cmd/compile/internal/gc: there are no -0 floating-point constants

Fixes #12577.

Change-Id: Id469cd92f5f9436b0ef948ee1a252ed1842bc7aa
Reviewed-on: https://go-review.googlesource.com/16133
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/mparith3.go

@@ -113,7 +113,7 @@ func mpgetflt(a *Mpflt) float64 {
 		Yyerror("mpgetflt ovf")
 	}
 
-	return x
+	return x + 0 // avoid -0 (should not be needed, but be conservative)
 }
 
 func mpgetflt32(a *Mpflt) float64 {
@@ -125,7 +125,7 @@ func mpgetflt32(a *Mpflt) float64 {
 		Yyerror("mpgetflt32 ovf")
 	}
 
-	return x
+	return x + 0 // avoid -0 (should not be needed, but be conservative)
 }
 
 func Mpmovecflt(a *Mpflt, c float64) {
@@ -133,6 +133,10 @@ func Mpmovecflt(a *Mpflt, c float64) {
 		fmt.Printf("\nconst %g", c)
 	}
 
+	// convert -0 to 0
+	if c == 0 {
+		c = 0
+	}
 	a.Val.SetFloat64(c)
 
 	if Mpdebug {
@@ -141,7 +145,10 @@ func Mpmovecflt(a *Mpflt, c float64) {
 }
 
 func mpnegflt(a *Mpflt) {
-	a.Val.Neg(&a.Val)
+	// avoid -0
+	if a.Val.Sign() != 0 {
+		a.Val.Neg(&a.Val)
+	}
 }
 
 //
@@ -163,15 +170,20 @@ func mpatoflt(a *Mpflt, as string) {
 		// - decimal point and binary point in constant
 		// TODO(gri) use different conversion function or check separately
 		Yyerror("malformed constant: %s", as)
-		a.Val.SetUint64(0)
+		a.Val.SetFloat64(0)
 		return
 	}
 
 	if f.IsInf() {
 		Yyerror("constant too large: %s", as)
-		a.Val.SetUint64(0)
+		a.Val.SetFloat64(0)
 		return
 	}
+
+	// -0 becomes 0
+	if f.Sign() == 0 && f.Signbit() {
+		a.Val.SetFloat64(0)
+	}
 }
 
 func (f *Mpflt) String() string {
@@ -188,7 +200,7 @@ func Fconv(fvp *Mpflt, flag int) string {
 	// determine sign
 	f := &fvp.Val
 	var sign string
-	if fvp.Val.Signbit() {
+	if f.Sign() < 0 {
 		sign = "-"
 		f = new(big.Float).Abs(f)
 	} else if flag&obj.FmtSign != 0 {

test/fixedbugs/issue12577.go

@@ -0,0 +1,66 @@
+// run
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 12577: Test that there are no -0 floating-point constants.
+
+package main
+
+import "math"
+
+const (
+	z0 = 0.0
+	z1 = -0.0
+	z2 = -z0
+	z3 = -z2
+)
+
+var (
+	x0 float32 = z0
+	x1 float32 = z1
+	x2 float32 = z2
+	x3 float32 = z3
+
+	y0 float64 = z0
+	y1 float64 = z1
+	y2 float64 = z2
+	y3 float64 = z3
+)
+
+func test32(f float32) {
+	if f != 0 || math.Signbit(float64(f)) {
+		println("BUG: got", f, "want 0.0")
+		return
+	}
+}
+
+func test64(f float64) {
+	if f != 0 || math.Signbit(f) {
+		println("BUG: got", f, "want 0.0")
+		return
+	}
+}
+
+func main() {
+	if f := -x0; f != 0 || !math.Signbit(float64(f)) {
+		println("BUG: got", f, "want -0.0")
+	}
+
+	test32(-0.0)
+	test32(x0)
+	test32(x1)
+	test32(x2)
+	test32(x3)
+
+	if f := -y0; f != 0 || !math.Signbit(f) {
+		println("BUG: got", f, "want -0.0")
+	}
+
+	test64(-0.0)
+	test64(y0)
+	test64(y1)
+	test64(y2)
+	test64(y3)
+}