slices: add sort benchmark for sorted strings

For #60777

Change-Id: I424535ce6454156c61af2f299228630ee304d165
Reviewed-on: https://go-review.googlesource.com/c/go/+/503815
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Eli Bendersky <eliben@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Eli Bendersky <eliben@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/slices/sort_benchmark_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"math/rand"
 	"sort"
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -50,6 +51,15 @@ func BenchmarkSortInts(b *testing.B) {
 	}
 }
 
+func makeSortedStrings(n int) []string {
+	x := make([]string, n)
+	for i := 0; i < n; i++ {
+		x[i] = strconv.Itoa(i)
+	}
+	Sort(x)
+	return x
+}
+
 func BenchmarkSlicesSortInts(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		b.StopTimer()
@@ -153,6 +163,15 @@ func BenchmarkSortStrings(b *testing.B) {
 	}
 }
 
+func BenchmarkSortStrings_Sorted(b *testing.B) {
+	ss := makeSortedStrings(N)
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		sort.Strings(ss)
+	}
+}
+
 func BenchmarkSlicesSortStrings(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		b.StopTimer()
@@ -162,6 +181,15 @@ func BenchmarkSlicesSortStrings(b *testing.B) {
 	}
 }
 
+func BenchmarkSlicesSortStrings_Sorted(b *testing.B) {
+	ss := makeSortedStrings(N)
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		Sort(ss)
+	}
+}
+
 // These benchmarks compare sorting a slice of structs with sort.Sort vs.
 // slices.SortFunc.
 type myStruct struct {