Test program:

package main

import (
        "fmt"
)

func s(k uint64) float32 {
        var x float32
        x = float32(k)
        return x
}

func main() {
        k := uint64(0x8234508000000001)
	x2 := float64(k)
	fmt.Printf("%x\n", x2)
        x := float64(s(k))
	fmt.Printf("%x\n", x)
}

On amd64 this prints

0x1.0468a1p+63
0x1.0468a2p+63

On 386 it prints

0x1.0468a1p+63
0x1.0468ap+63

The second number is different. This is happening because on 386 we convert a uint64 to float32 by first converting to float64 (via runtime.uint64tofloat64) and then converting that to float32. This introduces a double-rounding. Specifically the uint64 value 0x1.0468a1p+63 converts to float32 0x1.0468ap+63, but the correct result of the original rounding is 0x1.0468a2p+63.