From 8061c6c7f103ab66296238df89c0dc606aa38d76 Mon Sep 17 00:00:00 2001
From: Alexey Shabalin <shaba@altlinux.org>
Date: Sun, 28 Jun 2020 18:56:20 +0300
Subject: [PATCH] Link with -latomic if necessary

This commit largely comes from mips_mipsel_libatomic.patch
from Debian ceph package.
Thanks to iv@
---
 CMakeLists.txt                    |  6 ++++++
 build_tools/build_detect_platform |  4 ++++
 cmake/modules/CheckAtomic.cmake   | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 77 insertions(+)
 create mode 100644 cmake/modules/CheckAtomic.cmake

diff --git a/CMakeLists.txt b/CMakeLists.txt
index aeef92e..3d5c140 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -799,6 +799,12 @@ else()
   set(SYSTEM_LIBS ${CMAKE_THREAD_LIBS_INIT})
 endif()
 
+# check if linking against libatomic is necessary
+include(CheckAtomic)
+if(HAVE_CXX_ATOMIC_WITH_LIB OR HAVE_CXX_ATOMICS64_WITH_LIB)
+  set(SYSTEM_LIBS ${SYSTEM_LIBS} atomic)
+endif()
+
 add_library(${ROCKSDB_STATIC_LIB} STATIC ${SOURCES})
 target_link_libraries(${ROCKSDB_STATIC_LIB}
   ${THIRDPARTY_LIBS} ${SYSTEM_LIBS})
diff --git a/build_tools/build_detect_platform b/build_tools/build_detect_platform
index 158c161..493cd89 100755
--- a/build_tools/build_detect_platform
+++ b/build_tools/build_detect_platform
@@ -681,6 +681,10 @@ EOF
   fi
 fi
 
+if [ "$TARGET_ARCHITECTURE" = "mips" ] ; then
+  PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -latomic"
+fi
+
 PLATFORM_CCFLAGS="$PLATFORM_CCFLAGS $COMMON_FLAGS"
 PLATFORM_CXXFLAGS="$PLATFORM_CXXFLAGS $COMMON_FLAGS"
 
diff --git a/cmake/modules/CheckAtomic.cmake b/cmake/modules/CheckAtomic.cmake
new file mode 100644
index 0000000..67fde83
--- /dev/null
+++ b/cmake/modules/CheckAtomic.cmake
@@ -0,0 +1,67 @@
+# Checks if atomic operations are supported natively or if linking against
+# libatomic is needed.
+
+# Check inspired by LLVMs cmake/modules/CheckAtomic.cmake
+
+INCLUDE(CheckCXXSourceCompiles)
+INCLUDE(CheckLibraryExists)
+
+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;
+int main() {
+  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);
+  return 0;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics64)
+
+# Check if atomics work without libatomic
+check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+
+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()
+
+# Check if 64bit atomics work without libatomic
+check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+
+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 std::atomic!")
+    endif()
+  else()
+    message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+  endif()
+endif()
--
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