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

Fixes #12867

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

diff --git a/src/math/all_test.go b/src/math/all_test.go
index e18e45e020228d95d77e779f5f2a8d9e9e54cd6c..53e84765cb8723227dc075cd8be82c85345a986b 100644 (file)
--- a/src/math/all_test.go
+++ b/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()},
 }

diff --git a/src/math/modf.go b/src/math/modf.go
index 1e8376a93819f23d13a282bcae550f4efcb0a88e..81cb8b5a9ce6ef8cd004e376101e4e94352e019e 100644 (file)
--- a/src/math/modf.go
+++ b/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
        }

diff --git a/src/math/modf_386.s b/src/math/modf_386.s
index 3debd3b95dbdf8b767fbc9db1c576d0c548e0276..d549f1d1a0af31d945463abf27a3d16cdf130028 100644 (file)
--- a/src/math/modf_386.s
+++ b/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