see: https://github.com/golang/go/issues/65878
package main
var a, b [0]int
var p, q = &a, &b
func main() {
if (p == q) {
p, q = &a, &b
println(p == q) // false
}
}
On Thursday, June 20, 2024 at 12:59:43 AM UTC+8 Oliver Eikemeier wrote:
> I'm observing some strange behavior and could use some help understanding
> it.
>
> The specification <https://go.dev/ref/spec#Comparison_operators%E2%80%9D>
> says: *“Pointers to distinct zero-size variables may or may not be
> equal.”*
>
> With the following program (Go Playground
> <https://go.dev/play/p/D41jZhecbyK>):
> var ( a struct{} b struct{} eq = &a == &b ) func f1() { println("&a:",
> &a) println("&b:", &b) println("&a == &b:", eq) }
>
>
> I'll get
> &a: 0x537580 &b: 0x537580 &a == &b: false
>
>
> Okay, a and b are empty structs, do not escape, so they share the same
> address - fine. Also, some optimizer sees that a and b are different
> variables, so their addresses must be different, and decides to make &a
> == &b a constant - wrong, but I can live with that.
>
> My question would be: Is this behavior expected, somehow defined by the
> specification, or is it undefined behavior?
>
> Let's try to confuse the optimizer a little (Go Playground
> <https://go.dev/play/p/_r5WEULKPKd>):
> var ( a struct{} b struct{} aa = &a ba = &b eq = aa == ba ) func f1() {
> println("&a:", aa) println("&b:", ba) println("&a == &b:", eq) }
>
> results in
> &a: 0x5375a0 &b: 0x5375a0 &a == &b: true
>
>
> Mission accomplished, too complicated to calculate in advance. But globals
> are bad, so (Go Playground <https://go.dev/play/p/SleVYsVj3u5>):
> func f2() { var ( a struct{} b struct{} aa = &a ba = &b eq = aa == ba )
> println("&a:", aa) println("&b:", ba) println("&a == &b:", eq) }
>
> &a: 0xc000046740 &b: 0xc000046740 &a == &b: false
>
>
> Seems like inlining helps generate false answers.
>
> The interesting part here is that I can create two pointers (which may or
> may not be equal per specification), but depending on how I compare them I
> get different results.
>
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