diff --git a/compiler-rt/lib/builtins/fp_lib.h b/compiler-rt/lib/builtins/fp_lib.h
--- a/compiler-rt/lib/builtins/fp_lib.h
+++ b/compiler-rt/lib/builtins/fp_lib.h
@@ -26,6 +26,15 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+#if (defined(SINGLE_PRECISION) + defined(DOUBLE_PRECISION) +                   \
+     defined(QUAD_PRECISION)) != 1
+#error Exactly one of SINGLE_PRECISION, DOUBLE_PRECISION or QUAD_PRECISION must be defined.
+// Now, turn on some arbitrary one of the three precision settings (except for
+// QUAD_PRECISION that has some specific requirements), so fp_*_impl.inc files
+// could be validated by clang-tidy in a more meaningful way.
+#define SINGLE_PRECISION
+#endif
+
 // x86_64 FreeBSD prior v9.3 define fixed-width types incorrectly in
 // 32-bit mode.
 #if defined(__FreeBSD__) && defined(__i386__)
@@ -195,6 +204,8 @@
 #undef Word_FullMask
 #endif // __LDBL_MANT_DIG__ == 113 && __SIZEOF_INT128__
 #else
+// Should never be reached due to the check for precison-related macro
+// at the top of this file. Recheck it here just in case...
 #error SINGLE_PRECISION, DOUBLE_PRECISION or QUAD_PRECISION must be defined.
 #endif