Implement Math.log1p using port from fdlibm.

src/math.js

Index: src/math.js
diff --git a/src/math.js b/src/math.js
index 436a41f5c44400609fe7cc446f318ca36ca6aacd..9cf4f1c9bdbb6561431d8c7dc267149311ac5ad3 100644
--- a/src/math.js
+++ b/src/math.js
@@ -347,26 +347,6 @@ function MathExpm1(x) {
   }
 }
 
-// ES6 draft 09-27-13, section 20.2.2.20.
-// Use Taylor series to approximate. With y = x + 1;
-// log(y) at 1 == log(1) + log'(1)(y-1)/1! + log''(1)(y-1)^2/2! + ...
-//             == 0 + x - x^2/2 + x^3/3 ...
-// The closer x is to 0, the fewer terms are required.
-function MathLog1p(x) {
-  if (!IS_NUMBER(x)) x = NonNumberToNumber(x);
-  var xabs = MathAbs(x);
-  if (xabs < 1E-7) {
-    return x * (1 - x * (1/2));
-  } else if (xabs < 3E-5) {
-    return x * (1 - x * (1/2 - x * (1/3)));
-  } else if (xabs < 7E-3) {
-    return x * (1 - x * (1/2 - x * (1/3 - x * (1/4 -
-           x * (1/5 - x * (1/6 - x * (1/7)))))));
-  } else {  // Use regular log if not close enough to 0.
-    return MathLog(1 + x);
-  }
-}
-
 // -------------------------------------------------------------------
 
 function SetUpMath() {
@@ -428,7 +408,7 @@ function SetUpMath() {
     "fround", MathFroundJS,
     "clz32", MathClz32,
     "cbrt", MathCbrt,
-    "log1p", MathLog1p,
+    "log1p", MathLog1p,    // implemented by third_party/fdlibm
     "expm1", MathExpm1
   ));