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# 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;
}")
# https://gitlab.kitware.com/cmake/cmake/-/issues/24063
set(CMAKE_CXX_STANDARD 11)
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)
unset(CMAKE_CXX_STANDARD)
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