[OpenMP] Switch default C++ standard to C++ 14

[tianshilei1992]

Summary

Per discussion in https://reviews.llvm.org/D74145, switch default C++ standard to C++ 14

[JonChesterfield]

Thanks!

[Hahnfeld]

This should have a RFC on openmp-dev. As far as I understand the linked patch, the aim is to use std::make_unique which is just syntactic sugar IMO. Unless there are good reasons to switch, I'd object because it requires newer versions of compilers than many HPC systems have.

openmp/cmake/HandleOpenMPOptions.cmake

31–34

You should rename this, otherwise an incremental build won't test the new flag IIRC.

[JonChesterfield]

Unless there are good reasons to switch, I'd object because it requires newer versions of compilers than many HPC systems have.

I disagree. The use case we would be supporting is:

• building llvm libomptarget from source
• using it with a binary llvm dist, or some other toolchain
• won't download or build a newer toolchain

The advantage isn't in make_unique, it's in matching the rest of llvm which has required c++14 for months. It's not worth rehashing the stability-vs-progress discussion here when the parent project has already reached consensus.

[Hahnfeld]

In D74258#1865532, @JonChesterfield wrote:

Unless there are good reasons to switch, I'd object because it requires newer versions of compilers than many HPC systems have.

I disagree. The use case we would be supporting is:

• building llvm libomptarget from source
• using it with a binary llvm dist, or some other toolchain
• won't download or build a newer toolchain

You can do all that if the runtime only requires a subset of its parent project.

The advantage isn't in make_unique, it's in matching the rest of llvm which has required c++14 for months. It's not worth rehashing the stability-vs-progress discussion here when the parent project has already reached consensus.

I really hope that the OpenMP runtime has a broader usage than just with "the rest of llvm". It can be used standalone with Intel Compilers and GCC, and this is a great advantage for research.

[JonChesterfield]

In D74258#1865533, @Hahnfeld wrote:

You can do all that if the runtime only requires a subset of its parent project.

Yes, evidently. It's catering to a user who wants to build libomptarget from C++11 source, but is not willing to build a compiler from C++11 source, and is not willing to download a binary version of such a compiler. That's surely the empty set.

I really hope that the OpenMP runtime has a broader usage than just with "the rest of llvm". It can be used standalone with Intel Compilers and GCC, and this is a great advantage for research.

I believe it does have such usage. I just don't believe that requiring the same compiler capability as the rest of llvm will hinder this, and I definitely believe in the ongoing inefficiency induced by sticking with an older C++ revision. Exactly the same arguments that (eventually) moved the rest of llvm up to C++14 apply here.

[jdoerfert]

FWIW, I'm in strong favor of this, e.g., I suggested it in the other patch.

@Hahnfeld I do appreciate your concerns. Since you asked for an RFC, I wrote one: http://lists.llvm.org/pipermail/openmp-dev/2020-February/003176.html Let's move the discussion to the list and accept/reject this patch after we came to some kind of conclusion there.

[tianshilei1992]

openmp/cmake/HandleOpenMPOptions.cmake

31–34

Oh, exactly, sorry for missing that.

[JonChesterfield]

Issue https://github.com/llvm/llvm-project/issues/159 suggests we missed a file here. Perhaps plugins was previously relying on the compiler default?

[tianshilei1992]

In D74258#1899530, @JonChesterfield wrote:

Issue https://github.com/llvm/llvm-project/issues/159 suggests we missed a file here. Perhaps plugins was previously relying on the compiler default?

I’ve root cause his issue. It is environment issue.

openmp/CMakeLists.txt

 # Build host runtime library.
 add_subdirectory(runtime)
 
 
 set(ENABLE_LIBOMPTARGET ON)
 # Currently libomptarget cannot be compiled on Windows or MacOS X.
 # Since the device plugins are only supported on Linux anyway,
 # there is no point in trying to compile libomptarget on other OSes.
-if (APPLE OR WIN32 OR NOT OPENMP_HAVE_STD_CPP11_FLAG)
+if (APPLE OR WIN32 OR NOT OPENMP_HAVE_STD_CPP14_FLAG)
   set(ENABLE_LIBOMPTARGET OFF)
 endif()
 
