Harmony: implement Math.cbrt in Javascript.

src/harmony-math.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 156 matching lines @@
   // Binary search.
   if ((x & 0xFFFF0000) === 0) { x <<= 16; result += 16; };
   if ((x & 0xFF000000) === 0) { x <<=  8; result +=  8; };
   if ((x & 0xF0000000) === 0) { x <<=  4; result +=  4; };
   if ((x & 0xC0000000) === 0) { x <<=  2; result +=  2; };
   if ((x & 0x80000000) === 0) { x <<=  1; result +=  1; };
   return result;
 }
 
 
-//ES6 draft 09-27-13, section 20.2.2.9.
+// ES6 draft 09-27-13, section 20.2.2.9.
+// Cube root approximation, refer to: http://metamerist.com/cbrt/cbrt.htm
+// Using initial approximation adapted from Kahan's cbrt and 4 iterations
+// of Newton's method.
 function MathCbrt(x) {
-  return %Math_cbrt(TO_NUMBER_INLINE(x));
+  if (!IS_NUMBER(x)) x = NonNumberToNumber(x);
+  if (x == 0 || !NUMBER_IS_FINITE(x)) return x;
+  return x >= 0 ? CubeRoot(x) : -CubeRoot(-x);
+}
+
+macro NEWTON_ITERATION_CBRT(x, approx)
+  (1.0 / 3.0) * (x / (approx * approx) + 2 * approx);
+endmacro
+
+function CubeRoot(x) {
+  var approx_hi = MathFloor(%_DoubleHi(x) / 3) + 0x2A9F7893;
+  var approx = %_ConstructDouble(approx_hi, 0);
+  approx = NEWTON_ITERATION_CBRT(x, approx);
+  approx = NEWTON_ITERATION_CBRT(x, approx);
+  approx = NEWTON_ITERATION_CBRT(x, approx);
+  return NEWTON_ITERATION_CBRT(x, approx);
 }
 
 
+function NewtonIterationCbrt(x, approx) {
+  return (1.0 / 3.0) * (x / (approx * approx) + 2 * approx);
+}
+
+
 //ES6 draft 09-27-13, section 20.2.2.14.
 function MathExpm1(x) {
   return %Math_expm1(TO_NUMBER_INLINE(x));
 }
 
 
 //ES6 draft 09-27-13, section 20.2.2.20.
 function MathLog1p(x) {
   return %Math_log1p(TO_NUMBER_INLINE(x));
 }
@@ skipping 18 matching lines @@
     "fround", MathFround,
     "clz32", MathClz32,
     "cbrt", MathCbrt,
     "log1p", MathLog1p,
     "expm1", MathExpm1
   ));
 }
 
 
 ExtendMath();