[CMake] CMake + MPI

[Tim Gallagher]

That's good to know -- it looks like that capability was added 3 months after we wrote our CMakeLists.txt and we had to skip the find_package if we used a cray. That was 2011 and we never looked back at it because it worked. 

I guess I can go ahead and update that! 

Tim 

----- Original Message -----

From: "Chuck Atkins" <chuck.atkins at kitware.com> 
To: "tim gallagher" <tim.gallagher at gatech.edu> 
Cc: "Andreas Naumann" <Andreas-Naumann at gmx.net>, cmake at cmake.org 
Sent: Wednesday, September 9, 2015 11:11:27 AM 
Subject: Re: [CMake] CMake + MPI 






The only exception to that (which I am aware of) is if you are on a CRAY system where the MPI (and BLAS and LAPACK) are baked into the compiler wrappers (cc, ftn, etc). In those situations, you actually do not want to run find_package(MPI) or it will throw errors. 




Not true. The FindMPI.cmake module performs the appropriate introspection to check if your primary compiler wraps accordingly. For example, on a Cray XC30, with the PrgEnv-pgi and cray-mpich modules loaded: 

p02064 at swan:~/Code/tmp/build> cat ../source/CMakeLists.txt 
cmake_minimum_required(VERSION 3.3) 
project(TestMPI C CXX) 

find_package(MPI REQUIRED) 

add_executable(test-mpi-c test-mpi.c) 
target_include_directories(test-mpi-c PRIVATE ${MPI_C_INCLUDE_PATH}) 
target_link_libraries(test-mpi-c ${MPI_C_LIBRARIES}) 

add_executable(test-mpi-cxx test-mpi.cxx) 
target_include_directories(test-mpi-cxx PRIVATE ${MPI_CXX_INCLUDE_PATH}) 
target_link_libraries(test-mpi-cxx ${MPI_CXX_LIBRARIES}) 
p02064 at swan:~/Code/tmp/build> 

p02064 at swan:~/Code/tmp/build> cmake ../source 
-- The C compiler identification is PGI 15.7.0 
-- The CXX compiler identification is PGI 15.7.0 
-- Check for working C compiler: /opt/cray/craype/2.4.1/bin/cc 
-- Check for working C compiler: /opt/cray/craype/2.4.1/bin/cc -- works 
-- Detecting C compiler ABI info 
-- Detecting C compiler ABI info - done 
-- Check for working CXX compiler: /opt/cray/craype/2.4.1/bin/CC 
-- Check for working CXX compiler: /opt/cray/craype/2.4.1/bin/CC -- works 
-- Detecting CXX compiler ABI info 
-- Detecting CXX compiler ABI info - done 
-- Found MPI_C: /opt/cray/craype/2.4.1/bin/cc 
-- Found MPI_CXX: /opt/cray/craype/2.4.1/bin/CC 
-- Configuring done 
-- Generating done 
-- Build files have been written to: /home/users/p02064/Code/tmp/build 
p02064 at swan:~/Code/tmp/build> 

p02064 at swan:~/Code/tmp/build> grep '^MPI_.*' CMakeCache.txt | grep -v INTERNAL 
MPI_CXX_COMPILER:STRING=/opt/cray/craype/2.4.1/bin/CC 
MPI_CXX_COMPILE_FLAGS:STRING= 
MPI_CXX_INCLUDE_PATH:STRING= 
MPI_CXX_LIBRARIES:STRING= 
MPI_CXX_LINK_FLAGS:STRING= 
MPI_CXX_NO_INTERROGATE:STRING=/opt/cray/craype/2.4.1/bin/CC 
MPI_C_COMPILER:STRING=/opt/cray/craype/2.4.1/bin/cc 
MPI_C_COMPILE_FLAGS:STRING= 
MPI_C_INCLUDE_PATH:STRING= 
MPI_C_LIBRARIES:STRING= 
MPI_C_LINK_FLAGS:STRING= 
MPI_C_NO_INTERROGATE:STRING=/opt/cray/craype/2.4.1/bin/cc 
MPI_EXTRA_LIBRARY:STRING=MPI_EXTRA_LIBRARY-NOTFOUND 
MPI_LIBRARY:FILEPATH=MPI_LIBRARY-NOTFOUND 
p02064 at swan:~/Code/tmp/build> 

p02064 at swan:~/Code/tmp/build> make 
[ 25%] Building C object CMakeFiles/test-mpi-c.dir/test-mpi.c.o 
[ 50%] Linking C executable test-mpi-c 
[ 50%] Built target test-mpi-c 
[ 75%] Building CXX object CMakeFiles/test-mpi-cxx.dir/test-mpi.cxx.o 
[100%] Linking CXX executable test-mpi-cxx 
[100%] Built target test-mpi-cxx 
p02064 at swan:~/Code/tmp/build> 


As you can see from the resulting cache, the FindMPI module knows how to act accordingly even when mpi libraries are folded into the regular compiler. 

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