From 345069028b7d933a039aff91e254876677081471 Mon Sep 17 00:00:00 2001
From: Artur Sinila <freesoftware@logarithmus.dev>
Date: Tue, 29 Jun 2021 22:41:25 +0300
Subject: [PATCH] Fix atomic for architectures without support for lock-free
 atomics (e.g. armv6l)

---
 CMakeLists.txt     |  3 ++
 cmake/atomic.cmake | 97 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 100 insertions(+)
 create mode 100644 cmake/atomic.cmake

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b56953c4..65b898ae 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -203,6 +203,9 @@ else()
   endif()
 endif()
 
+include("cmake/atomic.cmake")
+list(APPEND mi_libraries ${CMAKE_REQUIRED_LIBRARIES})
+
 # -----------------------------------------------------------------------------
 # Install and output names
 # -----------------------------------------------------------------------------
diff --git a/cmake/atomic.cmake b/cmake/atomic.cmake
new file mode 100644
index 00000000..b2a8bb63
--- /dev/null
+++ b/cmake/atomic.cmake
@@ -0,0 +1,97 @@
+# Based on:
+# https://github.com/llvm/llvm-project/blob/d4dcb55c7050fd908af2378fa551078d859d994f/llvm/cmake/modules/DetermineGCCCompatible.cmake
+# https://github.com/llvm/llvm-project/blob/d4dcb55c7050fd908af2378fa551078d859d994f/llvm/cmake/modules/CheckAtomic.cmake
+
+INCLUDE(CheckCXXSourceCompiles)
+INCLUDE(CheckLibraryExists)
+
+# Determine if the compiler has GCC-compatible command-line syntax.
+
+if(CMAKE_COMPILER_IS_GNUCXX)
+  set(COMPILER_IS_GCC_COMPATIBLE ON)
+elseif( MSVC )
+  set(COMPILER_IS_GCC_COMPATIBLE OFF)
+elseif( "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
+  set(COMPILER_IS_GCC_COMPATIBLE ON)
+elseif( "${CMAKE_CXX_COMPILER_ID}" MATCHES "Intel" )
+  set(COMPILER_IS_GCC_COMPATIBLE ON)
+endif()
+
+# Sometimes linking against libatomic is required for atomic ops, if
+# the platform doesn't support lock-free atomics.
+
+function(check_working_cxx_atomics 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<int> x;
+std::atomic<short> y;
+std::atomic<char> z;
+int main() {
+  ++z;
+  ++y;
+  return ++x;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics)
+
+function(check_working_cxx_atomics64 varname)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11 ${CMAKE_REQUIRED_FLAGS}")
+  CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+#include <cstdint>
+std::atomic<uint64_t> x (0);
+int main() {
+  uint64_t i = x.load(std::memory_order_relaxed);
+  (void)i;
+  return 0;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics64)
+
+
+# Check for (non-64-bit) atomic operations.
+if(MSVC)
+  set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)
+elseif(COMPILER_IS_GCC_COMPATIBLE)
+  # First check if atomics work without the library.
+  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)
+    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()
+
+# Check for 64 bit atomic operations.
+if(MSVC)
+  set(HAVE_CXX_ATOMICS64_WITHOUT_LIB True)
+elseif(COMPILER_IS_GCC_COMPATIBLE)
+  # First check if atomics work without the library.
+  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)
+    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()