361. Requirements on Fortran Compilers

MPI-3.0 (and later) compliant Fortran bindings are not only a property of the MPI library itself, but rather a property of an MPI library together with the Fortran compiler suite for which it is compiled.
Advice to users.

Users must take appropriate steps to ensure that proper options are specified to compilers. MPI libraries must document these options. Some MPI libraries are shipped together with special compilation scripts (e.g., mpif90, mpicc) that set these options automatically. ( End of advice to users.)
An MPI library together with the Fortran compiler suite is only compliant with MPI-3.0 (and later), as referred by MPI_GET_VERSION, if all the solutions described in Sections Problems Due to Strong Typing through Permanent Data Movement work correctly. Based on this rule, major requirements for all three Fortran support methods (i.e., the mpi_f08 and mpi modules, and mpif.h) are:

• The language features assumed-type and assumed-rank from Fortran 2008 TS 29113 [41] are available. This is required only for mpi_f08. As long as this requirement is not supported by the compiler, it is valid to build an MPI library that implements the mpi_f08 module with MPI_SUBARRAYS_SUPPORTED set to .FALSE..
• ``Simply contiguous'' arrays and scalars must be passed to choice buffer dummy arguments of nonblocking routines with call by reference. This is needed only if one of the support methods does not use the ASYNCHRONOUS attribute. See Section Problems Due to Data Copying and Sequence Association with Subscript Triplets for more details.

• SEQUENCE and BIND(C) derived types are valid as actual arguments passed to choice buffer dummy arguments, and, in the case of MPI_SUBARRAYS_SUPPORTED== .FALSE., they are passed with call by reference, and passed by descriptor in the case of .TRUE..
• All actual arguments that are allowed for a dummy argument in an implicitly defined and separately compiled Fortran routine with the given compiler (e.g., CHARACTER(LEN=*) strings and array of strings) must also be valid for choice buffer dummy arguments with all Fortran support methods.
• The array dummy argument of the ISO_C_BINDING intrinsic module procedure C_F_POINTER is not restricted to Fortran types for which a corresponding type in C exists.
• The Fortran compiler shall not provide TYPE(*) unless the ASYNCHRONOUS attribute protects MPI communication as described in TS 29113. Specifically, the TS 29113 must be implemented as a whole.

• Separately compiled empty Fortran routines with implicit interfaces and separately compiled empty C routines with BIND(C) Fortran interfaces (e.g., MPI_F_SYNC_REG on page Calling MPI_F_SYNC_REG and Section Additional Support for Fortran Register-Memory-Synchronization , and DD on page A User Defined Routine Instead of MPI_F_SYNC_REG ) solve the problems described in Section Problems with Code Movement and Register Optimization .
• The problems with temporary data movement (described in detail in Section Temporary Data Movement and Temporary Memory Modification ) are solved as long as the application uses different sets of variables for the nonblocking communication (or nonblocking or split collective I/O) and the computation when overlapping communication and computation.
• Problems caused by automatic and permanent data movement (e.g., within a garbage collection, see Section Permanent Data Movement ) are resolved without any further requirements on the application program, neither on the usage of the buffers, nor on the declaration of application routines that are involved in invoking MPI procedures.

Some of these rules are already part of the Fortran 2003 standard, some of these requirements require the Fortran TS 29113 [41], and some of these requirements for MPI-3.0 are beyond the scope of TS 29113. ( End of advice to implementors.)