Methods for fetching structure fields in Go (golang)

gistfile1.go

package main

import "log"
import "time"
import "reflect"
import "unsafe"

// suggested via http://stackoverflow.com/a/8363629/87207
func trace(s string) (string, time.Time) {
  log.Println("START:", s)
    return s, time.Now()
}

// suggested via http://stackoverflow.com/a/8363629/87207
func un(s string, startTime time.Time) {
    endTime := time.Now()
    log.Println("  END:", s, "ElapsedTime in seconds:", endTime.Sub(startTime))
}

type S struct {
  A int
  B int
  C int
  D int
  E int
  F int
  G int
  H int
  X int
  Y int
}

type Getter func(*S, string) int

// get a field by static offset
func getField(s *S, _ string) int {
  return s.X
}

// dynamically lookup a field
// 40x slower than getField()
func reflectField(s *S, field string) int {
  r := reflect.ValueOf(s)
  f := reflect.Indirect(r).FieldByName(field)
  return int(f.Int())
}

// create a function that knows what slot to fetch given a name.
// 10x slower than getField()
func makeFieldGetter(v reflect.Value, field string) Getter {
  t := v.Type()
  for i := 0; i < v.NumField(); i++ {
    if t.Field(i).Name == field {
      return func(s *S, _ string) int {
        av := reflect.ValueOf(s).Elem()
        i, _ := av.Field(i).Interface().(int)
        return i
      }
    }
  }
  return nil
}

func makeFieldGetterUnsafe(t reflect.Type, field string) Getter {
  for i := 0; i < t.NumField(); i++ {
    if t.Field(i).Name == field {
      offset := t.Field(i).Offset
      return func(s *S, _ string) int {
        p := unsafe.Pointer(uintptr(unsafe.Pointer(s)) + offset)
        return *(*int)(p)
      }
    }
  }
  return nil
}

func testMethod(name string, s *S, f Getter) {
  defer un(trace(name))
  var x int
  for i := 0; i < 10 * 1000 * 1000; i++ {
    x = f(s, "X")
  }
  println(x)
}

func main() {
  s := S{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
  testMethod("direct", &s, getField)
  testMethod("getter", &s, makeFieldGetter(reflect.ValueOf(&s).Elem(), "X"))
  testMethod("getter_unsafe", &s, makeFieldGetterUnsafe(reflect.TypeOf(&s).Elem(), "X"))
  testMethod("reflect", &s, reflectField)
}

Added unsafe getter function, it gives same perf as "direct", but it's "unsafe":

2019/05/19 11:09:46 START: direct
2019/05/19 11:09:46   END: direct ElapsedTime in seconds: 40.745804ms
2019/05/19 11:09:46 START: getter
2019/05/19 11:09:46   END: getter ElapsedTime in seconds: 712.222534ms
2019/05/19 11:09:46 START: getter_unsafe
2019/05/19 11:09:46   END: getter_unsafe ElapsedTime in seconds: 38.501966ms
2019/05/19 11:09:46 START: reflect
2019/05/19 11:09:49   END: reflect ElapsedTime in seconds: 2.337854547s

Also let's print the result, so that we know it's correct:

2019/05/19 11:12:45 START: direct
9
2019/05/19 11:12:45   END: direct ElapsedTime in seconds: 38.518838ms
2019/05/19 11:12:45 START: getter
9
2019/05/19 11:12:46   END: getter ElapsedTime in seconds: 698.960467ms
2019/05/19 11:12:46 START: getter_unsafe
9
2019/05/19 11:12:46   END: getter_unsafe ElapsedTime in seconds: 40.986655ms
2019/05/19 11:12:46 START: reflect
9
2019/05/19 11:12:48   END: reflect ElapsedTime in seconds: 2.155271329s