Why does one string concatenation escape, but the other not?

[tapi...@gmail.com]

package main

import "testing"

var s33 = []byte{32: 'b'}

var a = string(s33)

func main() {
    x := a + a // a + a does not escape
    println(x)
}

func Benchmark_e_33(b *testing.B) {
    var x string
    for i := 0; i < b.N; i++ {
        x = a + a // a + a escapes to heap
    }
    println(x)
}

[tapi...@gmail.com]

Is this to avoid stack growing to fast if there are some stack allocations intervening with each other?

[jake...@gmail.com]

It does not answer your question, but possibly provides more clues: https://play.golang.org/p/s9Xnpcx8Mys

package main

func main() {
    var a = "b"
    x := a + a // does not escape
    x = a + a  // does not escape
    for i := 0; i < 1; i++ {

        x = a + a  // a + a escapes to heap

        y := a + a // does not escape
        println(y)
        println(x)
    }
}

Only when a variable outside the loop is set inside the loop does it escape. 

On Wednesday, June 23, 2021 at 12:39:17 AM UTC-4 tapi...@gmail.com wrote:

[tapi...@gmail.com]

On Wednesday, June 23, 2021 at 12:00:43 PM UTC-4 jake...@gmail.com wrote:

It does not answer your question, but possibly provides more clues: https://play.golang.org/p/s9Xnpcx8Mys

package main

func main() {
    var a = "b"
    x := a + a // does not escape
    x = a + a  // does not escape
    for i := 0; i < 1; i++ {

        x = a + a  // a + a escapes to heap

        y := a + a // does not escape
        println(y)
        println(x)
    }
}

Only when a variable outside the loop is set inside the loop does it escape. 

Yes, testing is not relevant here. It is the loop make effects here.

[tapi...@gmail.com]

Now, I think the escape analysis message should be "a + a does not escapes",

which accurate meaning is "the header part of a + a does not escape".

It is impossible for the compiler to determine whether or notthe underlying part of "a + a" will escape to heap.

On Wednesday, June 23, 2021 at 12:00:43 PM UTC-4 jake...@gmail.com wrote:

[tapi...@gmail.com]

Filled an issue: https://github.com/golang/go/issues/46908