如何计算golang溢出？

Question

I am a newbie to Golang

package main

import "fmt"

func main() {
    var u uint8 = 255
    var m uint16 = 255
    fmt.Println(u, u-1, u*(u-1))
    fmt.Println(m, m+1, m*m)
}

OUTPUT

./g2
255 254 2
255 256 65025

I understand that I have overflow in the first case. But why did I get 2 as a result?

Answer 1

You’ve already hinted at overflow, and in fact this is exactly what’s happening here. You already know what u and u-1 are (255 and 254 respectively), so let’s multiply those together. This gives us 64770, or in binary:

64770       = 1111 1101 0000 0010

This result is 16 bits wide, but we’re storing it in a uint8 which is only 8 bits wide. It is therefore truncated to the 8 least significant bits. Mathematically, this is equivalent to performing 64770 mod 2^8 or 64770 mod 256. Programatically, it’s equivalent to performing 64770 & (1 << 7) or 64770 & 256:

64770       = 1111 1101 0000 0010
256         =           1111 1111
64770 & 256 = 0000 0000 0000 0010 = 2

However you chose to imagine this operation, the result is 2.