Add an increment of a global variable after each op to ensure the Go compiler isn't being clever about optimisation.

bm_test.go

package bm

import (
	"testing"
)

var mb = map[string]bool{
	"alpha":   true,
	"beta":    true,
	"gamma":   true,
	"delta":   true,
	"epsilon": true,
	"zeta":    true,
	"eta":     true,
	"theta":   true,
	"iota":    true,
	"kappa":   true,
	"lambda":  true,
	"mu":      true,
	"nu":      true,
	"xi":      true,
	"omicron": true,
	"pi":      true,
	"rho":     true,
	"sigma":   true,
	"tau":     true,
	"upsilon": true,
	"phi":     true,
	"chi":     true,
	"psi":     true,
	"omega":   true,
}
var ms = map[string]struct{}{
	"alpha":   struct{}{},
	"beta":    struct{}{},
	"gamma":   struct{}{},
	"delta":   struct{}{},
	"epsilon": struct{}{},
	"zeta":    struct{}{},
	"eta":     struct{}{},
	"theta":   struct{}{},
	"iota":    struct{}{},
	"kappa":   struct{}{},
	"lambda":  struct{}{},
	"mu":      struct{}{},
	"nu":      struct{}{},
	"xi":      struct{}{},
	"omicron": struct{}{},
	"pi":      struct{}{},
	"rho":     struct{}{},
	"sigma":   struct{}{},
	"tau":     struct{}{},
	"upsilon": struct{}{},
	"phi":     struct{}{},
	"chi":     struct{}{},
	"psi":     struct{}{},
	"omega":   struct{}{},
}

var bb int

func inc(truthy bool) {
	if truthy {
		bb += 1
	}
}

func BenchmarkNoOp(B *testing.B) {
	bb = 0
	for i := 0; i < B.N; i++ {
		inc(true)
		inc(true)
		inc(true)
		inc(true)
		inc(true)
	}
}

func BenchmarkMapBool(B *testing.B) {
	bb = 0
	var b bool
	for i := 0; i < B.N; i++ {
		b = mb["alpha"]
		inc(b)
		b = mb["gamma"]
		inc(b)
		b = mb["mu"]
		inc(b)
		b = mb["pi"]
		inc(b)
		b = mb["omega"]
		inc(b)
	}
}

func BenchmarkMapStruct(B *testing.B) {
	bb = 0
	var b bool
	for i := 0; i < B.N; i++ {
		_, b = ms["alpha"]
		inc(b)
		_, b = ms["gamma"]
		inc(b)
		_, b = ms["mu"]
		inc(b)
		_, b = ms["pi"]
		inc(b)
		_, b = ms["omega"]
		inc(b)
	}
}

$ go test -bench . -count=10 -benchmem
goos: linux
goarch: amd64
pkg: github.com/hnakamur/go-map-benchmark
cpu: AMD Ryzen 7 PRO 5850U with Radeon Graphics
BenchmarkNoOp-16                916623603                1.295 ns/op           0 B/op          0 allocs/op
BenchmarkNoOp-16                947062818                1.296 ns/op           0 B/op          0 allocs/op
BenchmarkNoOp-16                913785105                1.280 ns/op           0 B/op          0 allocs/op
BenchmarkNoOp-16                941507858                1.273 ns/op           0 B/op          0 allocs/op
BenchmarkNoOp-16                911431180                1.276 ns/op           0 B/op          0 allocs/op
BenchmarkNoOp-16                946081177                1.259 ns/op           0 B/op          0 allocs/op
BenchmarkNoOp-16                1000000000               0.9821 ns/op          0 B/op          0 allocs/op
BenchmarkNoOp-16                1000000000               0.9820 ns/op          0 B/op          0 allocs/op
BenchmarkNoOp-16                1000000000               0.9694 ns/op          0 B/op          0 allocs/op
BenchmarkNoOp-16                1000000000               0.9641 ns/op          0 B/op          0 allocs/op
BenchmarkMapBool-16             32737788                36.38 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             32924523                36.64 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             29113693                37.01 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             32196271                36.60 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             33253494                37.05 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             30089841                36.68 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             27620382                37.79 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             27759393                36.82 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             32802170                36.63 ns/op            0 B/op          0 allocs/op
BenchmarkMapBool-16             28773949                37.24 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           27321225                43.19 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           28566975                43.33 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           27927062                43.58 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           28560314                42.62 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           27566887                43.75 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           24911251                42.26 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           26996860                43.26 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           28189378                43.84 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           24785774                43.39 ns/op            0 B/op          0 allocs/op
BenchmarkMapStruct-16           26041501                43.13 ns/op            0 B/op          0 allocs/op
PASS
ok      github.com/hnakamur/go-map-benchmark    36.187s
$ go version
go version go1.17 linux/amd64