Please answer these questions before submitting your issue. Thanks!

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

go version go1.10.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"

happens on GOOS="linux"

What did you do?

the high 32 bits of uint64(math.Float32bits(x)) are not zero

If possible, provide a recipe for reproducing the error.

package main

import (
	"fmt"
	"math"
)

func Foo(v float32) {
	fmt.Printf("%x\n", uint64(math.Float32bits(v)))
}

func main() {
	Foo(2.0)
}

this produces "c440000000"

What did you expect to see?

40000000

What did you see instead?

c40000000

The high order bits are not cleared in the cast to uint64

works fine in 1.9.6 and the generated code in 1.10.2 seems to optimize the cast out.