diff --git a/internal/compilers.h b/internal/compilers.h
index 8f32030c60..781dd9cfc9 100644
--- a/internal/compilers.h
+++ b/internal/compilers.h
@@ -85,6 +85,7 @@
 #  * granular. */
 # /* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66970 */
 # define __has_builtin(...) __has_builtin_##__VA_ARGS__
+# define __has_builtin____builtin_add_overflow   GCC_VERSION_SINCE(5, 1, 0)
 # define __has_builtin____builtin_bswap16        GCC_VERSION_SINCE(4, 8, 0) /* http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52624 */
 # define __has_builtin____builtin_bswap32        GCC_VERSION_SINCE(3, 6, 0)
 # define __has_builtin____builtin_bswap64        GCC_VERSION_SINCE(3, 6, 0)
@@ -94,12 +95,17 @@
 # define __has_builtin____builtin_ctz            GCC_VERSION_SINCE(3, 6, 0)
 # define __has_builtin____builtin_ctzl           GCC_VERSION_SINCE(3, 6, 0)
 # define __has_builtin____builtin_ctzll          GCC_VERSION_SINCE(3, 6, 0)
-# define __has_builtin____builtin_mul_overflow   GCC_VERSION_SINCE(5, 0, 0)
+# define __has_builtin____builtin_mul_overflow   GCC_VERSION_SINCE(5, 1, 0)
 # define __has_builtin____builtin_mul_overflow_p GCC_VERSION_SINCE(7, 0, 0)
 # define __has_builtin____builtin_popcount       GCC_VERSION_SINCE(3, 6, 0)
 # define __has_builtin____builtin_popcountl      GCC_VERSION_SINCE(3, 6, 0)
 # define __has_builtin____builtin_popcountll     GCC_VERSION_SINCE(3, 6, 0)
+# define __has_builtin____builtin_sub_overflow   GCC_VERSION_SINCE(5, 1, 0)
 # /* Take config.h definition when available */
+# ifdef HAVE_BUILTIN____BUILTIN_ADD_OVERFLOW
+#  undef __has_builtin____builtin_add_overflow
+#  define __has_builtin____builtin_add_overflow HAVE_BUILTIN____BUILTIN_ADD_OVERFLOW
+# endif
 # ifdef HAVE_BUILTIN____BUILTIN_BSWAP16
 #  undef __has_builtin____builtin_bswap16
 #  define __has_builtin____builtin_bswap16 HAVE_BUILTIN____BUILTIN_BSWAP16
@@ -156,6 +162,10 @@
 #  undef __has_builtin____builtin_popcountll
 #  define __has_builtin____builtin_popcountll HAVE_BUILTIN____BUILTIN_POPCOUNTLL
 # endif
+# ifdef HAVE_BUILTIN____BUILTIN_SUB_OVERFLOW
+#  undef __has_builtin____builtin_SUB_overflow
+#  define __has_builtin____builtin_sub_overflow HAVE_BUILTIN____BUILTIN_SUB_OVERFLOW
+# endif
 #endif
 
 #ifndef __has_feature
diff --git a/internal/fixnum.h b/internal/fixnum.h
index 08ca7bcec7..a388ddb36a 100644
--- a/internal/fixnum.h
+++ b/internal/fixnum.h
@@ -9,6 +9,12 @@
  *             modify this file, provided that  the conditions mentioned in the
  *             file COPYING are met.  Consult the file for details.
  */
+#include "ruby/config.h"        /* for HAVE_LONG_LONG */
+#include <limits.h>             /* for CHAR_BIT */
+#include "internal/compilers.h" /* for __has_builtin */
+#include "internal/stdbool.h"   /* for bool */
+#include "ruby/intern.h"        /* for rb_big_mul */
+#include "ruby/ruby.h"          /* for RB_FIXABLE */
 
 #if HAVE_LONG_LONG && SIZEOF_LONG * 2 <= SIZEOF_LONG_LONG
 # define DLONG LONG_LONG
@@ -16,9 +22,20 @@
 #elif defined(HAVE_INT128_T)
 # define DLONG int128_t
 # define DL2NUM(x) (RB_FIXABLE(x) ? LONG2FIX(x) : rb_int128t2big(x))
-VALUE rb_int128t2big(int128_t n);
+VALUE rb_int128t2big(int128_t n); /* in bignum.c */
 #endif
 
