Golang`copy`时间复杂度

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

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)))
}