Implement Math.tanh using fdlibm port.

test/mjsunit/es6/math-hyperbolic.js

 // Copyright 2013 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
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 //
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 assertEquals(512.00048828125, Math.cosh(-10*Math.LN2));
 // Case 22 <= |x| < log(maxdouble)
 assertEquals(2.1474836479999983e9, Math.cosh(32*Math.LN2));
 assertEquals(2.1474836479999983e9, Math.cosh(-32*Math.LN2));
 // Case log(maxdouble) <= |x| <= overflowthreshold
 assertEquals(1.7976931348621744e308, Math.cosh(710.4758600739439));
 assertEquals(1.7976931348621744e308, Math.cosh(-710.4758600739439));
 // Overflow.
 assertEquals(Infinity, Math.cosh(710.475860073944));
 assertEquals(Infinity, Math.cosh(-710.475860073944));
+
+// Implementation-specific tests for tanh.
+// Case |x| < 2^-55
+var two_56 = Math.pow(2, -56);
+assertEquals(two_56, Math.tanh(two_56));
+assertEquals(-two_56, Math.tanh(-two_56));
+// Case |x| < 1
+assertEquals(0.6, Math.tanh(Math.LN2));
+assertEquals(-0.6, Math.tanh(-Math.LN2));
+// Case  1 < |x| < 22
+assertEquals(15/17, Math.tanh(2 * Math.LN2));
+assertEquals(-15/17, Math.tanh(-2 * Math.LN2));
+// Case |x| > 22
+assertEquals(1, Math.tanh(100));
+assertEquals(-1, Math.tanh(-100));
+// Test against overflow
+assertEquals(1, Math.tanh(1e300));
+assertEquals(-1, Math.tanh(-1e300));