math: signed zero Sqrt special case

IEEE 754 says: sqrt(-0) = -0

R=rsc
CC=golang-dev
https://golang.org/cl/1098041

src/pkg/math/all_test.go

@@ -1335,12 +1335,16 @@ var sinhSC = []float64{
 var vfsqrtSC = []float64{
 	Inf(-1),
 	-Pi,
+	-1 / Inf(1), // -0
+	0,
 	Inf(1),
 	NaN(),
 }
 var sqrtSC = []float64{
 	NaN(),
 	NaN(),
+	-1 / Inf(1), // -0
+	0,
 	Inf(1),
 	NaN(),
 }
@@ -2018,7 +2022,7 @@ func TestSqrt(t *testing.T) {
 	for i := 0; i < len(vf); i++ {
 		a := Fabs(vf[i])
 		if f := SqrtGo(a); sqrt[i] != f {
-			t.Errorf("sqrtGo(%g) = %g, want %g\n", a, f, sqrt[i])
+			t.Errorf("SqrtGo(%g) = %g, want %g\n", a, f, sqrt[i])
 		}
 		a = Fabs(vf[i])
 		if f := Sqrt(a); sqrt[i] != f {
@@ -2026,7 +2030,10 @@ func TestSqrt(t *testing.T) {
 		}
 	}
 	for i := 0; i < len(vfsqrtSC); i++ {
-		if f := Log10(vfsqrtSC[i]); !alike(sqrtSC[i], f) {
+		if f := SqrtGo(vfsqrtSC[i]); !alike(sqrtSC[i], f) {
+			t.Errorf("SqrtGo(%g) = %g, want %g\n", vfsqrtSC[i], f, sqrtSC[i])
+		}
+		if f := Sqrt(vfsqrtSC[i]); !alike(sqrtSC[i], f) {
 			t.Errorf("Sqrt(%g) = %g, want %g\n", vfsqrtSC[i], f, sqrtSC[i])
 		}
 	}

src/pkg/math/sqrt.go

@@ -8,6 +8,7 @@ package math
 //
 // Special cases are:
 //	Sqrt(+Inf) = +Inf
+//	Sqrt(±0) = ±0
 //	Sqrt(x < 0) = NaN
 //	Sqrt(NaN) = NaN
 func Sqrt(x float64) float64 { return sqrtGo(x) }

src/pkg/math/sqrt_port.go

@@ -90,7 +90,7 @@ package math
 //
 // Special cases are:
 //	Sqrt(+Inf) = +Inf
-//	Sqrt(0) = 0
+//	Sqrt(±0) = ±0
 //	Sqrt(x < 0) = NaN
 //	Sqrt(NaN) = NaN
 func sqrtGo(x float64) float64 {
@@ -98,10 +98,8 @@ func sqrtGo(x float64) float64 {
 	// TODO(rsc): Remove manual inlining of IsNaN, IsInf
 	// when compiler does it for us
 	switch {
-	case x != x || x > MaxFloat64: // IsNaN(x) || IsInf(x, 1):
+	case x == 0 || x != x || x > MaxFloat64: // x == 0 || IsNaN(x) || IsInf(x, 1):
 		return x
-	case x == 0:
-		return 0
 	case x < 0:
 		return NaN()
 	}