解释预分配切片的基准

I've been trying to understand slice preallocation with make and why it's a good idea. I noticed a large performance difference between preallocating a slice and appending to it vs just initializing it with 0 length/capacity and then appending to it. I wrote a set of very simple benchmarks:

import "testing"

func BenchmarkNoPreallocate(b *testing.B) {
    for i := 0; i < b.N; i++ {
        // Don't preallocate our initial slice
        init := []int64{}
        init = append(init, 5)
    }
}

func BenchmarkPreallocate(b *testing.B) {
    for i := 0; i < b.N; i++ {
        // Preallocate our initial slice
        init := make([]int64, 0, 1)
        init = append(init, 5)
    }
}

and was a little puzzled with the results:

$ go test -bench=. -benchmem
goos: linux
goarch: amd64
BenchmarkNoPreallocate-4    30000000            41.8 ns/op         8 B/op          1 allocs/op
BenchmarkPreallocate-4      2000000000           0.29 ns/op        0 B/op          0 allocs/op

I have a couple of questions:

• Why are there no allocations (it shows 0 allocs/op) in the preallocation benchmark case? Certainly we're preallocating, but the allocation had to have happened at some point.
• I imagine this may become clearer after the first question is answered, but how is the preallocation case so much quicker? Am I misinterpetting this benchmark?

Please let me know if anything is unclear. Thank you!

Go has an optimizing compiler. Constants are evaluated at compile time. Variables are evaluated at runtime. Constant values can be used to optimize compiler generated code. For example,

package main

import "testing"

func BenchmarkNoPreallocate(b *testing.B) {
    for i := 0; i < b.N; i++ {
        // Don't preallocate our initial slice
        init := []int64{}
        init = append(init, 5)
    }
}

func BenchmarkPreallocateConst(b *testing.B) {
    const (
        l = 0
        c = 1
    )
    for i := 0; i < b.N; i++ {
        // Preallocate our initial slice
        init := make([]int64, l, c)
        init = append(init, 5)
    }
}

func BenchmarkPreallocateVar(b *testing.B) {
    var (
        l = 0
        c = 1
    )
    for i := 0; i < b.N; i++ {
        // Preallocate our initial slice
        init := make([]int64, l, c)
        init = append(init, 5)
    }
}

Output:

$ go test alloc_test.go -bench=. -benchmem
BenchmarkNoPreallocate-4         50000000    39.3 ns/op     8 B/op    1 allocs/op
BenchmarkPreallocateConst-4    2000000000     0.36 ns/op    0 B/op    0 allocs/op
BenchmarkPreallocateVar-4        50000000    28.2 ns/op     8 B/op    1 allocs/op

Another interesting set of benchmarks:

package main

import "testing"

func BenchmarkNoPreallocate(b *testing.B) {
    const (
        l = 0
        c = 8 * 1024
    )
    for i := 0; i < b.N; i++ {
        // Don't preallocate our initial slice
        init := []int64{}
        for j := 0; j < c; j++ {
            init = append(init, 42)
        }
    }
}

func BenchmarkPreallocateConst(b *testing.B) {
    const (
        l = 0
        c = 8 * 1024
    )
    for i := 0; i < b.N; i++ {
        // Preallocate our initial slice
        init := make([]int64, l, c)
        for j := 0; j < cap(init); j++ {
            init = append(init, 42)
        }
    }
}

func BenchmarkPreallocateVar(b *testing.B) {
    var (
        l = 0
        c = 8 * 1024
    )
    for i := 0; i < b.N; i++ {
        // Preallocate our initial slice
        init := make([]int64, l, c)
        for j := 0; j < cap(init); j++ {
            init = append(init, 42)
        }
    }
}

Output:

$ go test peter_test.go -bench=. -benchmem
BenchmarkNoPreallocate-4       20000   75656 ns/op   287992 B/op   19 allocs/op
BenchmarkPreallocateConst-4   100000   22386 ns/op    65536 B/op    1 allocs/op
BenchmarkPreallocateVar-4     100000   22112 ns/op    65536 B/op    1 allocs/op