diff -r 000000000000 -r 6474c204b198 js/src/tests/ecma_6/Math/log2-exact.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/tests/ecma_6/Math/log2-exact.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,30 @@
+// Properties of Math.log2 that are guaranteed by the spec.
+
+// If x is NaN, the result is NaN.
+assertEq(Math.log2(NaN), NaN);
+
+// If x is less than 0, the result is NaN.
+assertEq(Math.log2(-1e-10), NaN);
+assertEq(Math.log2(-1e-5), NaN);
+assertEq(Math.log2(-1e-1), NaN);
+assertEq(Math.log2(-Number.MIN_VALUE), NaN);
+assertEq(Math.log2(-Number.MAX_VALUE), NaN);
+assertEq(Math.log2(-Infinity), NaN);
+
+for (var i = -1; i > -10; i--)
+    assertEq(Math.log2(i), NaN);
+
+// If x is +0, the result is −∞.
+assertEq(Math.log2(+0), -Infinity);
+
+// If x is −0, the result is −∞.
+assertEq(Math.log2(-0), -Infinity);
+
+// If x is 1, the result is +0.
+assertEq(Math.log2(1), +0);
+
+// If x is +∞, the result is +∞.
+assertEq(Math.log2(Infinity), Infinity);
+
+
+reportCompare(0, 0, 'ok');