Cannot build with CUDA enabled and C++17

[Rombur]

Hey all,

I can't compile the library with CUDA enabled and C++17. Somehow it looks like we inject -std=c++14 in the cuda flags and nvcc does not like to have both c++14 and c++17. Unfortunately, I can't find where the -std=c++14 flag is added. You can see in the detailed.log that the CUDA flags are correct:

#            DEAL_II_CUDA_FLAGS = -arch=sm_70 -std=c++17
#            DEAL_II_CUDA_FLAGS_RELEASE = 
#            DEAL_II_CUDA_FLAGS_DEBUG = -G

but the CUDA flags that are used are different (from source/lac/CMakeFiles/obj_lac_debug.dir/flags.make)

-arch=sm_70 -std=c++17 -G -std=c++14

I obviously tried to grep for -std=c++14 but I couldn't find it. Looking at this I would expect -G to be the last flag. Any idea how to debug this?

[masterleinad]

Hmmm... I can't quite reproduce. Configuring with CMAKE_CXX_STANDARD=17 and DEAL_II_CUDA_FLAGS= -arch=sm_70 -std=c++17 gives me

CUDA_FLAGS =   -Xcompiler "-fPIC -Wall -Wextra -Wmissing-braces -Woverloaded-virtual -Wpointer-arith -Wsign-compare -Wsuggest-override -Wswitch -Wsynth -Wwrite-strings -Wno-placement-new -Wno-deprecated-declarations -Wno-literal-suffix -Wno-psabi -fopenmp-simd -pthread -Wno-parentheses -Wno-unused-local-typedefs -O0 -ggdb " -arch=sm_70 -std=c++17 -G

The only place I could think of that adds a -std=c++14 flag is

dealii/cmake/checks/check_01_cxx_features.cmake

Lines 307 to 312 in b956691

	IF(NOT DEAL_II_HAVE_CXX14)
	MESSAGE(STATUS "C++14 support not available. Try to set -std=c++14 explicitly")
	ENABLE_IF_SUPPORTED(DEAL_II_CXX_FLAGS "-std=c++14")
	_set_up_cmake_required()
	_test_cxx14_support()
	ENDIF()

.

[Rombur]

Thanks that's true it works with CMAKE_CXX_STANDARD=17. I was setting the flag through CMAKE_CXX_FLAGS. The reason I didn't use CMAKE_CXX_STANDARD=17 is because the std=c++17 flag is not passed to our C++17 compatibility check. If you set CMAKE_CXX_STANDARD=17 alone, our C++17 check will fail and you will see that deal.II disables C++17. You can see in your CUDA_FLAGS that the std=c++17 flag is not sent to the host compiler. I guess we need to set the flag explicitly for the host and the device compiler and set the CXX standard.

[masterleinad]

Fair enough. Still, if I set DEAL_II_CXX_FLAGS=-std=c++17 and DEAL_II_CUDA_FLAGS=-arch=sm_70 -std=c++17 (and don't specify CMAKE_CXX_STANDARD), I get

CUDA_FLAGS =   -Xcompiler "-fPIC -Wall -Wextra -Wmissing-braces -Woverloaded-virtual -Wpointer-arith -Wsign-compare -Wsuggest-override -Wswitch -Wsynth -Wwrite-strings -Wno-placement-new -Wno-deprecated-declarations -Wno-literal-suffix -Wno-psabi -fopenmp-simd -std=c++17 -pthread -Wno-parentheses -Wno-unused-local-typedefs -O0 -ggdb " -arch=sm_70 -std=c++17 -G

[Rombur]

That's so strange. I tried two different versions of CMake (3.17 and 3.18) but I always get the same problem.

[tamiko]

@Rombur Is this still an issue?

[Rombur]

I haven't tried in a while but since we are moving to Kokkos anyway this is irrelevant. I'm closing