3.2 Escape Analysis

1. Escape Analysis

Objects created in Java or C++ are generally allocated on the heap, while objects created with new in Golang are allocated on the heap or stack according to escape analysis.

2. Why Escape Analysis Is Needed

	Heap	Stack
Reclamation	Requires garbage collection	Destroyed when the function exits
Space	Large	Small

3. How Escape Analysis Works

If there is no reference outside the function, prefer placing it on the stack;
If there is a reference outside the function, it must be placed on the heap.

4. Example

package main

import "fmt"

func foo() *int {
	t := 3
	return &t;
}

func main() {
	x := foo()
	fmt.Println(*x)
}

• go build -gcflags '-m -l' main.go

# command-line-arguments
// t escaped; no problem
.\main.go:6:2: moved to heap: t
.\main.go:12:13: main ... argument does not escape
// Did x also escape? When a function parameter is an interface type, such as fmt.Println(a ...interface{}), it is difficult to determine the concrete parameter type during compilation, so escape can also occur
.\main.go:12:14: *x escapes to heap

• go tool compile -S test.go | grep -i "test.go:6"

0x0024 00036 (test.go:6)        PCDATA  $0, $1
0x0024 00036 (test.go:6)        PCDATA  $1, $0
0x0024 00036 (test.go:6)        LEAQ    type.int(SB), AX
0x002b 00043 (test.go:6)        PCDATA  $0, $0
0x002b 00043 (test.go:6)        MOVQ    AX, (SP)
0x002f 00047 (test.go:6)        CALL    runtime.newobject(SB)// newobject shows that x escaped
0x0034 00052 (test.go:6)        PCDATA  $0, $1
0x0034 00052 (test.go:6)        MOVQ    8(SP), AX
0x0039 00057 (test.go:6)        MOVQ    $3, (AX)

5. References

• Golang Memory Allocation and Escape Analysis
• Where Do Golang Variables Go? | qcrao
• Compilation and Linking - How Escape Analysis Works - Go Questions - BookStack