Testing interface overhead.

...and results with inlining disabled:

$ go test -gcflags=-l -bench=. 
testing: warning: no tests to run
PASS
BenchmarkDirect-4   	500000000	         3.36 ns/op
BenchmarkInterface-4	1000000000	         3.46 ns/op
ok  	misc/intb	5.830s

intb_test.go

package main

import "testing"

type Int int64

func (i *Int) Sum(i2 Int) { (*i) += i2 }

type Summer interface {
	Sum(i Int)
}

func BenchmarkDirect(b *testing.B) {
	var o Int
	addVal := Int(18)
	for i := 0; i < b.N; i++ {
		o.Sum(addVal)
	}
	if int(o) != b.N*int(addVal) {
		b.Errorf("got %v after %v iterations, wanted %v", o, b.N, b.N*int(addVal))
	}
}

func BenchmarkInterface(b *testing.B) {
	var p Int
	o := Summer(&p)
	addVal := Int(18)
	for i := 0; i < b.N; i++ {
		o.Sum(addVal)
	}
	if int(p) != b.N*int(addVal) {
		b.Errorf("got %v after %v iterations, wanted %v", o, b.N, b.N*int(addVal))
	}
}

Results on my machine

$ go test -bench=. 
testing: warning: no tests to run
PASS
BenchmarkDirect-4   	500000000	         2.34 ns/op
BenchmarkInterface-4	500000000	         3.85 ns/op
ok  	misc/intb	3.809s