cmd/compile: optimize []byte(string1 + string2)

This CL optimizes the compilation of string-to-bytes conversion in the
case of string additions.

Fixes #62407

Change-Id: Ic47df758478e5d061880620025c4ec7dbbff8a64
Reviewed-on: https://go-review.googlesource.com/c/go/+/527935
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Tim King <taking@google.com>

Author: tpaschalis

src/cmd/compile/internal/test/issue62407_test.go

@@ -0,0 +1,62 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"reflect"
+	"testing"
+)
+
+//go:noinline
+func foo() string { return "foo" }
+
+//go:noinline
+func empty() string { return "" }
+
+func TestConcatBytes(t *testing.T) {
+	empty := empty()
+	s := foo()
+	tests := map[string]struct {
+		got  []byte
+		want []byte
+	}{
+		"two empty elements":                 {got: []byte(empty + empty), want: []byte{}},
+		"two nonempty elements":              {got: []byte(s + s), want: []byte("foofoo")},
+		"one empty and one nonempty element": {got: []byte(s + empty), want: []byte("foo")},
+		"multiple empty elements":            {got: []byte(empty + empty + empty + empty + empty + empty), want: []byte{}},
+		"multiple nonempty elements":         {got: []byte("1" + "2" + "3" + "4" + "5" + "6"), want: []byte("123456")},
+	}
+
+	for name, test := range tests {
+		if !reflect.DeepEqual(test.got, test.want) {
+			t.Errorf("[%s] got: %s, want: %s", name, test.got, test.want)
+		}
+	}
+}
+
+func TestConcatBytesAllocations(t *testing.T) {
+	empty := empty()
+	s := foo()
+	tests := map[string]struct {
+		f      func() []byte
+		allocs float64
+	}{
+		"two empty elements":      {f: func() []byte { return []byte(empty + empty) }, allocs: 0},
+		"multiple empty elements": {f: func() []byte { return []byte(empty + empty + empty + empty + empty + empty) }, allocs: 0},
+
+		"two elements":                       {f: func() []byte { return []byte(s + s) }, allocs: 1},
+		"three elements":                     {f: func() []byte { return []byte(s + s + s) }, allocs: 1},
+		"four elements":                      {f: func() []byte { return []byte(s + s + s + s) }, allocs: 1},
+		"five elements":                      {f: func() []byte { return []byte(s + s + s + s + s) }, allocs: 1},
+		"one empty and one nonempty element": {f: func() []byte { return []byte(s + empty) }, allocs: 1},
+		"two empty and two nonempty element": {f: func() []byte { return []byte(s + empty + s + empty) }, allocs: 1},
+	}
+	for name, test := range tests {
+		allocs := testing.AllocsPerRun(100, func() { test.f() })
+		if allocs != test.allocs {
+			t.Errorf("concatbytes [%s]: %v allocs, want %v", name, allocs, test.allocs)
+		}
+	}
+}

src/cmd/compile/internal/typecheck/_builtin/runtime.go

@@ -71,6 +71,12 @@ func concatstring4(*[32]byte, string, string, string, string) string
 func concatstring5(*[32]byte, string, string, string, string, string) string
 func concatstrings(*[32]byte, []string) string
 
+func concatbyte2(string, string) []byte
+func concatbyte3(string, string, string) []byte
+func concatbyte4(string, string, string, string) []byte
+func concatbyte5(string, string, string, string, string) []byte
+func concatbytes([]string) []byte
+
 func cmpstring(string, string) int
 func intstring(*[4]byte, int64) string
 func slicebytetostring(buf *[32]byte, ptr *byte, n int) string