math.c: faster tanh

* math.c (tanh): make faster by the extract form if three
  hyperbolic functions are unavailable.  [Feature #12647]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55799 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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

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