 option(OPENMP_ENABLE_LIBOMPTARGET "Enable building libomptarget for offloading."
        ${ENABLE_LIBOMPTARGET})
 if (OPENMP_ENABLE_LIBOMPTARGET)
   # Check that the library can actually be built.
   if (APPLE OR WIN32)
     message(FATAL_ERROR "libomptarget cannot be built on Windows and MacOS X!")
-  elseif (NOT OPENMP_HAVE_STD_CPP11_FLAG)
-    message(FATAL_ERROR "Host compiler must support C++11 to build libomptarget!")
+  elseif (NOT OPENMP_HAVE_STD_CPP14_FLAG)
+    message(FATAL_ERROR "Host compiler must support C++14 to build libomptarget!")
   endif()
 
   add_subdirectory(libomptarget)
 endif()
 
 set(ENABLE_OMPT_TOOLS ON)
 # Currently tools are not tested well on Windows or MacOS X.
 if (APPLE OR WIN32)

openmp/cmake/HandleOpenMPOptions.cmake

 append_if(OPENMP_HAVE_WFORMAT_PEDANTIC_FLAG "-Wformat-pedantic" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 append_if(OPENMP_HAVE_WIMPLICIT_FALLTHROUGH_FLAG "-Wimplicit-fallthrough" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 append_if(OPENMP_HAVE_WSIGN_COMPARE_FLAG "-Wsign-compare" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
 # Warnings that we want to disable because they are too verbose or fragile.
 append_if(OPENMP_HAVE_WNO_EXTRA_FLAG "-Wno-extra" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 append_if(OPENMP_HAVE_WNO_PEDANTIC_FLAG "-Wno-pedantic" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 append_if(OPENMP_HAVE_WNO_MAYBE_UNINITIALIZED_FLAG "-Wno-maybe-uninitialized" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
-append_if(OPENMP_HAVE_STD_GNUPP11_FLAG "-std=gnu++11" CMAKE_CXX_FLAGS)
-if (NOT OPENMP_HAVE_STD_GNUPP11_FLAG)
-  append_if(OPENMP_HAVE_STD_CPP11_FLAG "-std=c++11" CMAKE_CXX_FLAGS)
+append_if(OPENMP_HAVE_STD_GNUPP14_FLAG "-std=gnu++14" CMAKE_CXX_FLAGS)
+if (NOT OPENMP_HAVE_STD_GNUPP14_FLAG)
+  append_if(OPENMP_HAVE_STD_CPP14_FLAG "-std=c++14" CMAKE_CXX_FLAGS)
 endif()

openmp/cmake/config-ix.cmake

 include(CheckCXXCompilerFlag)
 
 check_cxx_compiler_flag(-Wall OPENMP_HAVE_WALL_FLAG)
 check_cxx_compiler_flag(-Werror OPENMP_HAVE_WERROR_FLAG)
 
 # Additional warnings that are not enabled by -Wall.
 check_cxx_compiler_flag(-Wcast-qual OPENMP_HAVE_WCAST_QUAL_FLAG)
 check_cxx_compiler_flag(-Wformat-pedantic OPENMP_HAVE_WFORMAT_PEDANTIC_FLAG)
 check_cxx_compiler_flag(-Wimplicit-fallthrough OPENMP_HAVE_WIMPLICIT_FALLTHROUGH_FLAG)
 check_cxx_compiler_flag(-Wsign-compare OPENMP_HAVE_WSIGN_COMPARE_FLAG)
 
 # Warnings that we want to disable because they are too verbose or fragile.
 check_cxx_compiler_flag(-Wno-extra OPENMP_HAVE_WNO_EXTRA_FLAG)
 check_cxx_compiler_flag(-Wno-pedantic OPENMP_HAVE_WNO_PEDANTIC_FLAG)
 check_cxx_compiler_flag(-Wno-maybe-uninitialized OPENMP_HAVE_WNO_MAYBE_UNINITIALIZED_FLAG)
 
-check_cxx_compiler_flag(-std=gnu++11 OPENMP_HAVE_STD_GNUPP11_FLAG)
-check_cxx_compiler_flag(-std=c++11 OPENMP_HAVE_STD_CPP11_FLAG)
+check_cxx_compiler_flag(-std=gnu++14 OPENMP_HAVE_STD_GNUPP14_FLAG)
+check_cxx_compiler_flag(-std=c++14 OPENMP_HAVE_STD_CPP14_FLAG)

openmp/libomptarget/cmake/Modules/LibomptargetNVPTXBitcodeLibrary.cmake

if (${discard_cached} OR NOT DEFINED ${output})
     else()
       message(STATUS "Performing Test ${output} - Failed")
     endif()
   endif()
 endfunction()
 
