diff --git a/build/CMakeLists.txt b/build/CMakeLists.txt
index 4d462700..bc80423a 100644
--- a/build/CMakeLists.txt
+++ b/build/CMakeLists.txt
@@ -257,8 +257,6 @@ if(THREADS_HAVE_PTHREAD_ARG) # compile time flag
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
 endif()
 
-include(CheckLibcxxAtomic)
-
 if (WITH_STATIC)
   set(Boost_USE_STATIC_LIBS ON)
   set(Boost_USE_STATIC_RUNTIME ON)
@@ -384,11 +382,7 @@ if (WITH_MESHNET)
   message(WARNING "This build will NOT work on mainline i2p")
 endif()
 
-if (ARCHITECTURE MATCHES "arm")
-  set(ATOMIC_LIB -latomic)
-else()
-  set(ATMOIC_LIB "")
-endif()
+include(CheckAtomic)
 
 
 # show summary
@@ -457,7 +451,7 @@ if (WITH_BINARY)
   if (WITH_STATIC)
     set(DL_LIB ${CMAKE_DL_LIBS})
   endif()
-  target_link_libraries( "${PROJECT_NAME}" libi2pd i2pdclient ${DL_LIB} ${Boost_LIBRARIES} ${OPENSSL_LIBRARIES} ${ZLIB_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MINGW_EXTRA} ${DL_LIB} ${ATOMIC_LIB})
+  target_link_libraries( "${PROJECT_NAME}" libi2pd i2pdclient ${DL_LIB} ${Boost_LIBRARIES} ${OPENSSL_LIBRARIES} ${ZLIB_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MINGW_EXTRA} ${DL_LIB} ${CMAKE_REQUIRED_LIBRARIES})
 
   install(TARGETS "${PROJECT_NAME}" RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT Runtime)
   set (APPS "\${CMAKE_INSTALL_PREFIX}/bin/${PROJECT_NAME}${CMAKE_EXECUTABLE_SUFFIX}")
diff --git a/build/cmake_modules/CheckAtomic.cmake b/build/cmake_modules/CheckAtomic.cmake
new file mode 100644
index 00000000..b8296a1c
--- /dev/null
+++ b/build/cmake_modules/CheckAtomic.cmake
@@ -0,0 +1,106 @@
+# atomic builtins are required for threading support.
+
+INCLUDE(CheckCXXSourceCompiles)
+
+# 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 "-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)
+
+
+# This isn't necessary on MSVC, so avoid command-line switch annoyance
+# by only running on GCC-like hosts.
+if (LLVM_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)
+else()
+  check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+endif()
+
+# 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 std::atomic!")
+    endif()
+  else()
+    message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+  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.
+CHECK_CXX_SOURCE_COMPILES("
+#ifdef _MSC_VER
+#include <Intrin.h> /* Workaround for PR19898. */
+#include <windows.h>
+#endif
+int main() {
+#ifdef _MSC_VER
+        volatile LONG val = 1;
+        MemoryBarrier();
+        InterlockedCompareExchange(&val, 0, 1);
+        InterlockedIncrement(&val);
+        InterlockedDecrement(&val);
+#else
+        volatile unsigned long val = 1;
+        __sync_synchronize();
+        __sync_val_compare_and_swap(&val, 1, 0);
+        __sync_add_and_fetch(&val, 1);
+        __sync_sub_and_fetch(&val, 1);
+#endif
+        return 0;
+      }
+" LLVM_HAS_ATOMICS)
+
+if( NOT LLVM_HAS_ATOMICS )
+  message(STATUS "Warning: LLVM will be built thread-unsafe because atomic builtins are missing")
+endif()
diff --git a/build/cmake_modules/CheckLibcxxAtomic.cmake b/build/cmake_modules/CheckLibcxxAtomic.cmake
deleted file mode 100644
index a7ae667a..00000000
--- a/build/cmake_modules/CheckLibcxxAtomic.cmake
+++ /dev/null
@@ -1,47 +0,0 @@
-INCLUDE(CheckCXXSourceCompiles)
-
-# Sometimes linking against libatomic is required for atomic ops, if
-# the platform doesn't support lock-free atomics.
-#
-# We could modify LLVM's CheckAtomic module and have it check for 64-bit
-# atomics instead. However, we would like to avoid careless uses of 64-bit
-# atomics inside LLVM over time on 32-bit platforms.
-
-function(check_cxx_atomics varname)
-  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-nodefaultlibs -std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
-  if (${LIBCXX_GCC_TOOLCHAIN})
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
-  endif()
-  if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
-  endif()
-  if (CMAKE_C_FLAGS MATCHES -fsanitize-coverage OR CMAKE_CXX_FLAGS MATCHES -fsanitize-coverage)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")
-  endif()
-  check_cxx_source_compiles("
-#include <cstdint>
-#include <atomic>
-std::atomic<uintptr_t> x;
-std::atomic<uintmax_t> y;
-int main() {
-  return x + y;
-}
-" ${varname})
-  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
-endfunction(check_cxx_atomics)
-
-check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
-check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
-# If not, check if the library exists, and atomics work with it.
-if(NOT LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
-  if(LIBCXX_HAS_ATOMIC_LIB)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-    check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
-    if (NOT LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
-      message(WARNING "Host compiler must support std::atomic!")
-    endif()
-  else()
-    message(WARNING "Host compiler appears to require libatomic, but cannot find it.")
-  endif()
-endif()