Fix single-threaded CPU build with LLVM Clang/Flang (#24)

* Fix single-threaded CPU build with LLVM Clang/Flang

Tested with LLVM main branch at f905877f2039 and OpenMPI 5.0.2 built
by that LLVM.

* Add usual compiler options for LLVM Clang/Flang builds

Tested with LLVM main branch at d1a461dbb229 and OpenMPI 5.0.2 built
by that LLVM.

---------

Co-authored-by: Joel E. Denny <jdenny-ornl@gmail.com>

DEPENDENCIES.md

@@ -2,6 +2,22 @@ System libraries
 -----------------------
 UMT Requires MPI to be installed on your system to provide a C++ and Fortran compiler.
 
+To build UMT with LLVM Clang/Flang, you must also use them to build MPI so that
+Fortran mod files are compatible.  For example, you might build OpenMPI as
+follows:
+
+```
+$ CC=clang CXX=clang++ FC=flang-new \
+  CFLAGS=-O3 CXXFLAGS=-O3 FCFLAGS=-O3 \
+  ../configure --prefix=$PWD/../install-flang \
+    --without-knem --without-ucx --enable-pretty-print-stacktrace \
+    --enable-orterun-prefix-by-default --enable-mpi1-compatibility
+$ make -j
+$ make install
+```
+
+Reference: <https://github.com/llvm/llvm-project/issues/56348>
+
 Third party libraries
 -----------------------
 The code depends on several libraries.

build_and_run_umt.sh

@@ -28,6 +28,16 @@ FFLAGS=-fallow-argument-mismatch
 #CXX=icpx
 #FC=ifx
 
+# LLVM Clang/Flang build
+#
+# See DEPENDENCIES.md for a compatible MPI.
+#
+# So far this works only for single-threaded CPU execution.
+#CC=clang
+#CXX=clang++
+#FC=flang-new
+#FFLAGS=-flang-experimental-polymorphism
+
 # Get directory this script is located in.  This is assumed to be the UMT repo location.
 SOURCE="${BASH_SOURCE[0]}"
 while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
@@ -47,7 +57,7 @@ cd umt_workspace
 git clone --recurse-submodules  https://github.com/LLNL/conduit.git conduit -b v0.9.0
 mkdir build_conduit
 cd build_conduit
-cmake ${PWD}/../conduit/src -DCMAKE_INSTALL_PREFIX=${INSTALL_PATH} -DCMAKE_C_COMPILER=${CC} -DCMAKE_CXX_COMPILER=${CXX} -DCMAKE_Fortran_COMPILER=${FC} -DMPI_CXX_COMPILER=mpicxx -DMPI_Fortran_COMPILER=mpifort -DBUILD_SHARED_LIBS=OFF -DENABLE_TESTS=OFF -DENABLE_EXAMPLES=OFF -DENABLE_DOCS=OFF -DENABLE_FORTRAN=ON -DENABLE_MPI=ON -DENABLE_PYTHON=OFF
+cmake ${PWD}/../conduit/src -DCMAKE_Fortran_FLAGS="${FFLAGS}" -DCMAKE_INSTALL_PREFIX=${INSTALL_PATH} -DCMAKE_C_COMPILER=${CC} -DCMAKE_CXX_COMPILER=${CXX} -DCMAKE_Fortran_COMPILER=${FC} -DMPI_CXX_COMPILER=mpicxx -DMPI_Fortran_COMPILER=mpifort -DBUILD_SHARED_LIBS=OFF -DENABLE_TESTS=OFF -DENABLE_EXAMPLES=OFF -DENABLE_DOCS=OFF -DENABLE_FORTRAN=ON -DENABLE_MPI=ON -DENABLE_PYTHON=OFF
 gmake -j install
 cd ..
 

src/cmake/InitBuildTypeCompilerFlags.cmake

@@ -20,6 +20,9 @@ if (NOT "${CMAKE_BUILD_TYPE}" STREQUAL "")
       set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
 
 	elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+      set(CMAKE_CXX_FLAGS_RELEASE "-O3 -DNDEBUG")
+      set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O3 -g -DNDEBUG")
+      set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
 
  	elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "XL" OR "${CMAKE_CXX_COMPILER_ID}" STREQUAL "XLClang")
       set(CMAKE_CXX_FLAGS_RELEASE "-O3 -qstrict -qarch=auto -qtune=auto -qmaxmem=-1 -qsuppress=1500-036")
@@ -54,6 +57,10 @@ if (NOT "${CMAKE_BUILD_TYPE}" STREQUAL "")
       set(CMAKE_Fortran_FLAGS_DEBUG "-fcheck=all -O0 -g -ffree-line-length-none")
 
 	elseif("${CMAKE_Fortran_COMPILER_ID}" MATCHES "Clang") # For Clang or AppleClang
+	elseif("${CMAKE_Fortran_COMPILER_ID}" MATCHES "LLVMFlang")
+      set(CMAKE_Fortran_FLAGS_RELEASE "-O3 -DNDEBUG")
+      set(CMAKE_Fortran_FLAGS_RELWITHDEBINFO "-O3 -g -DNDEBUG")
+      set(CMAKE_Fortran_FLAGS_DEBUG "-O0 -g")
 
  	elseif("${CMAKE_Fortran_COMPILER_ID}" STREQUAL "XL")
       # Enable F2003 support via the below -qxlf2003 flag list.  This behavior is the default if xlf2003 compiler is used, but not if xlf is used.

src/cmake/InitLinkerWrapperFlags.cmake

@@ -24,6 +24,10 @@ if ("Fortran" IN_LIST languages AND "${CMAKE_Fortran_LINKER_WRAPPER_FLAG}" STREQ
       message(WARNING "Cray Fortran toolchain failed to set CMAKE_Fortran_LINKER_WRAPPER_FLAGS.  Report this to vendor.  Setting flag to '-Wl,' manually as workaround.")
       set(CMAKE_Fortran_LINKER_WRAPPER_FLAG "-Wl,")
       set(CMAKE_Fortran_LINKER_WRAPPER_FLAG_SEP ",")
+  elseif("${CMAKE_Fortran_COMPILER_ID}" STREQUAL "LLVMFlang")
+         message(WARNING "LLVM Flang toolchain failed to set CMAKE_Fortran_LINKER_WRAPPER_FLAGS.  Report this to vendor.  Setting flag to '-Wl,' manually as workaround.")
+      set(CMAKE_Fortran_LINKER_WRAPPER_FLAG "-Wl,")
+      set(CMAKE_Fortran_LINKER_WRAPPER_FLAG_SEP ",")
   else()
       message(FATAL_ERROR "Toolchain file failed to initialize CMAKE_Fortran_LINKER_WRAPPER_FLAG, please submit a ticket to vendor.")
   endif()

src/teton/CMakeLists.txt

@@ -124,6 +124,11 @@ if(ENABLE_TESTS)
   add_executable( test_driver
                   driver/test_driver.cc )
 
+  if("${CMAKE_Fortran_COMPILER_ID}" STREQUAL "LLVMFlang")
+    set_target_properties( test_driver PROPERTIES LINK_FLAGS -fno-fortran-main)
+    set(OpenMP_Fortran_FLAGS "${OpenMP_Fortran_FLAGS} -fno-fortran-main")
+  endif()
+
   if (ENABLE_FIND_MPI)
     target_link_libraries( test_driver PUBLIC MPI::MPI_Fortran MPI::MPI_CXX )
   endif()