 # These flags are required to emit LLVM Bitcode. We check them together because
 # if any of them are not supported, there is no point in finding out which are.
-set(compiler_flags_required -emit-llvm -O1 --cuda-device-only -std=c++11 --cuda-path=${CUDA_TOOLKIT_ROOT_DIR})
+set(compiler_flags_required -emit-llvm -O1 --cuda-device-only -std=c++14 --cuda-path=${CUDA_TOOLKIT_ROOT_DIR})
 set(compiler_flags_required_src "extern \"C\" __device__ int thread() { return threadIdx.x; }")
 check_bitcode_compilation(LIBOMPTARGET_NVPTX_CUDA_COMPILER_SUPPORTS_FLAGS_REQUIRED "${compiler_flags_required_src}" ${compiler_flags_required})
 
 # It makes no sense to continue given that the compiler doesn't support
 # emitting basic LLVM Bitcode
 if (NOT LIBOMPTARGET_NVPTX_CUDA_COMPILER_SUPPORTS_FLAGS_REQUIRED)
   return()
 endif()

openmp/libomptarget/deviceRTLs/amdgcn/CMakeLists.txt

 # create libraries
 set(mcpus gfx700 gfx701 gfx801 gfx803 gfx900)
 if (DEFINED LIBOMPTARGET_AMDGCN_GFXLIST)
   set(mcpus ${LIBOMPTARGET_AMDGCN_GFXLIST})
 endif()
 
 macro(add_cuda_bc_library)
   set(cu_cmd ${AOMP_BINDIR}/clang++
-    -std=c++11
+    -std=c++14
     -fcuda-rdc
     -fvisibility=default
     --cuda-device-only
     -Wno-unused-value
     -x hip
     -O${optimization_level}
     --cuda-gpu-arch=${mcpu}
     ${CUDA_DEBUG}

openmp/runtime/test/lit.cfg

 
 
 # substitutions
 config.substitutions.append(("%libomp-compile-and-run", \
     "%libomp-compile && %libomp-run"))
 config.substitutions.append(("%libomp-cxx-compile-and-run", \
     "%libomp-cxx-compile && %libomp-run"))
 config.substitutions.append(("%libomp-cxx-compile", \
-    "%clangXX %openmp_flags %flags -std=c++11 %s -o %t" + libs))
+    "%clangXX %openmp_flags %flags -std=c++14 %s -o %t" + libs))
 config.substitutions.append(("%libomp-compile", \
     "%clang %openmp_flags %flags %s -o %t" + libs))
 config.substitutions.append(("%libomp-run", "%t"))
 config.substitutions.append(("%clangXX", config.test_cxx_compiler))
 config.substitutions.append(("%clang", config.test_c_compiler))
 config.substitutions.append(("%openmp_flags", config.test_openmp_flags))
 config.substitutions.append(("%flags", config.test_flags))
 config.substitutions.append(("%python", '"%s"' % (sys.executable)))

openmp/tools/archer/tests/lit.cfg

 # Race Tests
 config.substitutions.append(("%libarcher-compile-and-run-race", \
     "%libarcher-compile && %libarcher-run-race"))
 config.substitutions.append(("%libarcher-compile-and-run", \
                              "%libarcher-compile && %libarcher-run"))
 config.substitutions.append(("%libarcher-cxx-compile-and-run", \
     "%libarcher-cxx-compile && %libarcher-run"))
 config.substitutions.append(("%libarcher-cxx-compile", \
-    "%clang-archerXX %openmp_flags %archer_flags %flags -std=c++11 %s -o %t" + libs))
+    "%clang-archerXX %openmp_flags %archer_flags %flags -std=c++14 %s -o %t" + libs))
 config.substitutions.append(("%libarcher-compile", \
                              "%clang-archer %openmp_flags %archer_flags %flags %s -o %t" + libs))
 config.substitutions.append(("%libarcher-run-race", "%suppression %deflake %t 2>&1 | tee %t.log"))
 config.substitutions.append(("%libarcher-run", "%suppression %t 2>&1 | tee %t.log"))
 config.substitutions.append(("%clang-archerXX", config.test_cxx_compiler))
 config.substitutions.append(("%clang-archer", config.test_c_compiler))
 config.substitutions.append(("%openmp_flags", config.test_openmp_flags))
 config.substitutions.append(("%archer_flags", config.archer_flags))