Implement Math.log1p using port from fdlibm.

test/mjsunit/es6/math-log1p.js

 // Copyright 2014 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.
 
 assertTrue(isNaN(Math.log1p(NaN)));
 assertTrue(isNaN(Math.log1p(function() {})));
 assertTrue(isNaN(Math.log1p({ toString: function() { return NaN; } })));
 assertTrue(isNaN(Math.log1p({ valueOf: function() { return "abc"; } })));
-assertEquals("Infinity", String(1/Math.log1p(0)));
-assertEquals("-Infinity", String(1/Math.log1p(-0)));
-assertEquals("Infinity", String(Math.log1p(Infinity)));
-assertEquals("-Infinity", String(Math.log1p(-1)));
+assertEquals(Infinity, 1/Math.log1p(0));
+assertEquals(-Infinity, 1/Math.log1p(-0));
+assertEquals(Infinity, Math.log1p(Infinity));
+assertEquals(-Infinity, Math.log1p(-1));
 assertTrue(isNaN(Math.log1p(-2)));
 assertTrue(isNaN(Math.log1p(-Infinity)));
 
-for (var x = 1E300; x > 1E-1; x *= 0.8) {
+for (var x = 1E300; x > 1E16; x *= 0.8) {
   var expected = Math.log(x + 1);
-  assertEqualsDelta(expected, Math.log1p(x), expected * 1E-14);
+  assertEqualsDelta(expected, Math.log1p(x), expected * 1E-16);
 }
 
 // Values close to 0:
 // Use Taylor expansion at 1 for log(x) as test expectation:
 // log1p(x) == log(x + 1) == 0 + x / 1 - x^2 / 2 + x^3 / 3 - ...
 function log1p(x) {
   var terms = [];
   var prod = x;
   for (var i = 1; i <= 20; i++) {
     terms.push(prod / i);
     prod *= -x;
   }
   var sum = 0;
   while (terms.length > 0) sum += terms.pop();
   return sum;
 }
 
 for (var x = 1E-1; x > 1E-300; x *= 0.8) {
   var expected = log1p(x);
-  assertEqualsDelta(expected, Math.log1p(x), expected * 1E-14);
+  assertEqualsDelta(expected, Math.log1p(x), expected * 1E-16);
 }
+
+// Issue 3481.
+assertEquals(6.9756137364252422e-03,
+             Math.log1p(8070450532247929/Math.pow(2,60)));
+
+// Tests related to the fdlibm implementation.
+// Test largest double value.
+assertEquals(709.782712893384, Math.log1p(1.7976931348623157e308));
+// Test small values.
+assertEquals(Math.pow(2, -55), Math.log1p(Math.pow(2, -55)));
+assertEquals(9.313225741817976e-10, Math.log1p(Math.pow(2, -30)));
+// Cover various code paths.
+// -.2929 < x < .41422, k = 0
+assertEquals(-0.2876820724517809, Math.log1p(-0.25));
+assertEquals(0.22314355131420976, Math.log1p(0.25));
+// 0.41422 < x < 9.007e15
+assertEquals(2.3978952727983707, Math.log1p(10));
+// x > 9.007e15
+assertEquals(36.841361487904734, Math.log1p(10e15));
+// Normalize u.
+assertEquals(37.08337388996168, Math.log1p(12738099905822720));
+// Normalize u/2.
+assertEquals(37.08336444902049, Math.log1p(12737979646738432));
+// |f| = 0, k != 0
+assertEquals(1.3862943611198906, Math.log1p(3));
+// |f| != 0, k != 0
+assertEquals(1.3862945995384413, Math.log1p(3 + Math.pow(2,-20)));
+// final if-clause: k = 0
+assertEquals(0.5596157879354227, Math.log1p(0.75));
+// final if-clause: k != 0
+assertEquals(0.8109302162163288, Math.log1p(1.25));