# Adapted from
# https://github.com/llvm-mirror/llvm/blob/master/cmake/modules/CheckAtomic.cmake commit ebe4432
# LLVM License is compatible with
# University of Illinois/NCSA Open Source License
#
# To use std::atomic
# libatomic is explicitly required with some compilers
#

include(CheckCXXSourceCompiles)
include(CheckLibraryExists)

# Sometimes linking against libatomic is required for atomic ops, if
# the platform doesn't support lock-free atomics.

function(check_working_cxx_atomics varname)
  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++14")
  check_cxx_source_compiles(
    "
#include <atomic>
std::atomic<int> x;
int main() {
  return x;
}
"
    ${varname})
  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
endfunction(check_working_cxx_atomics)

check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
# If not, check if the library exists, and atomics work with it.
if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
  check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
  if(HAVE_LIBATOMIC)
    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
    check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
    if(NOT HAVE_CXX_ATOMICS_WITH_LIB)
      message(FATAL_ERROR "Host compiler must support std::atomic!")
    endif()
  else()
    message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
  endif()
endif()