bench_unroll.txt

$ go version
go version go1.12.5 darwin/amd64
$ go test -bench .
goos: darwin
goarch: amd64
BenchmarkContainsUint64-4          	  500000	      2286 ns/op
BenchmarkContainsUint64Unroll2-4   	 1000000	      1988 ns/op
BenchmarkContainsUint64Unroll4-4   	 1000000	      1589 ns/op
BenchmarkContainsUint64Unroll8-4   	 1000000	      1795 ns/op
PASS

$ go version
go version go1.13rc1 linux/amd64
$ go test -bench .
goos: linux
goarch: amd64
BenchmarkContainsUint64-2          	  596392	      1937 ns/op
BenchmarkContainsUint64Unroll2-2   	  579490	      1895 ns/op
BenchmarkContainsUint64Unroll4-2   	  700369	      1567 ns/op
BenchmarkContainsUint64Unroll8-2   	  667084	      2268 ns/op
PASS

unroll.go

package unroll

func ContainsUint64(ids []uint64, id uint64) bool {
	for i := 0; i < len(ids); i++ {
		if ids[i] == id {
			return true
		}
	}
	return false
}

func ContainsUint64Unroll2(ids []uint64, id uint64) bool {
	var i int
	for i = len(ids) - 1; i >= 1; i -= 2 {
		if ids[i] == id || ids[i-1] == id {
			return true
		}
	}
	for ; i >= 0; i-- {
		if ids[i] == id {
			return true
		}
	}
	return false
}

func ContainsUint64Unroll4(ids []uint64, id uint64) bool {
	var i int
	for i = len(ids) - 1; i >= 3; i -= 4 {
		if ids[i] == id ||
			ids[i-1] == id ||
			ids[i-2] == id ||
			ids[i-3] == id {
			return true
		}
	}
	for ; i >= 0; i-- {
		if ids[i] == id {
			return true
		}
	}
	return false
}

func ContainsUint64Unroll8(ids []uint64, id uint64) bool {
	var i int
	for i = len(ids) - 1; i >= 7; i -= 8 {
		if ids[i] == id ||
			ids[i-1] == id ||
			ids[i-2] == id ||
			ids[i-3] == id ||
			ids[i-4] == id ||
			ids[i-5] == id ||
			ids[i-6] == id ||
			ids[i-7] == id {
			return true
		}
	}
	for ; i >= 0; i-- {
		if ids[i] == id {
			return true
		}
	}
	return false
}

unroll_test.go

package unroll

import (
	"testing"
)

func makeBenchData(n int) []uint64 {
	ids := make([]uint64, n)
	for i := 0; i < n; i++ {
		ids[i] = uint64(i)
	}
	return ids
}

func BenchmarkContainsUint64(b *testing.B) {
	ids := makeBenchData(10000)
	for i := 0; i < b.N; i++ {
		ContainsUint64(ids, 5000)
	}
}

func BenchmarkContainsUint64Unroll2(b *testing.B) {
	ids := makeBenchData(10000)
	for i := 0; i < b.N; i++ {
		ContainsUint64Unroll2(ids, 5000)
	}
}

func BenchmarkContainsUint64Unroll4(b *testing.B) {
	ids := makeBenchData(10000)
	for i := 0; i < b.N; i++ {
		ContainsUint64Unroll4(ids, 5000)
	}
}

func BenchmarkContainsUint64Unroll8(b *testing.B) {
	ids := makeBenchData(10000)
	for i := 0; i < b.N; i++ {
		ContainsUint64Unroll8(ids, 5000)
	}
}