Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -232,6 +232,7 @@
 
 if (NOT MSVC)
   set(x86_64_SOURCES
+      ${GENERIC_TF_SOURCES}
       x86_64/floatdidf.c
       x86_64/floatdisf.c
       x86_64/floatdixf.c
Index: lib/builtins/fp_lib.h
===================================================================
--- lib/builtins/fp_lib.h
+++ lib/builtins/fp_lib.h
@@ -103,11 +103,12 @@
 COMPILER_RT_ABI fp_t __adddf3(fp_t a, fp_t b);
 
 #elif defined QUAD_PRECISION
-#if __LDBL_MANT_DIG__ == 113
+// __LDBL_MANT_DIG__ is set to 64 for x86_64.
+#if __LDBL_MANT_DIG__ == 113 || defined(__x86_64__)
 #define CRT_LDBL_128BIT
 typedef __uint128_t rep_t;
 typedef __int128_t srep_t;
-typedef long double fp_t;
+typedef __float128 fp_t;
 #define REP_C (__uint128_t)
 // Note: Since there is no explicit way to tell compiler the constant is a
 // 128-bit integer, we let the constant be casted to 128-bit integer