build/cmake: update CheckAtomic.cmake

cmake/CheckAtomic.cmake

@@ -49,10 +49,15 @@ elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL")
   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)
-    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!")
+    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()
 endif()
@@ -65,14 +70,32 @@ elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL")
   check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
   # If not, check if the library exists, and atomics work with it.
   if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-    check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITH_LIB)
-    if (NOT HAVE_CXX_ATOMICS64_WITH_LIB)
-      message(FATAL_ERROR "Host compiler must support 64-bit std::atomic!")
+    check_library_exists(atomic __atomic_load_8 "" HAVE_CXX_LIBATOMICS64)
+    if(HAVE_CXX_LIBATOMICS64)
+      list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+      check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITH_LIB)
+      if (NOT HAVE_CXX_ATOMICS64_WITH_LIB)
+        message(FATAL_ERROR "Host compiler must support 64-bit std::atomic!")
+      endif()
+    else()
+      #message(FATAL_ERROR "Host compiler appears to require libatomic for 64-bit operations, but cannot find it.")
     endif()
   endif()
 endif()
 
+# Set variable LLVM_ATOMIC_LIB specifying flags for linking against libatomic.
+if(HAVE_CXX_ATOMICS_WITH_LIB OR HAVE_CXX_ATOMICS64_WITH_LIB)
+  # Use options --push-state, --as-needed and --pop-state if linker is known to support them.
+  # Use single option -Wl of compiler driver to avoid incorrect re-ordering of options by CMake.
+  if(LLVM_LINKER_IS_GNULD OR LLVM_LINKER_IS_GOLD OR LLVM_LINKER_IS_LLD OR LLVM_LINKER_IS_MOLD)
+    set(LLVM_ATOMIC_LIB "-Wl,--push-state,--as-needed,-latomic,--pop-state")
+  else()
+    set(LLVM_ATOMIC_LIB "-latomic")
+  endif()
+else()
+  set(LLVM_ATOMIC_LIB)
+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.