Detect availability of OpenMP in CMake and Make files (#1001)

All of the secondary cmakelists.txt files contained code equivalent to:

```cmake
target_link_libraries(qsim_decide OpenMP::OpenMP_CXX)
```

This PR updates the form to use the recommended approach of configuring
the use in the top-level `CMakeLists.txt` and then using `INTERFACE` in
the calls to `target_link_libraries`. The code there and also in the
updated `Makefile` checks that OpenMP is supported on the host before
adding the link flag.

Author: mhucka

CMakeLists.txt

@@ -52,6 +52,15 @@ else()
     endif()
 endif()
 
+# Use OpenMP if it can be found.
+find_package(OpenMP COMPONENTS CXX NO_POLICY_SCOPE)
+add_library(qsim_openmp_config INTERFACE)
+if(OpenMP_CXX_FOUND)
+    target_link_libraries(qsim_openmp_config INTERFACE OpenMP::OpenMP_CXX)
+else()
+    message(STATUS "${MSG_PREFIX} cannot use thread parallelization")
+endif()
+
 # Add optimization flags to any configuration that is NOT Debug. Note: CMake
 # will automatically use the appropriate debug flags (e.g., /Od and /Zi for
 # MSVC, or -O0 -g for GCC/Clang), so there's no need to do that part.
@@ -61,8 +70,6 @@ else()
     add_compile_options($<$<NOT:$<CONFIG:Debug>>:-O3>)
 endif()
 
-find_package(OpenMP REQUIRED)
-
 # Always build the basic part.
 add_subdirectory(pybind_interface/basic)
 add_subdirectory(pybind_interface/decide)
@@ -90,5 +97,4 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 
 # Print additional useful info.
-message(STATUS "${MSG_PREFIX} OpenMP found = ${OPENMP_FOUND}")
 message(STATUS "${MSG_PREFIX} shell $PATH = $ENV{PATH}")

Makefile

@@ -38,7 +38,7 @@ CXX ?= g++
 NVCC ?= nvcc
 HIPCC ?= hipcc
 
-BASE_CXXFLAGS := -std=c++17 -fopenmp
+BASE_CXXFLAGS := -std=c++17
 BASE_NVCCFLAGS := -std c++17 -Wno-deprecated-gpu-targets
 BASE_HIPCCFLAGS :=
 
@@ -49,7 +49,15 @@ HIPCCFLAGS := $(BASE_HIPCCFLAGS) $(HIPCCFLAGS)
 LTO_FLAGS := -flto=auto
 USING_CLANG := $(shell $(CXX) --version | grep -isq clang && echo "true")
 ifeq ($(USING_CLANG),"true")
-	LTO_FLAGS := -flto
+    LTO_FLAGS := -flto
+endif
+
+# Test if OpenMP header files are available and we can link with the library.
+OMP_CHECK_CMD := echo "int main() { return 0; }" | \
+    $(CXX) -fopenmp -x c++ - -o /dev/null 2>/dev/null
+HAVE_OPENMP := $(shell $(OMP_CHECK_CMD) && echo "true")
+ifeq ($(HAVE_OPENMP),true)
+    OPENMP_FLAGS := -fopenmp
 endif
 
 ifdef DEBUG
@@ -58,7 +66,7 @@ ifdef DEBUG
     NVCCFLAGS += $(DEBUG_FLAGS)
     HIPCCFLAGS += $(DEBUG_FLAGS)
 else
-    CXXFLAGS += -O3 $(LTO_FLAGS)
+    CXXFLAGS += -O3 $(OPENMP_FLAGS) $(LTO_FLAGS)
     NVCCFLAGS += -O3
     HIPCCFLAGS += -O3
 endif

pybind_interface/avx2/CMakeLists.txt

@@ -46,4 +46,4 @@ endif()
 include(../GetPybind11.cmake)
 pybind11_add_module(qsim_avx2 pybind_main_avx2.cpp)
 
-target_link_libraries(qsim_avx2 PUBLIC OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_avx2 PRIVATE qsim_openmp_config)

pybind_interface/avx512/CMakeLists.txt

@@ -39,4 +39,4 @@ endif()
 include(../GetPybind11.cmake)
 pybind11_add_module(qsim_avx512 pybind_main_avx512.cpp)
 
-target_link_libraries(qsim_avx512 PUBLIC OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_avx512 PRIVATE qsim_openmp_config)

pybind_interface/basic/CMakeLists.txt

@@ -39,4 +39,4 @@ endif()
 include(../GetPybind11.cmake)
 pybind11_add_module(qsim_basic pybind_main_basic.cpp)
 
-target_link_libraries(qsim_basic PUBLIC OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_basic PRIVATE qsim_openmp_config)

pybind_interface/cuda/CMakeLists.txt

@@ -57,4 +57,4 @@ set_target_properties(qsim_cuda PROPERTIES
 )
 set_source_files_properties(pybind_main_cuda.cpp PROPERTIES LANGUAGE CUDA)
 
-target_link_libraries(qsim_cuda OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_cuda PRIVATE qsim_openmp_config)

pybind_interface/custatevec/CMakeLists.txt

@@ -53,4 +53,4 @@ set_target_properties(qsim_custatevec PROPERTIES
 )
 set_source_files_properties(pybind_main_custatevec.cpp PROPERTIES LANGUAGE CUDA)
 
-target_link_libraries(qsim_custatevec OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_custatevec PRIVATE qsim_openmp_config)

pybind_interface/custatevecex/CMakeLists.txt

@@ -53,4 +53,4 @@ set_target_properties(qsim_custatevecex PROPERTIES
 )
 set_source_files_properties(pybind_main_custatevecex.cpp PROPERTIES LANGUAGE CUDA)
 
-target_link_libraries(qsim_custatevecex OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_custatevecex PRIVATE qsim_openmp_config)

pybind_interface/decide/CMakeLists.txt

@@ -61,7 +61,7 @@ if(CMAKE_CUDA_COMPILER)
         SUFFIX "${PYTHON_MODULE_EXTENSION}"
     )
     set_source_files_properties(decide.cpp PROPERTIES LANGUAGE CUDA)
-    target_link_libraries(qsim_decide OpenMP::OpenMP_CXX)
+
 elseif(has_hipcc)
     list(APPEND CMAKE_MODULE_PATH "/opt/rocm/lib/cmake/hip")
     find_package(HIP REQUIRED)
@@ -73,8 +73,9 @@ elseif(has_hipcc)
         PREFIX "${PYTHON_MODULE_PREFIX}"
         SUFFIX "${PYTHON_MODULE_EXTENSION}"
     )
-    target_link_libraries(qsim_decide PUBLIC OpenMP::OpenMP_CXX)
+
 else()
     pybind11_add_module(qsim_decide decide.cpp)
-    target_link_libraries(qsim_decide PUBLIC OpenMP::OpenMP_CXX)
 endif()
+
+target_link_libraries(qsim_decide PRIVATE qsim_openmp_config)

pybind_interface/hip/CMakeLists.txt

@@ -37,4 +37,4 @@ set_target_properties(qsim_hip PROPERTIES
 )
 set_source_files_properties(pybind_main_hip.cpp PROPERTIES LANGUAGE HIP)
 
-target_link_libraries(qsim_hip PUBLIC OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_hip PRIVATE qsim_openmp_config)