cmake: check atomic for `RISC-V` architecture

CMakeLists.txt

@@ -320,6 +320,9 @@ if (OPENDHT_STATIC)
             PUBLIC ${CMAKE_THREAD_LIBS_INIT} ${GNUTLS_LIBRARIES} ${Nettle_STATIC_LIBRARIES}
                    ${Jsoncpp_STATIC_LIBRARIES} ${FMT_LIBRARY} ${HTTP_PARSER_LIBRARY}
                    ${OPENSSL_STATIC_LIBRARIES})
+        if (NOT HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB)
+            target_link_libraries(opendht-static PUBLIC "atomic")
+        endif ()
     else ()
         if (OPENDHT_TOOLS)
             function (add_obj_lib name libfile)
@@ -393,6 +396,9 @@ if (OPENDHT_SHARED)
             PRIVATE ${GNUTLS_LIBRARIES} ${Nettle_LIBRARIES}
                     ${Jsoncpp_LIBRARIES}
                     ${FMT_LIBRARY} ${HTTP_PARSER_LIBRARY} ${argon2_LIBRARIES})
+        if (NOT HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB)
+            target_link_libraries(opendht PUBLIC "atomic")
+        endif ()
     endif ()
 
     install (TARGETS opendht DESTINATION ${CMAKE_INSTALL_LIBDIR} EXPORT opendht)
@@ -497,6 +503,9 @@ if (OPENDHT_TESTS)
        ${GNUTLS_LIBRARIES}
        ${Jsoncpp_LIBRARIES}
     )
+    if (NOT HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB)
+        target_link_libraries(opendht_unit_tests "atomic")
+    endif ()
     if (OPENDHT_PROXY_OPENSSL)
         target_link_libraries(opendht_unit_tests ${OPENSSL_LIBRARIES})
     endif()

cmake/CheckAtomic.cmake

@@ -34,6 +34,21 @@ int main() {
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_working_cxx_atomics64)
 
+function(check_working_cxx_atomics_for_risc_v varname)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++11")
+  CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+std::atomic<uint8_t> x;
+std::atomic<uint16_t> y;
+int main() {
+  x++;
+  y++;
+  return 0;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics_for_risc_v)
 
 # This isn't necessary on MSVC, so avoid command-line switch annoyance
 # by only running on GCC-like hosts.
@@ -76,6 +91,22 @@ if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
   endif()
 endif()
 
+# Check for RISC-V atomic operations.
+if(MSVC)
+  set(HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB True)
+else()
+  check_working_cxx_atomics_for_risc_v(HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB)
+endif()
+
+# If not, check if the library exists, and atomics work with it.
+if(NOT HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB)
+  list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+  check_working_cxx_atomics_for_risc_v(HAVE_CXX_ATOMICS_RISC_V_WITH_LIB)
+  if (NOT HAVE_CXX_ATOMICS_RISC_V_WITH_LIB)
+    message(FATAL_ERROR "Host compiler must support RISC-V std::atomic!")
+  endif()
+endif()
+
 ## TODO: This define is only used for the legacy atomic operations in
 ## llvm's Atomic.h, which should be replaced.  Other code simply
 ## assumes C++11 <atomic> works.