Why Do Comparisons of Arrays of Zero-Sized Structs in Go Produce Unexpected Results?

Puzzling Comparison Results for Arrays of Zero-Sized Structs

The comparison of arrays of empty structs can yield unexpected results, as seen in the following code snippet:

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

To comprehend these results, we turn to the Go specification. It states that pointer values are comparable and that distinct zero-size variables may or may not have identical addresses in memory. In our case, the empty structs s and ss have zero size, so the pointers &s and &ss may not be equal. This explains why &s == &ss evaluates to false.

However, the specification also indicates that two zero-size variables may have the same address in memory, which could potentially lead to arr1 == arr2 evaluating to true. This behavior is influenced by escape analysis, which optimizes the memory allocation and storage of variables for faster performance.

In the simplified version of our code, both &s and &ss do not escape, meaning they are only used locally and are not passed to any external functions. This allows the compiler to optimize their allocation and potential storage at the same address, resulting in arr1 == arr2 evaluating to true.

However, if &s or &ss were to be used in a way that causes them to escape, such as by assigning them to a function parameter, the compiler would optimize their storage differently. They would be moved to the heap, effectively breaking any accidental equality between their addresses. In such cases, arr1 == arr2 would evaluate to false.

This behavior change demonstrates the complex interplay between allocation optimization and variable storage, and highlights the importance of understanding escape analysis and its potential impact on code execution.

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