From a18bfe9c156be16f231e0bb380fc70fad22da20c Mon Sep 17 00:00:00 2001
From: Ken Haase <kh@beingmeta.com>
Date: Fri, 14 Feb 2020 10:18:53 -0500
Subject: Added atomic check for building on ARMs (#1196)

---
 CMakeLists.txt             |  7 +++++
 cmake/CheckAtomicLib.cmake | 70 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 cmake/CheckAtomicLib.cmake

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 70177449..d71e72b9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -148,6 +148,13 @@ if (NOT NNG_SANITIZER STREQUAL "none")
     set(NNG_SANITIZER_FLAGS "-fsanitize=${NNG_SANITIZER}")
 endif ()
 
+include(CheckAtomicLib)
+CheckAtomicLib()
+if (NOT HAVE_C_ATOMICS_WITHOUT_LIB AND HAVE_C_ATOMICS_WITH_LIB)
+     list(APPEND NNG_LIBS "atomic")
+endif ()
+
+
 if (NNG_ENABLE_COVERAGE)
     # NB: This only works for GCC and Clang 3.0 and newer.  If your stuff
     # is older than that, you will need to find something newer.  For
diff --git a/cmake/CheckAtomicLib.cmake b/cmake/CheckAtomicLib.cmake
new file mode 100644
index 00000000..06979305
--- /dev/null
+++ b/cmake/CheckAtomicLib.cmake
@@ -0,0 +1,70 @@
+#
+# Copyright 2020 Kenneth Haase <kh@beingmeta.com>
+#
+# This software is supplied under the terms of the MIT License, a
+# copy of which should be located in the distribution where this
+# file was obtained (LICENSE.txt).  A copy of the license may also be
+# found online at https://opensource.org/licenses/MIT.
+#
+
+# atomic builtins are required for threading support.
+
+INCLUDE(CheckIncludeFiles)
+INCLUDE(CheckCSourceCompiles)
+INCLUDE(CheckLibraryExists)
+
+# Sometimes linking against libatomic is required for atomic ops, if
+# the platform doesn't support lock-free atomics.
+
+function(check_c_atomics_without_lib varname)
+  CHECK_C_SOURCE_COMPILES("
+#include <stdatomic.h>
+int main() {
+  _Atomic long long x;
+  atomic_store(&x,3);
+  long long y = atomic_load(&x);
+  atomic_fetch_add(&x,12);
+  long long z = atomic_load(&x);
+  
+  return x;
+}
+" ${varname})
+endfunction(check_c_atomics_without_lib)
+
+function(check_c_atomics_with_lib varname)
+  SET(SAVED_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
+  list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+  CHECK_C_SOURCE_COMPILES("
+#include <stdatomic.h>
+int main() {
+  _Atomic long long x;
+  atomic_store(&x,3);
+  long long y = atomic_load(&x);
+  atomic_fetch_add(&x,12);
+  long long z = atomic_load(&x);
+  
+  return x;
+}
+" ${varname})
+  SET(CMAKE_REQUIRED_LIBRARIES "${SAVED_CMAKE_REQUIRED_LIBRARIES}")
+endfunction(check_c_atomics_with_lib)
+
+macro (CheckAtomicLib)
+  # First check if atomics work without the library.
+  if(MSVC)
+    set(HAVE_C_ATOMICS_WITHOUT_LIB True)
+  else()
+    check_c_atomics_without_lib(HAVE_C_ATOMICS_WITHOUT_LIB)
+    if(NOT HAVE_C_ATOMICS_WITHOUT_LIB)
+      check_library_exists(atomic __atomic_fetch_add_8 "" HAVE_LIBATOMIC)
+      if( HAVE_LIBATOMIC )
+	check_c_atomics_with_lib(HAVE_C_ATOMICS_WITH_LIB)
+	if (NOT HAVE_C_ATOMICS_WITH_LIB)
+	  message(FATAL_ERROR "Host compiler must support atomic types!")
+	endif()
+      else()
+	message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+      endif()
+    endif()
+  endif()
+endmacro (CheckAtomicLib)
-- 
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