Merge pull request swiftlang#25762 from compnerd/cmpxchg

build: add a workaround for android NDK[<21]

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -288,6 +288,21 @@ function(_add_variant_c_compile_flags)
     endif()
   endif()
 
+  if(CFLAGS_SDK STREQUAL ANDROID)
+    if(CFLAGS_ARCH STREQUAL x86_64)
+      # NOTE(compnerd) Android NDK 21 or lower will generate library calls to
+      # `__sync_val_compare_and_swap_16` rather than lowering to the CPU's
+      # `cmpxchg16b` instruction as the `cx16` feature is disabled due to a bug
+      # in Clang.  This is being fixed in the current master Clang and will
+      # hopefully make it into Clang 9.0.  In the mean time, workaround this in
+      # the build.
+      if(CMAKE_C_COMPILER_ID MATCHES Clang AND CMAKE_C_COMPILER_VERSION
+          VERSION_LESS 9.0.0)
+        list(APPEND result -mcx16)
+      endif()
+    endif()
+  endif()
+
   if(CFLAGS_ENABLE_ASSERTIONS)
     list(APPEND result "-UNDEBUG")
   else()