Go copy vs append

copy_vs_append.go

package benchcp_test

import (
	"fmt"
	"testing"
)
import (
	"fmt"
	"testing"
)

func BenchmarkAppendAllocInline(b *testing.B) {
	for _, size := range []int{100, 1000, 10000} {
		size := size
		existing := make([]int64, size, size)
		b.Run(fmt.Sprintf("size=%d", size), func(b *testing.B) {
			b.ResetTimer()
			for i := 0; i < b.N; i++ {
				target := make([]int64, 0, size)
				target = append(target, existing...)
			}
		})
	}
}

func BenchmarkCopyInline(b *testing.B) {
	for _, size := range []int{100, 1000, 10000} {
		size := size
		existing := make([]int64, size, size)
		b.Run(fmt.Sprintf("size=%d", size), func(b *testing.B) {
			b.ResetTimer()
			for i := 0; i < b.N; i++ {
				target := make([]int64, size, size)
				copy(target, existing)
			}
		})
	}
}

Results

$ go version
go version go1.18.3 darwin/amd64

$ go test -bench=. -benchmem
goos: darwin
goarch: amd64
pkg: play/bench
cpu: Intel(R) Core(TM) i7-6567U CPU @ 3.30GHz
BenchmarkAppendAllocInline/size=100-4         	 7442314	       146.4 ns/op	     896 B/op	       1 allocs/op
BenchmarkAppendAllocInline/size=1000-4        	  929802	      1397 ns/op	    8192 B/op	       1 allocs/op
BenchmarkAppendAllocInline/size=10000-4       	  105584	     10860 ns/op	   81920 B/op	       1 allocs/op
BenchmarkCopyInline/size=100-4                	 7351207	       150.4 ns/op	     896 B/op	       1 allocs/op
BenchmarkCopyInline/size=1000-4               	  920683	      1296 ns/op	    8192 B/op	       1 allocs/op
BenchmarkCopyInline/size=10000-4              	  107530	     10376 ns/op	   81920 B/op	       1 allocs/op
PASS
ok  	play/bench	9.644s

For some reason, with my tests using Go 1.23.2, BenchmarkAppendAllocInline appears to consistently win by a very thin margin with the benchmark code as is here. But also for some reason, if the make([]int64, size, size) in BenchmarkCopyInline is changed to make([]int64, size), it starts to consistently win, by a slightly larger margin than it was the other way around.