Why Do Comparisons of Arrays of Pointers to Empty Structs Produce Inconsistent Results?

Why Array Comparison with Empty Structs Yields Different Results

Given an array of pointers to empty structs, why does the equality comparison of the arrays sometimes evaluate to true and sometimes to false?

Exploration of Behavior

package main

import "fmt"

type myStruct struct{}

func main() {
    s, ss := myStruct{}, myStruct{}
    arr1 := [6]*myStruct{&s}
    arr2 := [6]*myStruct{&ss}
    fmt.Println(&s == &ss, arr1 == arr2)  // Produces mixed results (e.g., false, true or true, false)

    l, ll := myStruct{A: 1}, myStruct{A: 1}
    arr3 := [6]*myStruct{&l}
    arr4 := [6]*myStruct{&ll}
    fmt.Println(&l == &ll, arr3 == arr4)  // Always evaluates to false
}

Explanation

The Go language specification states that:

• Pointers to distinct zero-size variables may or may not be equal.
• A struct or array has zero size if it contains no non-zero-size fields (or elements).
• Two distinct zero-size variables may have the same address in memory.

Dynamic Behavior and Escape Analysis

The behavior can be explained by considering the escape analysis performed by the Go compiler.

• In the first example, the variables s and ss do not escape (i.e., they are not passed by reference to other functions). This means the compiler has more flexibility in allocating them in memory and may assign them the same address.
• In the second example, adding a fmt.Printf call (which escapes both s and ss) causes the compiler to move the variables to the heap, resulting in different memory addresses and thus yielding false for &l == &ll.

Implications

• It is not reliable to compare arrays of empty structs for equality based on the equality of their pointers.
• Escape analysis can significantly impact the behavior of such comparisons.

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