Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -502,6 +502,9 @@
   set(BUILTIN_CFLAGS "")
 
   append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 BUILTIN_CFLAGS)
+  if("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "riscv")
+    append_list_if(COMPILER_RT_HAS_FINT128_FLAG -fforce-enable-int128 BUILTIN_CFLAGS)
+  endif()
 
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm
   # cmake step. Add them manually if this is a standalone build.