Fix constant in Math.tan implementation.

src/third_party/fdlibm/fdlibm.js

 // The following is adapted from fdlibm (http://www.netlib.org/fdlibm),
 //
 // ====================================================
 // Copyright (C) 1993-2004 by Sun Microsystems, Inc. All rights reserved.
 //
 // Developed at SunSoft, a Sun Microsystems, Inc. business.
 // Permission to use, copy, modify, and distribute this
 // software is freely granted, provided that this notice
 // is preserved.
 // ====================================================
@@ skipping 249 matching lines @@
         var w = x + y;
         var z = %_ConstructDouble(%_DoubleHi(w), 0);
         var v = y - (z - x);
         var a = -1 / w;
         var t = %_ConstructDouble(%_DoubleHi(a), 0);
         var s = 1 + t * z;
         return t + a * (s + t * v);
       }
     }
   }
-  if (ix >= 0x3fe59429) {  // |x| > .6744
+  if (ix >= 0x3fe59428) {  // |x| > .6744

[Raymond Toy, 2014/11/04 16:49:52]

Sorry about the typo!

More tests should be added for each of the functions wherever there is a branch
is taken. I think there are tests for each branch, but not at the branch values.

     if (x < 0) {
       x = -x;
       y = -y;
     }
     z = PIO4 - x;
     w = PIO4LO - y;
     x = z + w;
     y = 0;
   }
   z = x * x;
   w = z * z;
 
   // Break x^5 * (T1 + x^2*T2 + ...) into
   // x^5 * (T1 + x^4*T3 + ... + x^20*T11) +
   // x^5 * (x^2 * (T2 + x^4*T4 + ... + x^22*T12))
   var r = KTAN(1) + w * (KTAN(3) + w * (KTAN(5) +
                     w * (KTAN(7) + w * (KTAN(9) + w * KTAN(11)))));
   var v = z * (KTAN(2) + w * (KTAN(4) + w * (KTAN(6) +
                          w * (KTAN(8) + w * (KTAN(10) + w * KTAN(12))))));
   var s = z * x;
   r = y + z * (s * (r + v) + y);
   r = r + KTAN(0) * s;
   w = x + r;
   if (ix >= 0x3fe59428) {

[Yang, 2014/11/03 09:35:27]

compare to this.

     return (1 - ((hx >> 30) & 2)) *
       (returnTan - 2.0 * (x - (w * w / (w + returnTan) - r)));
   }
   if (returnTan == 1) {
     return w;
   } else {
     z = %_ConstructDouble(%_DoubleHi(w), 0);
     v = r - (z - x);
     var a = -1 / w;
     var t = %_ConstructDouble(%_DoubleHi(a), 0);
@@ skipping 500 matching lines @@
   // |x| in [log(maxdouble), overflowthreshold]
   if (MathAbs(x) <= KCOSH_OVERFLOW) {
     var w = MathExp(0.5 * MathAbs(x));
     var t = 0.5 * w;
     return t * w;
   }
   if (NUMBER_IS_NAN(x)) return x;
   // |x| > overflowthreshold.
   return INFINITY;
 }