math: fix special cases in Nextafter

Nextafter(0, -1) != -0.

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/5467060

diff --git a/src/pkg/math/all_test.go b/src/pkg/math/all_test.go
index 7256ca42d34e8fb3bfe25ffd4957029e0f7b266b..98a4df0c1f072d1a6ab4cd9f5ee73fe9c1ef7856 100644 (file)
--- a/src/pkg/math/all_test.go
+++ b/src/pkg/math/all_test.go
@@ -1328,12 +1328,26 @@ var modfSC = [][2]float64{
 }
 
 var vfnextafterSC = [][2]float64{
+       {0, 0},
+       {0, Copysign(0, -1)},
+       {0, -1},
        {0, NaN()},
+       {Copysign(0, -1), 1},
+       {Copysign(0, -1), 0},
+       {Copysign(0, -1), Copysign(0, -1)},
+       {Copysign(0, -1), -1},
        {NaN(), 0},
        {NaN(), NaN()},
 }
 var nextafterSC = []float64{
+       0,
+       0,
+       -4.9406564584124654418e-324, // Float64frombits(0x8000000000000001)
        NaN(),
+       4.9406564584124654418e-324, // Float64frombits(0x0000000000000001)
+       Copysign(0, -1),
+       Copysign(0, -1),
+       -4.9406564584124654418e-324, // Float64frombits(0x8000000000000001)
        NaN(),
        NaN(),
 }
@@ -2259,7 +2273,7 @@ func TestNextafter(t *testing.T) {
                        t.Errorf("Nextafter(%g, %g) = %g want %g", vf[i], 10.0, f, nextafter[i])
                }
        }
-       for i := 0; i < len(vfmodfSC); i++ {
+       for i := 0; i < len(vfnextafterSC); i++ {
                if f := Nextafter(vfnextafterSC[i][0], vfnextafterSC[i][1]); !alike(nextafterSC[i], f) {
                        t.Errorf("Nextafter(%g, %g) = %g want %g", vfnextafterSC[i][0], vfnextafterSC[i][1], f, nextafterSC[i])
                }

diff --git a/src/pkg/math/nextafter.go b/src/pkg/math/nextafter.go
index ae1267f752e38029ccffe0d16bed3fc55cc71eea..e7723baacfb7e108e00df99de6b5726402a6c820 100644 (file)
--- a/src/pkg/math/nextafter.go
+++ b/src/pkg/math/nextafter.go
@@ -8,9 +8,8 @@ package math
 // If x == y, then x is returned.
 //
 // Special cases are:
-//     Nextafter(NaN, y) = NaN
-//     Nextafter(x, NaN) = NaN
-//     Nextafter(0, y) = -0, if y < 0
+//      Nextafter(NaN, y) = NaN
+//      Nextafter(x, NaN) = NaN
 func Nextafter(x, y float64) (r float64) {
        // TODO(rsc): Remove manual inlining of IsNaN
        // when compiler does it for us
@@ -26,5 +25,5 @@ func Nextafter(x, y float64) (r float64) {
        default:
                r = Float64frombits(Float64bits(x) - 1)
        }
-       return r
+       return
 }