how to compile MPI and non-MPI version of the same program with automake?

Question

I have a C++ code that can be compiled with MPI support depending on a certain preprocessor flag; missing the appropriate flag, the sources compile to a non-parallel version.

I would like to setup the Makefile.am so that it compiles both the MPI-parallel and the sequential version, if an option to ./configure is given.

Here's the catch: MPI has its own C++ compiler wrapper, and insists that sources are compiled and linked using it rather than the standard C++ compiler. If I were to write the Makefile myself, I would have to do something like this:

myprog.seq: myprog.cxx
    $(CXX) ... myprog.cxx

myprog.mpi: myprog.cxx
    $(MPICXX) -DWITH_MPI ... myprog.cxx

Is there a way to tell automake that it has to use $(MPICXX) instead of $(CXX) when compiling the MPI-enabled version of the program?

Answer 1

MPI installations do (usually) ship with compiler wrappers, but there is no requirement that you use them -- MPI does not insist on it. If you want to go your own way you can write your own makefile to ensure that the C++ compiler gets the right libraries (etc). To figure out what the right libraries (etc) are, inspect the compiler wrapper which is, on all the systems I've used, a shell script.

At first sight the compiler wrappers which ship with products such as the Intel compilers are a little daunting but stop and think about what is going on -- you are simply compiling a program which makes use of an external library or two. Writing a makefile to use the MPI libraries is no more difficult than writing a makefile to use any other library.

Answer 2

If you have disabled the subdir-objects option to automake, something like this might work:

configure.ac:

AC_ARG_ENABLE([seq], ...)
AC_ARG_ENABLE([mpi], ...)
AM_CONDITIONAL([ENABLE_SEQ], [test $enable_seq = yes])
AM_CONDITIONAL([ENABLE_MPI], [test $enable_mpi = yes])
AC_CONFIG_FILES([Makefile seq/Makefile mpi/Makefile])

Makefile.am:

SUBDIRS =
if ENABLE_SEQ
SUBDIRS += seq
endif
if ENABLE_MPI
SUBDIRS += mpi
endif

sources.am:

ALL_SOURCES = src/foo.c src/bar.cc src/baz.cpp

seq/Makefile.am:

include $(top_srcdir)/sources.am

bin_PROGRAMS = seq
seq_SOURCES = $(ALL_SOURCES)

mpi/Makefile.am:

include $(top_srcdir)/sources.am

CXX = $(MPICXX)
AM_CPPFLAGS = -DWITH_MPI

bin_PROGRAMS = mpi
mpi_SOURCES = $(ALL_SOURCES)

The only thing stopping you from doing both of these in the same directory is the override of $(CXX). You could, for instance, set mpi_CPPFLAGS and automake would handle that gracefully, but the compiler switch makes it a no-go here.

Answer 3

I have the same problem, and I've found that there's no really good way to get autotools to conditionally use MPI compilers for particular targets. Autotools is good at figuring out which compiler to use based on what language your source is written in (CC, CXX, FC, F77, etc.), but it really isn't good at figuring out whether or not to use MPI compiler for a particular target. You can set MPICC, MPICXX, etc., but you essentially have to rewrite all your Makefile rules for your target (as you've done above) if you use the compiler this way. If you do that, what's the point of writing an automake file?

Someone else suggested using MPI like an external library, and this is the approach I'd advocate, but you shouldn't do it by hand, because different MPI installations have different sets of flags they pass to the compiler, and they can depend on the language you're compiling.

The good thing is that all the currently shipping MPI compilers that I know of support introspection arguments, like -show, -show-compile or -show-link. You can automatically extract the arguments from the scripts.

So, what I did to deal with this was to make an m4 script that extracts the defines, includes, library paths, libs, and linker flags from the MPI compilers, then assigns them to variables you can use in your Makefile.am. Here's the script:

lx_find_mpi.m4

This makes MPI work the way automake expects it to. Incidentally, this is the approach CMake uses in their FindMPI module, and I find it works quite well there. It makes the build much more convenient because you can just do something like this for your targets:

bin_PROGRAMS = mpi_exe seq_exe

# This is all you need for a sequential program
seq_exe_SOURCES = seq_exe.C

# For an MPI program you need special LDFLAGS and INCLUDES
mpi_exe_SOURCES = mpi_exe.C
mpi_exe_LDFLAGS = $(MPI_CXXLDFLAGS)

INCLUDES = $(MPI_CXXFLAGS)

There are similar flags for the other languages since, like I said, the particular flags and libraries can vary depending on which language's MPI compiler you use.

lx_find_mpi.m4 also sets some shell variables so that you can test in your configure.ac file whether MPI was found. e.g., if you are looking for MPI C++ support, you can test $have_CXX_mpi to see if the macro found it.

I've tested this macro with mvapich and OpenMPI, as well as the custom MPICH2 implementation on BlueGene machines (though it does not address all the cross-compiling issues you'll see there). Let me know if something doesn't work. I'd like to keep the macro as robust as possible.