Why Does Mixing `println` and `fmt.Println` Impact Stack Growth in Go?

Mixing Println and Fmt.Println: Understanding Stack Growth

In Go, the stack grows dynamically to accommodate the storage of variables and function arguments. However, this behavior can be affected when using different printing functions.

Consider the following example:

package main

import "fmt"

const size = 1024

func main() {
    fmt.Println("Start")
    s := "HELLO"
    stackCopy(&s, 0, [size]int{})
}

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 only println is used, the address of the string s changes as the stack grows:

Start
println:  0xc000107f58 HELLO
println:  0xc000117f58 HELLO
println:  0xc000117f58 HELLO

However, when fmt.Println is mixed with println, or used exclusively, the address of s remains the same:

Start
println:  0xc00010a040 HELLO
fmt:      0xc00010a040 HELLO
println:  0xc00010a040 HELLO
fmt:      0xc00010a040 HELLO

Understanding the Difference

The difference arises from the way println and fmt.Println handle their arguments.

println is a built-in function that does not retain any arguments passed to it. As a result, the arguments do not escape to the heap and are allocated on the stack.

fmt.Println, on the other hand, is from the standard library and is treated like a custom function. The compiler does not assume that fmt.Println does not retain its arguments, so they may escape to the heap. Consequently, arguments to fmt.Println are allocated on the heap instead of the stack.

Since the stackCopy function takes a large argument ([size]int), the initial stack can become insufficient and a larger one will be allocated. If the variables allocated on the stack are passed as arguments to functions where they can escape to the heap, they will be moved to the heap as the stack grows. This is why we observe the change in the address of s when only println is used.

Conclusion

Mixing println and fmt.Println can influence stack growth due to the difference in how they handle their arguments. println retains arguments on the stack, while fmt.Println may allocate them to the heap, which can lead to changes in the stack layout and variable addresses.

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