Golang `copy` time complexity - go

Question

I was wondering about the time complexity of go's copy function?
Intuitively I would assume the worst case of linear time. But I was wondering if there was any magic that was able to bulk allocate, or something, which would allow it to perform better?
https://golang.org/ref/spec#Appending_and_copying_slices
I figured the assembly would explain something but I'm not sure what I"m reading :p
$ GOOS=linux GOARCH=amd64 go tool compile -S main.go
func main() {
src := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
dst := make([]int, len(src))
numCopied := copy(dst, src)
if numCopied != 10 {
panic(fmt.Sprintf("expected 5 copied received: %d", numCopied))
}
}
With the following output from the copy line:
0x007a 00122 (main.go:23) CMPQ AX, $10
0x007e 00126 (main.go:23) JLE 133
0x0080 00128 (main.go:23) MOVL $10, AX
0x0085 00133 (main.go:23) MOVQ AX, "".numCopied+56(SP)
0x008a 00138 (main.go:23) MOVQ CX, (SP)
0x008e 00142 (main.go:23) LEAQ ""..autotmp_8+72(SP), CX
0x0093 00147 (main.go:23) MOVQ CX, 8(SP)
0x0098 00152 (main.go:23) SHLQ $3, AX
0x009c 00156 (main.go:23) MOVQ AX, 16(SP)
0x00a1 00161 (main.go:23) PCDATA $0, $0
0x00a1 00161 (main.go:23) CALL runtime.memmove(SB)
0x00a6 00166 (main.go:23) MOVQ "".numCopied+56(SP), AX
I then tried with 5 elements as well:
func main() {
src := []int{1, 2, 3, 4, 5}
dst := make([]int, len(src))
numCopied := copy(dst, src)
if numCopied != 5 {
panic(fmt.Sprintf("expected 5 copied received: %d", numCopied))
}
}
With the following output from the copy line:
0x0086 00134 (main.go:9) CMPQ AX, $5
0x008a 00138 (main.go:9) JLE 145
0x008c 00140 (main.go:9) MOVL $5, AX
0x0091 00145 (main.go:9) MOVQ AX, "".numCopied+56(SP)
0x0096 00150 (main.go:9) MOVQ CX, (SP)
0x009a 00154 (main.go:9) LEAQ ""..autotmp_8+72(SP), CX
0x009f 00159 (main.go:9) MOVQ CX, 8(SP)
0x00a4 00164 (main.go:9) SHLQ $3, AX
0x00a8 00168 (main.go:9) MOVQ AX, 16(SP)
0x00ad 00173 (main.go:9) PCDATA $0, $0
0x00ad 00173 (main.go:9) CALL runtime.memmove(SB)
0x00b2 00178 (main.go:9) MOVQ "".numCopied+56(SP), AX

Answer 1

I suggest benchmarking the time it takes to copy array/slices of different sizes. Here is something to get the ball rolling:
package main
import (
"fmt"
"math"
"testing"
)
func main() {
for i := 0; i < 16; i++ {
size := powerOfTwo(i)
runBench(size)
}
}
func runBench(size int) {
bench := func(b *testing.B) {
src := make([]int, size, size)
dst := make([]int, size, size)
// we don't want to measure the time
// it takes to make the arrays, so reset timer
b.ResetTimer()
for i := 0; i < b.N; i++ {
copy(dst, src)
}
}
fmt.Printf("size = %d, %s", size, testing.Benchmark(bench))
}
func powerOfTwo(i int) int {
return int(math.Pow(float64(2), float64(i)))
}