[Libcxx][MIPS] Use LLVM CheckAtomic.cmake module to figure out whether we need to link with libatomic.

[nitesh.jain]

Summary

The patch http://reviews.llvm.org/D20896 add check for 64 bit atomic operations in LLVM CheckAtomic.cmake module. We can than used this module to figure out whether we need to link with libatomic.

[nitesh.jain]

Hi Daniel/Vasileios

Please could you find sometime to review this?

Thanks

[dsanders]

What sets HAVE_CXX_LIBATOMICS64? I don't see a reference to LLVM's CheckAtomic.

[nitesh.jain]

In D21599#481553, @dsanders wrote:

What sets HAVE_CXX_LIBATOMICS64? I don't see a reference to LLVM's CheckAtomic.

The CheckAtomic.cmake module (http://reviews.llvm.org/D20896) in llvm will set HAVE_CXX_LIBATOMICS64 if library exit and atomics work with it. This value then can be read from LLVM CMake cache.

CMakeCache.txt74 KBDownload

[dsanders]

Have you tried a standalone build? I suspect HAVE_CXX_LIBATOMICS64 will never be set for that case. If that's the case, adding 'include(CheckAtomic)' will probably fix it.

[nitesh.jain]

In D21599#481588, @dsanders wrote:

Have you tried a standalone build? I suspect HAVE_CXX_LIBATOMICS64 will never be set for that case. If that's the case, adding 'include(CheckAtomic)' will probably fix it.

I never tried a standalone build. Please can you shared standalone build steps (I can't find any doc's for that) ?

Thanks

[dsanders]

I haven't used it that way either and I'm not sure it's a supported build for libcxx but most LLVM projects support a standalone build as far as I know. Can someone from the libcxx project confirm whether standalone builds are supported?

[EricWF]

In D21599#481593, @dsanders wrote:

I haven't used it that way either and I'm not sure it's a supported build for libcxx but most LLVM projects support a standalone build as far as I know. Can someone from the libcxx project confirm whether standalone builds are supported?

Standalone builds are supported. So this breaks those.

cmake/Modules/CheckLibcxxAtomic.cmake

-INCLUDE(CheckCXXSourceCompiles)
-
-# Sometimes linking against libatomic is required for atomic ops, if
-# the platform doesn't support lock-free atomics.
-#
-# We could modify LLVM's CheckAtomic module and have it check for 64-bit
-# atomics instead. However, we would like to avoid careless uses of 64-bit
-# atomics inside LLVM over time on 32-bit platforms.
-
-function(check_cxx_atomics varname)
-  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
-  if (${LIBCXX_GCC_TOOLCHAIN})
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
-  endif()
-  check_cxx_source_compiles("
-#include <cstdint>
-#include <atomic>
-std::atomic<uintptr_t> x;
-std::atomic<uintmax_t> y;
-int main() {
-  return x + y;
-}
-" ${varname})
-  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
-endfunction(check_cxx_atomics)
-
-check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
-# If not, check if the library exists, and atomics work with it.
-if(NOT LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
-  check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
-  if(LIBCXX_HAS_ATOMIC_LIB)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-    check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
-    if (NOT LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
-      message(WARNING "Host compiler must support std::atomic!")
-    endif()
-  else()
-    message(WARNING "Host compiler appears to require libatomic, but cannot find it.")
-  endif()
-endif()

cmake/config-ix.cmake

 include(CheckLibraryExists)
 include(CheckCXXCompilerFlag)
-include(CheckLibcxxAtomic)
 
 # Check compiler flags
 
 check_cxx_compiler_flag(/WX                     LIBCXX_HAS_WX_FLAG)
 check_cxx_compiler_flag(/WX-                    LIBCXX_HAS_NO_WX_FLAG)
 check_cxx_compiler_flag(/EHsc                   LIBCXX_HAS_EHSC_FLAG)
 check_cxx_compiler_flag(/EHs-                   LIBCXX_HAS_NO_EHS_FLAG)
 check_cxx_compiler_flag(/EHa-                   LIBCXX_HAS_NO_EHA_FLAG)

lib/CMakeLists.txt

 endif()
 
 # Generate library list.
 add_library_flags_if(LIBCXX_HAS_PTHREAD_LIB pthread)
 add_library_flags_if(LIBCXX_HAS_C_LIB c)
 add_library_flags_if(LIBCXX_HAS_M_LIB m)
 add_library_flags_if(LIBCXX_HAS_RT_LIB rt)
 add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
-add_library_flags_if(LIBCXX_HAS_ATOMIC_LIB atomic)
+add_library_flags_if(HAVE_CXX_LIBATOMICS64 atomic)
 
 # Setup flags.
 add_flags_if_supported(-fPIC)
 add_link_flags_if_supported(-nodefaultlibs)
 
 if ( APPLE AND (LIBCXX_CXX_ABI_LIBNAME STREQUAL "libcxxabi" OR
                 LIBCXX_CXX_ABI_LIBNAME STREQUAL "none"))
   if (NOT DEFINED LIBCXX_LIBCPPABI_VERSION)

test/CMakeLists.txt

 pythonize_bool(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
 pythonize_bool(LIBCXX_ENABLE_FILESYSTEM)
 pythonize_bool(LIBCXX_ENABLE_RTTI)
 pythonize_bool(LIBCXX_ENABLE_SHARED)
 pythonize_bool(LIBCXX_BUILD_32_BITS)
 pythonize_bool(LIBCXX_GENERATE_COVERAGE)
 pythonize_bool(LIBCXXABI_ENABLE_SHARED)
 pythonize_bool(LIBCXXABI_USE_LLVM_UNWINDER)
-pythonize_bool(LIBCXX_HAS_ATOMIC_LIB)
+pythonize_bool(HAVE_CXX_LIBATOMICS64)
 
 # The tests shouldn't link to any ABI library when it has been linked into
 # libc++ statically or via a linker script.
 if (LIBCXX_ENABLE_STATIC_ABI_LIBRARY OR LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
   set(LIBCXX_CXX_ABI_LIBNAME "none")
 endif()
 
 # By default, for non-standalone builds, libcxx and libcxxabi share a library

test/lit.site.cfg.in

 config.configuration_variant    = "@LIBCXX_LIT_VARIANT@"
 config.target_triple            = "@LIBCXX_TARGET_TRIPLE@"
 config.sysroot                  = "@LIBCXX_SYSROOT@"
 config.gcc_toolchain            = "@LIBCXX_GCC_TOOLCHAIN@"
 config.generate_coverage        = "@LIBCXX_GENERATE_COVERAGE@"
 config.target_info              = "@LIBCXX_TARGET_INFO@"
 config.executor                 = "@LIBCXX_EXECUTOR@"
 config.llvm_unwinder            = "@LIBCXXABI_USE_LLVM_UNWINDER@"
-config.use_libatomic            = "@LIBCXX_HAS_ATOMIC_LIB@"
+config.use_libatomic            = "@HAVE_CXX_LIBATOMICS64@"
 config.libcxxabi_shared         = "@LIBCXXABI_ENABLE_SHARED@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")