math: Modf(-0) returns -0,-0

Fixes #12867

Change-Id: I8ba81c622bce2a77a6142f941603198582eaf8a4
Reviewed-on: https://go-review.googlesource.com/15570Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/all_test.go

@@ -447,7 +447,7 @@ var log2 = []float64{
 var modf = [][2]float64{
 	{4.0000000000000000e+00, 9.7901192488367350108546816e-01},
 	{7.0000000000000000e+00, 7.3887247457810456552351752e-01},
-	{0.0000000000000000e+00, -2.7688005719200159404635997e-01},
+	{Copysign(0, -1), -2.7688005719200159404635997e-01},
 	{-5.0000000000000000e+00, -1.060361827107492160848778e-02},
 	{9.0000000000000000e+00, 6.3629370719841737980004837e-01},
 	{2.0000000000000000e+00, 9.2637723924396464525443662e-01},
@@ -1356,11 +1356,13 @@ var log1pSC = []float64{
 
 var vfmodfSC = []float64{
 	Inf(-1),
+	Copysign(0, -1),
 	Inf(1),
 	NaN(),
 }
 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)},
 	{NaN(), NaN()},
 }

src/math/modf.go

@@ -14,9 +14,12 @@ func Modf(f float64) (int float64, frac float64)
 
 func modf(f float64) (int float64, frac float64) {
 	if f < 1 {
-		if f < 0 {
+		switch {
+		case f < 0:
 			int, frac = Modf(-f)
 			return -int, -frac
+		case f == 0:
+			return f, f // Return -0, -0 when f == -0
 		}
 		return 0, f
 	}

src/math/modf_386.s

@@ -6,6 +6,19 @@
 
 // func Modf(f float64) (int float64, frac float64)
 TEXT ·Modf(SB),NOSPLIT,$0
+	// special case for f == -0.0
+	MOVL f+4(FP), DX	// high word
+	MOVL f+0(FP), AX	// low word
+	CMPL DX, $(1<<31)	// beginning of -0.0
+	JNE notNegativeZero
+	CMPL AX, $0			// could be denormalized
+	JNE notNegativeZero
+	MOVL AX, int+8(FP)
+	MOVL DX, int+12(FP)
+	MOVL AX, frac+16(FP)
+	MOVL DX, frac+20(FP)
+	RET
+notNegativeZero:
 	FMOVD   f+0(FP), F0  // F0=f
 	FMOVD   F0, F1       // F0=f, F1=f
 	FSTCW   -2(SP)       // save old Control Word