+static inline long rb_overflowed_fix_to_int(long x);
+static inline VALUE rb_fix_plus_fix(VALUE x, VALUE y);
+static inline VALUE rb_fix_minus_fix(VALUE x, VALUE y);
+static inline VALUE rb_fix_mul_fix(VALUE x, VALUE y);
+static inline void rb_fix_divmod_fix(VALUE x, VALUE y, VALUE *divp, VALUE *modp);
+static inline VALUE rb_fix_div_fix(VALUE x, VALUE y);
+static inline VALUE rb_fix_mod_fix(VALUE x, VALUE y);
+static inline bool FIXNUM_POSITIVE_P(VALUE num);
+static inline bool FIXNUM_NEGATIVE_P(VALUE num);
+static inline bool FIXNUM_ZERO_P(VALUE num);
+
 static inline long
 rb_overflowed_fix_to_int(long x)
 {
@@ -28,7 +45,10 @@ rb_overflowed_fix_to_int(long x)
 static inline VALUE
 rb_fix_plus_fix(VALUE x, VALUE y)
 {
-#ifdef HAVE_BUILTIN___BUILTIN_ADD_OVERFLOW
+#if !__has_builtin(__builtin_add_overflow)
+    long lz = FIX2LONG(x) + FIX2LONG(y);
+    return LONG2NUM(lz);
+#else
     long lz;
     /* NOTE
      * (1) `LONG2FIX(FIX2LONG(x)+FIX2LONG(y))`
@@ -56,16 +76,16 @@ rb_fix_plus_fix(VALUE x, VALUE y)
     else {
         return (VALUE)lz;
     }
-#else
-    long lz = FIX2LONG(x) + FIX2LONG(y);
-    return LONG2NUM(lz);
 #endif
 }
 
 static inline VALUE
 rb_fix_minus_fix(VALUE x, VALUE y)
 {
-#ifdef HAVE_BUILTIN___BUILTIN_SUB_OVERFLOW
+#if !__has_builtin(__builtin_sub_overflow)
+    long lz = FIX2LONG(x) - FIX2LONG(y);
+    return LONG2NUM(lz);
+#else
     long lz;
     if (__builtin_sub_overflow((long)x, (long)y-1, &lz)) {
         return rb_int2big(rb_overflowed_fix_to_int(lz));
@@ -73,9 +93,6 @@ rb_fix_minus_fix(VALUE x, VALUE y)
     else {
         return (VALUE)lz;
     }
-#else
-    long lz = FIX2LONG(x) - FIX2LONG(y);
-    return LONG2NUM(lz);
 #endif
 }
 
@@ -147,4 +164,22 @@ rb_fix_mod_fix(VALUE x, VALUE y)
     rb_fix_divmod_fix(x, y, NULL, &mod);
     return mod;
 }
+
+static inline bool
+FIXNUM_POSITIVE_P(VALUE num)
+{
+    return (SIGNED_VALUE)num > (SIGNED_VALUE)INT2FIX(0);
+}
+
+static inline bool
+FIXNUM_NEGATIVE_P(VALUE num)
+{
+    return (SIGNED_VALUE)num < 0;
+}
+
+static inline bool
+FIXNUM_ZERO_P(VALUE num)
+{
+    return num == INT2FIX(0);
+}
 #endif /* INTERNAL_FIXNUM_H */
diff --git a/internal/numeric.h b/internal/numeric.h
index 3e64b82daf..c85327ea82 100644
--- a/internal/numeric.h
+++ b/internal/numeric.h
@@ -9,6 +9,7 @@
  *             modify this file, provided that  the conditions mentioned in the
  *             file COPYING are met.  Consult the file for details.
  */
+#include "internal/fixnum.h"    /* for FIXNUM_POSITIVE_P */
 
 struct RFloat {
     struct RBasic basic;
@@ -19,10 +20,6 @@ struct RFloat {
 
 /* numeric.c */
 
-#define FIXNUM_POSITIVE_P(num) ((SIGNED_VALUE)(num) > (SIGNED_VALUE)INT2FIX(0))
-#define FIXNUM_NEGATIVE_P(num) ((SIGNED_VALUE)(num) < 0)
-#define FIXNUM_ZERO_P(num) ((num) == INT2FIX(0))
-
 #define INT_NEGATIVE_P(x) (FIXNUM_P(x) ? FIXNUM_NEGATIVE_P(x) : BIGNUM_NEGATIVE_P(x))
 
 #define FLOAT_ZERO_P(x) (RFLOAT_VALUE(x) == 0.0)