Why Does Mixing Println and Fmt.Println Impact Stack Growth in Go?

Mix Print and Fmt.Println: Impact on Stack Growth

In Go, understanding the difference between the built-in println function and the fmt.Println function is crucial when analyzing stack growth behavior.

Printf vs. Fmt.Println

println is a built-in function that operates under the assumption that its arguments are not retained. Consequently, arguments passed to println do not escape to the heap. On the other hand, fmt.Println is a standard library function treated like any user-defined function. The compiler assumes that arguments passed to fmt.Println may escape to the heap, so they are allocated in the heap rather than the stack.

Implications for Stack Growth

This distinction becomes relevant when using recursion and passing large arguments on the stack. Consider the following recursion:

func stackCopy(s *string, c int, a [size]int) {
    println("println: ", s, *s)
    // fmt.Println("fmt:     ", s, *s)
    c++
    if c == 10 {
        return
    }
    stackCopy(s, c, a)
}

When using println, the address of s changes because the stack is growing, and data is moved to a different location. However, when fmt.Println is mixed into the recursion or used exclusively, the address of s remains constant.

Reason for the Behavior

The reason for this disparity lies in Go's dynamic stack. The stack initially starts small but can expand as needed. When a large argument is passed to a recursive function like stackCopy, the initial stack may be insufficient, leading to stack growth and relocation of stack-allocated variables. This does not occur when using fmt.Println because the compiler places s on the heap due to the possibility of it escaping to the heap.

Compiler Insight

Using the -gcflags '-m' flag reveals the compiler's escape analysis. For the case using only println, s does not escape. However, when fmt.Println is used, s and *s are marked as escaping to the heap.

Conclusion

Understanding the nuances between println and fmt.Println and their impact on stack growth is essential for optimizing Go code and avoiding unexpected behavior. By utilizing the compiler's escape analysis, developers can gain deeper insights into the memory allocation of their programs.

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