some 32 bit systems (armel, m68k, powerpc, sh4) don't link to atomic symbols

[drew-parsons]

Some 32 bit systems (armel, m68k, powerpc, sh4) don't link to atomic symbols. The problem is under discussion in gcc at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81358

This patch works around it for netgen, linking libngcore to libatomic where needed

Index: netgen/libsrc/core/CMakeLists.txt
===================================================================
--- netgen.orig/libsrc/core/CMakeLists.txt      2023-11-12 16:18:12.039784202 +0100
+++ netgen/libsrc/core/CMakeLists.txt   2023-11-12 16:20:53.737110429 +0100
@@ -60,6 +60,24 @@
     target_link_libraries(ngcore PRIVATE ${NUMA_LIBRARY})
 endif(USE_NUMA)

+# some 32-bit arches do not automatically link to atomic symbols (util.hpp)
+# cf. https://github.com/google/highway/pull/1008
+check_cxx_source_compiles(
+"#include <atomic>
+#include <cstdint>
+std::atomic<uint8_t> n8 (0);   // basic (should not fail)
+std::atomic<uint64_t> n64 (0); // expected to fail on armel, mipsel, powerpc (undefined reference to `__atomic_fetch_add_8')
+int main() {
+  ++n8;
+  ++n64;
+  return 0;
+}"
+ HAS_INTERNAL_ATOMIC
+)
+if(NOT HAS_INTERNAL_ATOMIC)
+  target_link_libraries(ngcore INTERFACE atomic)
+endif()
+
 install(TARGETS ngcore DESTINATION ${NG_INSTALL_DIR} COMPONENT netgen)

 target_link_libraries(ngcore PUBLIC netgen_mpi PRIVATE "$<BUILD_INTERFACE:netgen_python>" ${CMAKE_THREAD_LIBS_INIT})

atomic needs to be linked to ngcore as INTERFACE not PRIVATE (or PUBLIC) since libngcore.so itself does not use atomic symbols (they are used inline in core/utils.hpp).

The cmake test for atomic linking was adapted from https://github.com/google/highway/pull/1008

Originally posted by @drew-parsons in https://github.com/NGSolve/netgen/issues/168#issuecomment-1813500874

[barracuda156]

@drew-parsons Is this still an issue? I have just built netgen on MacOS PowerPC (32-bit), which normally does need libatomic linking. Looks like the build does not use 8-byte atomics.

P. S. Or maybe some of our Macports options disable usage of those.

[drew-parsons]

Still needed for 6.2.2305 building with gcc 13.2.0.

[barracuda156]

ngsolve 6.2.2307 by the way needs it for sure.