This is a weird corner case in string concatenation optimization.
runtime.concatstrings (what the + in this code gets rewritten to) has an
optimization where if all the strings but one that it is concatenating are
0 length, then it returns the remaining one without doing any allocation.
Because of that optimization, runtime.concatstrings might return its
argument. Thus, we can't do the zero-copy conversion of []byte to string
for arguments to runtime.concatstrings.
Except when we know that at least one of the input strings has non-zero
length. Then we can. That's what is happening in f2.
On Monday, April 17, 2017 at 11:06:26 PM UTC-7, T L wrote:
>
>
> package main
>
> import "fmt"
> import "testing"
>
> var s string
> var a = make([]byte, 1000)
>
> func f0() {
> x := string(a)
> s = x + x + x +
> x + x + x +
> x + x + x +
> x
> }
>
> func f1() {
> s = string(a) + string(a) + string(a) +
> string(a) + string(a) + string(a) +
> string(a) + string(a) + string(a) +
> string(a)
> }
>
> func f2() {
> s = (" " +
> string(a) + string(a) + string(a) +
> string(a) + string(a) + string(a) +
> string(a) + string(a) + string(a) +
> string(a) )[1:]
> }
>
> func main() {
> fmt.Println(testing.AllocsPerRun(1, f0)) // 2
> fmt.Println(testing.AllocsPerRun(1, f1)) // 11
> fmt.Println(testing.AllocsPerRun(1, f2)) // 1
> }
>
> why doesn't gc make optimization for f1?
>
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