Escape analysis
- Go course
- Expert
- Lesson 15/34
- 4 min
Learn how the compiler decides stack vs heap.
Escape analysis
For each variable the compiler asks: can this value still be reached after the function returns? If yes, the value escapes to the heap; if not, it stays on the stack.
func a() *int { x := 1; return &x } // x escapes to the heap
func b() int { x := 1; return x } // x stays on the stackTo see the decisions: go build -gcflags=-m → moved to heap: x.
Common escape causes
Unexpected escapes: passing to functions with any parameters such as fmt.Println(x) (the value is boxed into an interface), slices whose size isn't known at compile time (make([]int, n)), variables captured by closures that outlive the function, and interface method calls. Reading the -gcflags=-m output on hot paths reveals needless heap allocations.
Practice Measure the allocation
Write two functions: isaret() *int returns a local variable's address and deger() int returns its value. Assign the results to package-level variables, measure with testing.AllocsPerRun and print both numbers.
Starter code
package main
import (
"fmt"
"testing"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"testing"
)
var (
p *int
v int
)
//go:noinline
func isaret() *int {
x := 1
return &x
}
//go:noinline
func deger() int {
x := 1
return x
}
func main() {
a := testing.AllocsPerRun(10, func() { p = isaret() })
b := testing.AllocsPerRun(10, func() { v = deger() })
fmt.Println(a, b)
}Expected output
1 0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which does <strong>not</strong> make a value escape to the heap?
- Passing its pointer to a goroutine
- Returning the value by copy Correct answer
- Returning its address
- Assigning it to a global
Explanation: Returning a copy gives the caller a new value; the local variable can disappear with the function.
2 Escape report
go build -gcflags=____ ./...go build -gcflags=-m ./...Explanation: -gcflags=-m prints the compiler's escape and inlining decisions.
3 Code and result
| Term | Meaning |
|---|---|
return &x |
Escapes to heap |
return x |
Stays on stack |
make([]int, 8) |
May stay on stack |
make([]int, n) |
Usually heap |