran >= v4.9), Open MPI v5.0.0 removed the names from the MPI interfaces when there is only a single subroutine in the interface, and that subroutine name exactly matches the interface name. This change is likely to make Open MPI v6.1.x’s mpi module bindings less restrictive than Open MPI v4.x, but it may also have ABI implications, depending on your Fortran compiler.

      See this git commit message for more details.

      Important

      This is likely a compiler-specific issue, and may need to be tested in your environment.

    • Some Fortran interfaces changed INTENT declarations, added ASYNCHRONOUS, or changed from named to unnamed interfaces. These source-level interface changes may require changes to an application’s source code, followed by recompilation and relinking.

    • When using the Fortran mpi_f08 module bindings in an environment where a Fortran INTEGER is 8 bytes but a C int is 4 bytes, the size of a Type(MPI_Status) was increased starting with Open MPI v5.0.0. Applications built in this configuration must be rebuilt and relinked against Open MPI v6.1.x.

  • Open MPI v5.0.0 removed support for the MPI C++ bindings. If an application was using the deprecated and now removed C++ bindings, it will not be able to compile or execute with Open MPI v5.0.0. For details on deprecated and removed functions see Removed MPI constructs and Deprecation warnings