diff --git a/ChangeLog b/ChangeLog
index 084e5805b6..0ecb01e03f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Aug  2 21:42:40 2016  Chia-sheng Chen  <qitar888@gmail.com>
+
+	* math.c (tanh): make faster by the extract form if three
+	  hyperbolic functions are unavailable.  [Feature #12647]
+
 Tue Aug  2 12:37:00 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/socket/option.c, ext/socket/rubysocket.h (inet_ntop): share
diff --git a/math.c b/math.c
index eefcec3c93..b15e87113c 100644
--- a/math.c
+++ b/math.c
@@ -283,8 +283,13 @@ math_sinh(VALUE obj, VALUE x)
 double
 tanh(double x)
 {
+# if defined(HAVE_SINH) && defined(HAVE_COSH)
     const double c = cosh(x);
     if (!isinf(c)) return sinh(x) / c;
+# else
+    const double e = exp(x+x);
+    if (!isinf(e)) return (e - 1) / (e + 1);
+# endif
     return x > 0 ? 1.0 : -1.0;
 }
 #endif