cmd/internal/obj: add SortSlice

sort.Slice was added in Go 1.8.
It's nice to use, and faster than sort.Sort,
so it'd be nice to be able to use it in the toolchain.
This CL adds obj.SortSlice, which is sort.Slice,
but with a slower fallback version for bootstrapping.

This CL also includes a single demo+test use.

Change-Id: I2accc60b61f8e48c8ab4f1a63473e3b87af9b691
Reviewed-on: https://go-review.googlesource.com/40114
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -49,16 +49,7 @@ type Sig struct {
 	offset int32
 }
 
-// byMethodNameAndPackagePath sorts method signatures by name, then package path.
-type byMethodNameAndPackagePath []*Sig
-
-func (x byMethodNameAndPackagePath) Len() int      { return len(x) }
-func (x byMethodNameAndPackagePath) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
-func (x byMethodNameAndPackagePath) Less(i, j int) bool {
-	return siglt(x[i], x[j])
-}
-
-// siglt reports whether a < b
+// siglt sorts method signatures by name, then package path.
 func siglt(a, b *Sig) bool {
 	if a.name != b.name {
 		return a.name < b.name
@@ -382,7 +373,7 @@ func methods(t *types.Type) []*Sig {
 		}
 	}
 
-	sort.Sort(byMethodNameAndPackagePath(ms))
+	obj.SortSlice(ms, func(i, j int) bool { return siglt(ms[i], ms[j]) })
 	return ms
 }
 

src/cmd/compile/internal/gc/reflect_test.go

@@ -6,12 +6,12 @@ package gc
 
 import (
 	"cmd/compile/internal/types"
+	"cmd/internal/obj"
 	"reflect"
-	"sort"
 	"testing"
 )
 
-func TestSortingByMethodNameAndPackagePath(t *testing.T) {
+func TestSortingBySigLT(t *testing.T) {
 	data := []*Sig{
 		&Sig{name: "b", pkg: &types.Pkg{Path: "abc"}},
 		&Sig{name: "b", pkg: nil},
@@ -38,11 +38,10 @@ func TestSortingByMethodNameAndPackagePath(t *testing.T) {
 	if reflect.DeepEqual(data, want) {
 		t.Fatal("data must be shuffled")
 	}
-	sort.Sort(byMethodNameAndPackagePath(data))
+	obj.SortSlice(data, func(i, j int) bool { return siglt(data[i], data[j]) })
 	if !reflect.DeepEqual(data, want) {
 		t.Logf("want: %#v", want)
 		t.Logf("data: %#v", data)
 		t.Errorf("sorting failed")
 	}
-
 }

src/cmd/internal/obj/bootstrap.go

@@ -0,0 +1,34 @@
+// Copyright 2017 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.
+
+// +build !go1.8
+
+package obj
+
+import (
+	"reflect"
+	"sort"
+)
+
+func SortSlice(slice interface{}, less func(i, j int) bool) {
+	val := reflect.ValueOf(slice)
+	tmp := reflect.New(val.Type().Elem()).Elem()
+	x := sliceByFn{val: val, tmp: tmp, less: less}
+	sort.Sort(x)
+}
+
+type sliceByFn struct {
+	val  reflect.Value
+	tmp  reflect.Value
+	less func(i, j int) bool
+}
+
+func (x sliceByFn) Len() int           { return x.val.Len() }
+func (x sliceByFn) Less(i, j int) bool { return x.less(i, j) }
+func (x sliceByFn) Swap(i, j int) {
+	a, b := x.val.Index(i), x.val.Index(j)
+	x.tmp.Set(a)
+	a.Set(b)
+	b.Set(x.tmp)
+}

src/cmd/internal/obj/sort.go

@@ -0,0 +1,13 @@
+// Copyright 2017 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.
+
+// +build go1.8
+
+package obj
+
+import "sort"
+
+func SortSlice(slice interface{}, less func(i, j int) bool) {
+	sort.Slice(slice, less)
+}