math: portable FMA implementation incorrectly returns -0 in some situations

[mundaym]

What version of Go are you using (go version)?

8b5fe59

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

linux-amd64

What did you do?

https://go.dev/play/p/KGZMg3ATQtx

package main

import (
	"fmt"
	"math"
)

var portableFMA = math.FMA

func main() {
	fmt.Printf("%g\n", math.FMA(-1, 1, 1))
	fmt.Printf("%g\n", portableFMA(-1, 1, 1))
}

What did you expect to see?

0
0

What did you see instead?

0
-0

FMA calculates x*y+z. In the portable implementation of FMA if |x*y| is equal to |z| the sign bit of the result is set to the sign bit of x*y. If x*y is negative and z is positive this results in the incorrect result -0.

FMA is an intrinsic on some platforms. This issue only affects the portable implementation (hence the different results when calling FMA directly vs. indirectly).

[gopherbot]

Change https://go.dev/cl/507376 mentions this issue: math: fix portable FMA when x*y < 0 and x*y == -z

[mpx]

@Nasfame - Go follows IEEE-754 which defines -0 == +0. The portable implementation has a bug where -1 * 1 + 1 returns -0, it should return +0.