Harmony: move implementation of Math.log1p and Math.expm1 to Javascript.

src/harmony-math.js

Index: src/harmony-math.js
diff --git a/src/harmony-math.js b/src/harmony-math.js
index 7856917890b5f3f4c0369a8731c6892a8ef57346..e7afa4d6075152df991219094db514fd56fefc66 100644
--- a/src/harmony-math.js
+++ b/src/harmony-math.js
@@ -174,21 +174,51 @@ function MathClz32(x) {
 }
 
 
-//ES6 draft 09-27-13, section 20.2.2.9.
+// ES6 draft 09-27-13, section 20.2.2.9.
 function MathCbrt(x) {
   return %Math_cbrt(TO_NUMBER_INLINE(x));
 }
 
 
-//ES6 draft 09-27-13, section 20.2.2.14.
+// ES6 draft 09-27-13, section 20.2.2.14.
+// Use Taylor series to approximate.
+// exp(x) - 1 at 0 == -1 + exp(0) + exp'(0)*x/1! + exp''(0)*x^2/2! + ...
+//                 == x/1! + x^2/2! + x^3/3! + ...
+// The closer x is to 0, the fewer terms are required.
 function MathExpm1(x) {
-  return %Math_expm1(TO_NUMBER_INLINE(x));
+  if (!IS_NUMBER(x)) x = NonNumberToNumber(x);
+  var xabs = MathAbs(x);
+  if (xabs < 2E-7) {
+    return x * (1 + x * (1/2));
+  } else if (xabs < 6E-5) {
+    return x * (1 + x * (1/2 + x * (1/6)));
+  } else if (xabs < 2E-2) {
+    return x * (1 + x * (1/2 + x * (1/6 +
+           x * (1/24 + x * (1/120 + x * (1/720))))));
+  } else {  // Use regular exp if not close enough to 0.
+    return MathExp(x) - 1;
+  }
 }
 
 
-//ES6 draft 09-27-13, section 20.2.2.20.
+// 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) {
-  return %Math_log1p(TO_NUMBER_INLINE(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);
+  }
 }