math: exp sometimes returns wrong values on overflow on amd64

[ALTree]

go version devel +12c58bbf81 Fri Feb 3 06:52:46 2017 +0000 linux/amd64

Documentation says:

Very large values overflow to 0 or +Inf

But that's not always true (at least not when using the assembly implementation on amd64). Example:

package main

import (
	"fmt"
	"math"
)

func main() {
	fmt.Println(math.Exp(1.48852223e+09))
	fmt.Println(math.Exp(1.4885222e+09))
	fmt.Println(math.Exp(1.48852e+09))
}

Gives

0.0027127673070219977
2.5385055740655363e-16
+Inf

[gopherbot]

CL https://golang.org/cl/36271 mentions this issue.