Array Internals and the Call Stack in Go

1. HELLO, MY NAME IS GEORGI @GeorgiCodes
2. array declaration var elements [4]int
3. func main() { a := [4]int{1, 2, 3, 4} println("a addr:", &a) for i := range a { print("Value: ", a[i]) println(" Address:", &a[i]) } }
4. on 64bit architecture, each int = 8 bytes func main() { a := [4]int{1, 2, 3, 4} println("a addr:", &a) for i := range a { print("Value: ", a[i]) println(" Address:", &a[i]) } } a addr: 0x220822bf20 Value: 1 IndexAddr: 0x220822bf20 Value: 2 IndexAddr: 0x220822bf28 Value: 3 IndexAddr: 0x220822bf30 Value: 4 IndexAddr: 0x220822bf38
5. arrays are contiguous in memory
7. CPU cache
8. Pass by value
9. In Go, everything is pass by value. When we pass an array as an argument, we pass a copy of the array not a reference to the array. package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(names) println(names[0]) } func f1(a [4]string) { a[0] = "marie" }
10. 1. A copy of names is made when f1 is called 1 package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(names) println(names[0]) } func f1(a [4]string) { a[0] = "marie" }
11. 2. The copy of names is assigned to local var a 1 package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(names) println(names[0]) } func f1(a [4]string) { a[0] = "marie" } 2
12. 3. We make a change to the copy 3 2 1 package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(names) println(names[0]) } func f1(a [4]string) { a[0] = "marie" }
13. 3 2 1 package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(names) println(names[0]) } func f1(a [4]string) { a[0] = "marie" } // ada
14. call stack
15. Pointers
16. If we want to share names with f1 so f1 can modify it, then we pass the address of names to f1. package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(&names) println(names[0]) } func f1(a *[4]string) { a[0] = "marie" }
17. 1. Pass the address of names package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(&names) println(names[0]) } func f1(a *[4]string) { a[0] = "marie" } 1
18. 2. f1 accepts a pointer variable package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(&names) println(names[0]) } func f1(a *[4]string) { a[0] = "marie" } 2 1
19. 3. Changes to a update the array that a points to, names. package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(&names) println(names[0]) } func f1(a *[4]string) { a[0] = "marie" } 3 2 1
20. package main func main() { names := [4]string {"ada", "lovelace", "tom", "jerry"} f1(&names) println(names[0]) } func f1(a *[4]string) { a[0] = "marie" } 3 2 1 // marie
21. call stack using a pointer
22. the Go compiler likes to optimise all the things!
23. func main() { a := [4]int{1, 2, 3, 4} println("a addr:", &a) for i := range a { print("Value: ", a[i]) println(" Address:", &a[i]) } } print
24. func main() { a := [4]int{1, 2, 3, 4} println("a addr:", &a) for i := range a { print("Value: ", a[i]) println(" Address:", &a[i]) } } print func main() { a := [4]int{1, 2, 3, 4} println("a addr:", &a) for i := range a { print("Value: ", a[i]) println(" Address:", &a[i]) } } import "fmt" func main() { a := [4]int{1, 2, 3, 4} fmt.Printf("a addr: %p \n", &a) for i := range a { fmt.Printf("V: %v A: %p \n", a[i], &a[i]) } } import "fmt" func main() { a := [4]int{1, 2, 3, 4} fmt.Printf("a addr: %p \n", &a) for i := range a { fmt.Printf("V: %v A: %p \n", a[i], &a[i]) } } fmt.Print
25. 1. escape analysis go build -gcflags=-m main.go:6: moved to heap: a main.go:8: &a escapes to heap main.go:11: &a[i] escapes to heap main.go:8: main ... argument does not escape main.go:11: main ... argument does not escape with ft.Printf
26. 1. inlining func main() { names := [2]string{"ada", "lovelace"} f1(names) println(names[0]) } func f1(a [2]string) { a[0] = "marie" }
27. inlining func main() { names := [2]string{"ada", "lovelace"} f1(names) println(names[0]) } func f1(a [2]string) { a[0] = "marie" // Do this to prevent inlining. var x int fmt.Sprintf("Prevent Inlining: %d", x) }
28. thank-you! @GeorgiCodes georgi.io