math: fix normalization bug in pure-Go sqrt

Fixes #13013

Change-Id: I6cf500eacdce76e303fc1cd92dd1c80eef0986bc
Reviewed-on: https://go-review.googlesource.com/16158Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/all_test.go

@@ -1363,7 +1363,7 @@ var vfmodfSC = []float64{
 var modfSC = [][2]float64{
 	{Inf(-1), NaN()}, // [2]float64{Copysign(0, -1), Inf(-1)},
 	{Copysign(0, -1), Copysign(0, -1)},
-	{Inf(1), NaN()},  // [2]float64{0, Inf(1)},
+	{Inf(1), NaN()}, // [2]float64{0, Inf(1)},
 	{NaN(), NaN()},
 }
 
@@ -1611,6 +1611,7 @@ var vfsqrtSC = []float64{
 	0,
 	Inf(1),
 	NaN(),
+	Float64frombits(2), // subnormal; see https://golang.org/issue/13013
 }
 var sqrtSC = []float64{
 	NaN(),
@@ -1619,6 +1620,7 @@ var sqrtSC = []float64{
 	0,
 	Inf(1),
 	NaN(),
+	3.1434555694052576e-162,
 }
 
 var vftanhSC = []float64{

src/math/sqrt.go

@@ -108,7 +108,7 @@ func sqrt(x float64) float64 {
 	// normalize x
 	exp := int((ix >> shift) & mask)
 	if exp == 0 { // subnormal x
-		for ix&1<<shift == 0 {
+		for ix&(1<<shift) == 0 {
 			ix <<= 1
 			exp--
 		}