Implement Math.log2 via ported extract from fdlibm.

src/math.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 "use strict";
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Object = global.Object;
 
@@ skipping 209 matching lines @@
 // ES6 draft 09-27-13, section 20.2.2.7.
 function MathAtanh(x) {
   if (!IS_NUMBER(x)) x = NonNumberToNumber(x);
   // Idempotent for +/-0.
   if (x === 0) return x;
   // Returns NaN for NaN and +/- Infinity.
   if (!NUMBER_IS_FINITE(x)) return NAN;
   return 0.5 * MathLog((1 + x) / (1 - x));
 }
 
-// ES6 draft 09-27-13, section 20.2.2.22.
-function MathLog2(x) {
-  return MathLog(x) * 1.442695040888963407;  // log2(x) = log(x)/log(2).
-}
-
 // ES6 draft 09-27-13, section 20.2.2.17.
 function MathHypot(x, y) {  // Function length is 2.
   // We may want to introduce fast paths for two arguments and when
   // normalization to avoid overflow is not necessary.  For now, we
   // simply assume the general case.
   var length = %_ArgumentsLength();
   var args = new InternalArray(length);
   var max = 0;
   for (var i = 0; i < length; i++) {
     var n = %_Arguments(i);
@@ skipping 112 matching lines @@
     "imul", MathImul,
     "sign", MathSign,
     "trunc", MathTrunc,
     "sinh", MathSinh,     // implemented by third_party/fdlibm
     "cosh", MathCosh,     // implemented by third_party/fdlibm
     "tanh", MathTanh,
     "asinh", MathAsinh,
     "acosh", MathAcosh,
     "atanh", MathAtanh,
     "log10", MathLog10,   // implemented by third_party/fdlibm
-    "log2", MathLog2,
+    "log2", MathLog2,     // implemented by third_party/fdlibm
     "hypot", MathHypot,
     "fround", MathFroundJS,
     "clz32", MathClz32,
     "cbrt", MathCbrt,
     "log1p", MathLog1p,   // implemented by third_party/fdlibm
     "expm1", MathExpm1    // implemented by third_party/fdlibm
   ));
 
   %SetInlineBuiltinFlag(MathCeil);
   %SetInlineBuiltinFlag(MathRandom);
   %SetInlineBuiltinFlag(MathSin);
   %SetInlineBuiltinFlag(MathCos);
 }
 
 SetUpMath();