From ad3387143aaf1ee18c111d7fb1f9889f1c4f153f Mon Sep 17 00:00:00 2001
From: Axel Kohlmeyer <akohlmey@gmail.com>
Date: Fri, 24 Jun 2022 06:18:22 -0400
Subject: [PATCH] add crosscompiling with MPI support to plugins package

---
 cmake/Modules/LAMMPSInterfacePlugin.cmake |  1 +
 examples/plugins/LAMMPSInterfaceCXX.cmake | 57 +++++++++++++++++++++--
 2 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/cmake/Modules/LAMMPSInterfacePlugin.cmake b/cmake/Modules/LAMMPSInterfacePlugin.cmake
index d80d7609b7..95bb707e86 100644
--- a/cmake/Modules/LAMMPSInterfacePlugin.cmake
+++ b/cmake/Modules/LAMMPSInterfacePlugin.cmake
@@ -99,6 +99,7 @@ endif()
 ################################################################################
 # MPI configuration
 if(NOT CMAKE_CROSSCOMPILING)
+  set(MPI_CXX_SKIP_MPICXX TRUE)
   find_package(MPI QUIET)
   option(BUILD_MPI "Build MPI version" ${MPI_FOUND})
 else()
diff --git a/examples/plugins/LAMMPSInterfaceCXX.cmake b/examples/plugins/LAMMPSInterfaceCXX.cmake
index d52cf8f4e5..a3313215d6 100644
--- a/examples/plugins/LAMMPSInterfaceCXX.cmake
+++ b/examples/plugins/LAMMPSInterfaceCXX.cmake
@@ -1,6 +1,9 @@
 # Cmake script code to define the LAMMPS C++ interface
 # settings required for building LAMMPS plugins
 
+set(LAMMPS_THIRDPARTY_URL "https://download.lammps.org/thirdparty"
+  CACHE STRING "URL for thirdparty package downloads")
+
 ################################################################################
 # helper function
 function(validate_option name values)
@@ -37,10 +40,56 @@ else()
 endif()
 
 if(BUILD_MPI)
-  find_package(MPI REQUIRED)
-  option(LAMMPS_LONGLONG_TO_LONG "Workaround if your system or MPI version does not recognize 'long long' data types" OFF)
-  if(LAMMPS_LONGLONG_TO_LONG)
-    target_compile_definitions(lammps INTERFACE -DLAMMPS_LONGLONG_TO_LONG)
+  # do not include the (obsolete) MPI C++ bindings which makes
+  # for leaner object files and avoids namespace conflicts
+  set(MPI_CXX_SKIP_MPICXX TRUE)
+  # We use a non-standard procedure to cross-compile with MPI on Windows
+  if((CMAKE_SYSTEM_NAME STREQUAL "Windows") AND CMAKE_CROSSCOMPILING)
+    # Download and configure custom MPICH files for Windows
+    message(STATUS "Downloading and configuring MPICH-1.4.1 for Windows")
+    set(MPICH2_WIN64_DEVEL_URL "${LAMMPS_THIRDPARTY_URL}/mpich2-win64-devel.tar.gz" CACHE STRING "URL for MPICH2 (win64) tarball")
+    set(MPICH2_WIN32_DEVEL_URL "${LAMMPS_THIRDPARTY_URL}/mpich2-win32-devel.tar.gz" CACHE STRING "URL for MPICH2 (win32) tarball")
+    set(MPICH2_WIN64_DEVEL_MD5 "4939fdb59d13182fd5dd65211e469f14" CACHE STRING "MD5 checksum of MPICH2 (win64) tarball")
+    set(MPICH2_WIN32_DEVEL_MD5 "a61d153500dce44e21b755ee7257e031" CACHE STRING "MD5 checksum of MPICH2 (win32) tarball")
+    mark_as_advanced(MPICH2_WIN64_DEVEL_URL)
+    mark_as_advanced(MPICH2_WIN32_DEVEL_URL)
+    mark_as_advanced(MPICH2_WIN64_DEVEL_MD5)
+    mark_as_advanced(MPICH2_WIN32_DEVEL_MD5)
+
+    include(ExternalProject)
+    if(CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64")
+      ExternalProject_Add(mpi4win_build
+        URL     ${MPICH2_WIN64_DEVEL_URL}
+        URL_MD5 ${MPICH2_WIN64_DEVEL_MD5}
+        CONFIGURE_COMMAND "" BUILD_COMMAND "" INSTALL_COMMAND ""
+        BUILD_BYPRODUCTS <SOURCE_DIR>/lib/libmpi.a)
+    else()
+      ExternalProject_Add(mpi4win_build
+        URL     ${MPICH2_WIN32_DEVEL_URL}
+        URL_MD5 ${MPICH2_WIN32_DEVEL_MD5}
+        CONFIGURE_COMMAND "" BUILD_COMMAND "" INSTALL_COMMAND ""
+        BUILD_BYPRODUCTS <SOURCE_DIR>/lib/libmpi.a)
+    endif()
+
+    ExternalProject_get_property(mpi4win_build SOURCE_DIR)
+    file(MAKE_DIRECTORY "${SOURCE_DIR}/include")
+    add_library(MPI::MPI_CXX UNKNOWN IMPORTED)
+    set_target_properties(MPI::MPI_CXX PROPERTIES
+      IMPORTED_LOCATION "${SOURCE_DIR}/lib/libmpi.a"
+      INTERFACE_INCLUDE_DIRECTORIES "${SOURCE_DIR}/include"
+      INTERFACE_COMPILE_DEFINITIONS "MPICH_SKIP_MPICXX")
+    add_dependencies(MPI::MPI_CXX mpi4win_build)
+
+    # set variables for status reporting at the end of CMake run
+    set(MPI_CXX_INCLUDE_PATH "${SOURCE_DIR}/include")
+    set(MPI_CXX_COMPILE_DEFINITIONS "MPICH_SKIP_MPICXX")
+    set(MPI_CXX_LIBRARIES "${SOURCE_DIR}/lib/libmpi.a")
+  else()
+    find_package(MPI REQUIRED)
+    option(LAMMPS_LONGLONG_TO_LONG "Workaround if your system or MPI version does not recognize 'long long' data types" OFF)
+    if(LAMMPS_LONGLONG_TO_LONG)
+      target_compile_definitions(lammps INTERFACE -DLAMMPS_LONGLONG_TO_LONG)
+    endif()
   endif()
   target_link_libraries(lammps INTERFACE MPI::MPI_CXX)
 else()