Why Do Array Comparisons of Empty Structures in Go Sometimes Yield Unexpected Results?

Why Array Comparisons of Structures Yield Unequal Results

In Go, comparing arrays of empty structures can produce unexpected results. This behavior stems from the subtle nature of pointers and zero-size variables in the language.

Pointers to Zero-Size Variables

According to the Go specification, pointers to distinct zero-size variables do not necessarily have unique addresses. This is because zero-size variables may be optimized away by the compiler, leading to unpredictable memory allocation.

Example

Consider the following code:

var s, ss struct{} // two empty structs
arr1 := [6]*struct{}{&s} // array with empty struct pointer
arr2 := [6]*struct{}{&ss} // array with empty struct pointer
fmt.Println(&s == &ss, arr1 == arr2)  // false, true

In this example, arr1 and arr2 are two arrays containing pointers to distinct empty structures, s and ss. Surprisingly, the comparison arr1 == arr2 returns true. This is because the compiler may have placed s and ss at the same address, despite their distinct identities. However, &s == &ss evaluates to false because it directly compares the pointers themselves.

Int Variables in Structures

However, when non-zero values are stored in the structures, such as integers, the comparison behavior changes.

var l, ll struct{A int}{}
arr3 := [6]*struct{A int}{&l} // array with empty struct pointer
arr4 := [6]*struct{A int}{&ll} // array with empty struct pointer
fmt.Println(&l == &ll, arr3 == arr4)  // false, false

In this case, the comparison of arr3 and arr4 returns false, indicating that the arrays are not equal. This is because the non-zero integers stored in the structures give them distinct addresses.

Escape Analysis and Variable Relocation

The unexpected behavior can also be influenced by escape analysis, a compiler optimization that determines whether variables need to be allocated on the heap or stack. If a variable escapes its local scope, it is allocated on the heap to ensure its accessibility in other parts of the program.

In the case of pointers to empty structures, if they escape, the compiler may relocate them to different heap addresses. This relocation affects their equality comparison, resulting in false.

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