Golang foreach difference - go

Question

func Benchmark_foreach1(b *testing.B) {
var test map[int]int
test = make(map[int]int)
for i := 0; i < 100000; i++ {
test[i] = 1
}
for i := 0; i < b.N; i++ {
for i, _ := range test {
if test[i] != 1 {
panic("ds")
}
}
}
}
func Benchmark_foreach2(b *testing.B) {
var test map[int]int
test = make(map[int]int)
for i := 0; i < 100000; i++ {
test[i] = 1
}
for i := 0; i < b.N; i++ {
for _, v := range test {
if v != 1 {
panic("heh")
}
}
}
}
run with result as below
goos: linux
goarch: amd64
Benchmark_foreach1-2 500 3172323 ns/op
Benchmark_foreach2-2 1000 1707214 ns/op
why is foreach-2 slow?

Answer 1

I think Benchmark_foreach2-2 is about 2 times faster - it requires 1707214 nanoseconds per operation, and first one takes 3172323. So second one is 3172323 / 1707214 = 1.85 times faster.
Reason: second doesn't need to take value from a memory again, it already used value in v variable.

Answer 2

The test[k] statement in BenchmarkForeachK takes time to randomly read the value, so BenchmarkForeachK takes more time than BenchmarkForeachV, 9362945 ns/op versus 4213940 ns/op .
For example,
package main
import "testing"
func testMap() map[int]int {
test := make(map[int]int)
for i := 0; i < 100000; i++ {
test[i] = 1
}
return test
}
func BenchmarkForeachK(b *testing.B) {
test := testMap()
b.ReportAllocs()
b.ResetTimer()
for i := 0; i < b.N; i++ {
for k := range test {
if test[k] != 1 {
panic("eh")
}
}
}
}
func BenchmarkForeachV(b *testing.B) {
test := testMap()
b.ReportAllocs()
b.ResetTimer()
for i := 0; i < b.N; i++ {
for _, v := range test {
if v != 1 {
panic("heh")
}
}
}
}
Output:
$ go test foreach_test.go -bench=.
BenchmarkForeachK-4 200 9362945 ns/op 0 B/op 0 allocs/op
BenchmarkForeachV-4 300 4213940 ns/op 0 B/op 0 allocs/op