# Atomics

find_package(Atomics REQUIRED)

list(APPEND DEPLIBS ${ATOMICS_LIBRARIES})

# Original issue:

# * https://gitlab.kitware.com/cmake/cmake/-/issues/23021#note_1098733

#

# For reference:

# * https://gcc.gnu.org/wiki/Atomic/GCCMM

#

# riscv64 specific:

# * https://lists.debian.org/debian-riscv/2022/01/msg00009.html

#

# ATOMICS_FOUND        - system has c++ atomics

# ATOMICS_LIBRARIES    - libraries needed to use c++ atomics

include(CheckCXXSourceCompiles)

# RISC-V only has 32-bit and 64-bit atomic instructions. GCC is supposed

# to convert smaller atomics to those larger ones via masking and

# shifting like LLVM, but it’s a known bug that it does not. This means

# anything that wants to use atomics on 1-byte or 2-byte types needs

# -latomic, but not 4-byte or 8-byte (though it does no harm).

set(atomic_code

    "

     #include <atomic>

     #include <cstdint>

     std::atomic<uint8_t> n8 (0); // riscv64

     std::atomic<uint64_t> n64 (0); // armel, mipsel, powerpc

     int main() {

       ++n8;

       ++n64;

       return 0;

     }")

check_cxx_source_compiles("${atomic_code}" ATOMICS_LOCK_FREE_INSTRUCTIONS)

if(ATOMICS_LOCK_FREE_INSTRUCTIONS)

  set(ATOMICS_FOUND TRUE)

  set(ATOMICS_LIBRARIES)

else()

  set(CMAKE_REQUIRED_LIBRARIES "-latomic")

  check_cxx_source_compiles("${atomic_code}" ATOMICS_IN_LIBRARY)

  set(CMAKE_REQUIRED_LIBRARIES)

  if(ATOMICS_IN_LIBRARY)

    set(ATOMICS_LIBRARY atomic)

    include(FindPackageHandleStandardArgs)

    find_package_handle_standard_args(Atomics DEFAULT_MSG ATOMICS_LIBRARY)

    set(ATOMICS_LIBRARIES ${ATOMICS_LIBRARY})

    unset(ATOMICS_LIBRARY)

  else()

    if(Atomics_FIND_REQUIRED)

      message(FATAL_ERROR "Neither lock free instructions nor -latomic found.")

    endif()

  endif()

endif()

unset(atomic_code)