slices: make Clone preallocate and copy instead of solely invoking append

src/slices/slices.go

@@ -340,7 +340,9 @@ func Clone[S ~[]E, E any](s S) S {
 	if s == nil {
 		return nil
 	}
-	return append(S([]E{}), s...)
+	t := make([]E, len(s))
+	copy(t, s)
+	return t
 }
 
 // Compact replaces consecutive runs of equal elements with a single copy.

src/slices/slices_benchmark_test.go

@@ -0,0 +1,56 @@
+// Copyright 2023 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 slices
+
+import (
+	"testing"
+)
+
+var sink any = nil
+
+func BenchmarkIntClone_10B(b *testing.B) {
+	benchmarkClone[int](b, 10)
+}
+
+func BenchmarkIntClone_1Kb(b *testing.B) {
+	benchmarkClone[int](b, 1<<10)
+}
+
+func BenchmarkIntClone_10Kb(b *testing.B) {
+	benchmarkClone[int](b, 10<<10)
+}
+
+func BenchmarkIntClone_1Mb(b *testing.B) {
+	benchmarkClone[int](b, 1<<20)
+}
+
+func BenchmarkIntClone_10Mb(b *testing.B) {
+	benchmarkClone[int](b, 10<<20)
+}
+
+func BenchmarkByteClone_10B(b *testing.B) {
+	benchmarkClone[byte](b, 10)
+}
+
+func BenchmarkByteClone_10Kb(b *testing.B) {
+	benchmarkClone[byte](b, 10<<10)
+}
+
+func benchmarkClone[T int | byte](b *testing.B, n int) {
+	s1 := make([]T, n)
+	for i := 0; i < n/2; i++ {
+		s1[i] = T(i)
+		s1[n-i-1] = T(i * 2)
+	}
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		sink = Clone(s1)
+	}
+	if sink == nil {
+		b.Fatal("Benchmark did not run!")
+	}
+	sink = nil
+}