Implement Math.log2 via ported extract from fdlibm.

test/mjsunit/es6/math-log2-log10.js

Index: test/mjsunit/es6/math-log2-log10.js
diff --git a/test/mjsunit/es6/math-log2-log10.js b/test/mjsunit/es6/math-log2-log10.js
index 863506c7055d77127d4694a6c461fc54a7b28d4e..fa3f46807a10169ec36b99297ae24cf0fd080e1c 100644
--- a/test/mjsunit/es6/math-log2-log10.js
+++ b/test/mjsunit/es6/math-log2-log10.js
@@ -25,6 +25,8 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+// Flags: --allow-natives-syntax
+
 [Math.log10, Math.log2].forEach( function(fun) {
   assertTrue(isNaN(fun(NaN)));
   assertTrue(isNaN(fun(fun)));
@@ -41,8 +43,57 @@
 
 for (var i = -310; i <= 308; i += 0.5) {
   assertEquals(i, Math.log10(Math.pow(10, i)));
-  assertEqualsDelta(i, Math.log2(Math.pow(2, i)), 1E-13);
+  // Square roots are tested below.
+  if (i != -0.5 && i != 0.5) assertEquals(i, Math.log2(Math.pow(2, i)));
 }
 
 // Test denormals.
 assertEquals(-307.77759430519706, Math.log10(1.5 * Math.pow(2, -1023)));
+
+// Test Math.log2(2^k) for -1074 <= k <= 1023.
+var n = -1074;
+// This loop covers n from -1074 to -1043
+for (var lowbits = 1; lowbits <= 0x80000000; lowbits *= 2) {
+  var x = %_ConstructDouble(0, lowbits);
+  assertEquals(n, Math.log2(x));
+  n++;
+}
+// This loop covers n from -1042 to -1023
+for (var hibits = 1; hibits <= 0x80000; hibits *= 2) {
+  var x = %_ConstructDouble(hibits, 0);
+  assertEquals(n, Math.log2(x));
+  n++;
+}
+// The rest of the normal values of 2^n
+var x = 1;
+for (var n = -1022; n <= 1023; ++n) {
+  var x = Math.pow(2, n);
+  assertEquals(n, Math.log2(x));
+}
+
+// Test special values.
+// Expectation isn't exactly 1/2 because Math.SQRT2 isn't exactly sqrt(2).
+assertEquals(0.5000000000000001, Math.log2(Math.SQRT2));
+
+// Expectation isn't exactly -1/2 because Math.SQRT1_2 isn't exactly sqrt(1/2).
+assertEquals(-0.4999999999999999, Math.log2(Math.SQRT1_2));
+
+assertEquals(3.321928094887362, Math.log2(10));
+assertEquals(6.643856189774724, Math.log2(100));
+
+// Test relationships
+x = 1;
+for (var k = 0; k < 1000; ++k) {
+  var y = Math.abs(Math.log2(x) + Math.log2(1/x));
+  assertEqualsDelta(0, y, 1.5e-14);
+  x *= 1.1;
+}
+
+x = Math.pow(2, -100);
+for (var k = 0; k < 1000; ++k) {
+  var y = Math.log2(x);
+  var expected = Math.log(x) / Math.LN2;
+  var err = Math.abs(y - expected) / expected;
+  assertEqualsDelta(0, err, 1e-15);
+  x *= 1.1;
+}