Implement Math.tanh using fdlibm port.

src/third_party/fdlibm/fdlibm.js

Index: src/third_party/fdlibm/fdlibm.js
diff --git a/src/third_party/fdlibm/fdlibm.js b/src/third_party/fdlibm/fdlibm.js
index d89f69b5f5900fe76adfeb69e25be89d84f641aa..983a86c151ac36e38f30610ef6f20570e82c9831 100644
--- a/src/third_party/fdlibm/fdlibm.js
+++ b/src/third_party/fdlibm/fdlibm.js
@@ -843,6 +843,63 @@ function MathCosh(x) {
   return INFINITY;
 }
 
+// ES6 draft 09-27-13, section 20.2.2.33.
+// Math.tanh(x)
+// Method :
+//                                    x    -x
+//                                   e  - e
+//     0. tanh(x) is defined to be -----------
+//                                    x    -x
+//                                   e  + e
+//     1. reduce x to non-negative by tanh(-x) = -tanh(x).
+//     2.  0      <= x <= 2**-55 : tanh(x) := x*(one+x)
+//                                             -t
+//         2**-55 <  x <=  1     : tanh(x) := -----; t = expm1(-2x)
+//                                            t + 2
+//                                                  2
+//         1      <= x <=  22.0  : tanh(x) := 1-  ----- ; t = expm1(2x)
+//                                                t + 2
+//         22.0   <  x <= INF    : tanh(x) := 1.
+//
+// Special cases:
+//     tanh(NaN) is NaN;
+//     only tanh(0) = 0 is exact for finite argument.
+//
+
+define TWO_M55 = 2.77555756156289135105e-17;  // 2^-55, empty lower half

[Raymond Toy, 2015/10/16 17:07:25]

A bit late, but this comment is incorrect.  Since 2^-55 is a power of two, the
lower half is always empty.  The comment here implies to me that you've forcibly
made the lower half zero, but it was already zero anyway.

+
+function MathTanh(x) {
+  x = x * 1;  // Convert to number.
+  // x is Infinity or NaN
+  if (!NUMBER_IS_FINITE(x)) {
+    if (x > 0) return 1;
+    if (x < 0) return -1;
+    return x;
+  }
+
+  var ax = MathAbs(x);
+  var z;
+  // |x| < 22
+  if (ax < 22) {
+    if (ax < TWO_M55) {
+      // |x| < 2^-55, tanh(small) = small.
+      return x;
+    }
+    if (ax >= 1) {
+      // |x| >= 1
+      var t = MathExpm1(2 * ax);
+      z = 1 - 2 / (t + 2);
+    } else {
+      var t = MathExpm1(-2 * ax);
+      z = -t / (t + 2);
+    }
+  } else {
+    // |x| > 22, return +/- 1
+    z = 1;
+  }
+  return (x >= 0) ? z : -z;
+}
+
 // ES6 draft 09-27-13, section 20.2.2.21.
 // Return the base 10 logarithm of x
 //
@@ -1029,6 +1086,7 @@ utils.InstallFunctions(GlobalMath, DONT_ENUM, [
   "tan", MathTan,
   "sinh", MathSinh,
   "cosh", MathCosh,
+  "tanh", MathTanh,
   "log10", MathLog10,
   "log2", MathLog2,
   "log1p", MathLog1p,