Implement Math.log2 via ported extract from fdlibm.

src/third_party/fdlibm/fdlibm.cc

Index: src/third_party/fdlibm/fdlibm.cc
diff --git a/src/third_party/fdlibm/fdlibm.cc b/src/third_party/fdlibm/fdlibm.cc
index 23c13c0f058ba3091e9217de466650edb9153b00..cc5dbc2f9793255229003e44911859ae876ae131 100644
--- a/src/third_party/fdlibm/fdlibm.cc
+++ b/src/third_party/fdlibm/fdlibm.cc
@@ -63,26 +63,36 @@ const double MathConstants::constants[] = {
     3.06161699786838301793e-17,   // pio4lo    33
     6.93147180369123816490e-01,   // ln2_hi    34
     1.90821492927058770002e-10,   // ln2_lo    35
-    1.80143985094819840000e+16,   // 2^54      36
-    6.666666666666666666e-01,     // 2/3       37
-    6.666666666666735130e-01,     // LP1       38  coefficients for log1p
-    3.999999999940941908e-01,     //           39
-    2.857142874366239149e-01,     //           40
-    2.222219843214978396e-01,     //           41
-    1.818357216161805012e-01,     //           42
-    1.531383769920937332e-01,     //           43
-    1.479819860511658591e-01,     // LP7       44
-    7.09782712893383973096e+02,   //           45  overflow threshold for expm1
-    1.44269504088896338700e+00,   // 1/ln2     46
-    -3.33333333333331316428e-02,  // Q1        47  coefficients for expm1
-    1.58730158725481460165e-03,   //           48
-    -7.93650757867487942473e-05,  //           49
-    4.00821782732936239552e-06,   //           50
-    -2.01099218183624371326e-07,  // Q5        51
-    710.4758600739439,            //           52  overflow threshold sinh, cosh
-    4.34294481903251816668e-01,   // ivln10    53  coefficients for log10
-    3.01029995663611771306e-01,   // log10_2hi 54
-    3.69423907715893078616e-13    // log10_2lo 55
+    6.666666666666666666e-01,     // 2/3       36
+    6.666666666666735130e-01,     // LP1       37  coefficients for log1p
+    3.999999999940941908e-01,     //           38
+    2.857142874366239149e-01,     //           39
+    2.222219843214978396e-01,     //           40
+    1.818357216161805012e-01,     //           41
+    1.531383769920937332e-01,     //           42
+    1.479819860511658591e-01,     // LP7       43
+    7.09782712893383973096e+02,   //           44  overflow threshold for expm1
+    1.44269504088896338700e+00,   // 1/ln2     45
+    -3.33333333333331316428e-02,  // Q1        46  coefficients for expm1
+    1.58730158725481460165e-03,   //           47
+    -7.93650757867487942473e-05,  //           48
+    4.00821782732936239552e-06,   //           49
+    -2.01099218183624371326e-07,  // Q5        50
+    710.4758600739439,            //           51  overflow threshold sinh, cosh
+    4.34294481903251816668e-01,   // ivln10    52  coefficients for log10
+    3.01029995663611771306e-01,   // log10_2hi 53
+    3.69423907715893078616e-13,   // log10_2lo 54
+    5.99999999999994648725e-01,   // L1        55  coefficients for log2

[Raymond Toy, 2014/12/10 17:15:53]

Nit: Certainly would be nice to add (or move) a comment to a new line before
line 85 saying these are for the log2 implementation.  Same goes for the other
tables above now that there are so many entries.

+    4.28571428578550184252e-01,   //           56
+    3.33333329818377432918e-01,   //           57
+    2.72728123808534006489e-01,   //           58
+    2.30660745775561754067e-01,   //           59
+    2.06975017800338417784e-01,   // L6        60
+    9.61796693925975554329e-01,   // cp        61  2/(3*ln(2))
+    9.61796700954437255859e-01,   // cp_h      62
+    -7.02846165095275826516e-09,  // cp_l      63
+    5.84962487220764160156e-01,   // dp_h      64
+    1.35003920212974897128e-08    // dp_l      65
 };