cmd/compile: redundant allocation & copy in return string([]byte)

[jfcg]

What version of Go are you using (go version)?

go1.17.8

Does this issue reproduce with the latest release?

yes also with go1.18

What operating system and processor architecture are you using (go env)?

x86_64 Linux

What did you do?

pkg.go

package pkg

// ToLower converts ascii string s to lower-case
func ToLower(s string) string {
	if s == "" {
		return ""
	}
	buf := make([]byte, len(s))

	for i := 0; i < len(s); i++ {
		c := s[i]
		if 'A' <= c && c <= 'Z' {
			c |= 32
		}
		buf[i] = c
	}
	return string(buf)
}

pkg_test.go

package pkg

import "testing"

func BenchmarkToLower(b *testing.B) {
	str := "SomE StrInG"
	want := "some string"
	var res string

	for i := 0; i < b.N; i++ {
		res = ToLower(str)
	}
	if res != want {
		b.Fatal("ToLower error")
	}
}

go test -v -bench ToLower -benchmem -count=3 prints

BenchmarkToLower-4      18710726                66.86 ns/op           32 B/op          2 allocs/op
BenchmarkToLower-4      17420910                65.57 ns/op           32 B/op          2 allocs/op
BenchmarkToLower-4      17085405                63.16 ns/op           32 B/op          2 allocs/op

What did you expect to see?

I was expecting to see a single memory allocation in ToLower() since there is no other reference to buf.

What did you see instead?

As is clear in bench output, there must a redundant allocation & copy in return string(buf).

[seankhliao]

I think this is a dup of #50846
Please comment if you disagree