Showcase of Go "1.14beta1" vs "1.13.1" `defer` performance improvement.

defer_benchmark.go

package main

import (
	"testing"
)

var sink1 = 0

func BenchmarkNotDefered(b *testing.B) {
	for n := 0; n < b.N; n++ {
		notDefered()
	}
	b.Log(sink1)
}

func notDefered() {
	func() {
		sink1++
	}()
}

var sink2 = 0

func BenchmarkDefered(b *testing.B) {
	for n := 0; n < b.N; n++ {
		defered()
	}
	b.Log(sink2)
}

func defered() {
	defer func() {
		sink2++
	}()
}

defer_benchmark_results.txt

# Hardware:
#
# Model: Lenovo ThinkPad X1 Extreme 2nd
# CPU: 12x Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz
# RAM memory: 32GB  Intel Corporation Cannon Lake PCH Shared SRAM (rev 10)
# Linux pop-os 5.3.0-20-generic #21+system76~1572304854~19.10~8caa3e6-Ubuntu SMP Tue Oct 29 00:4 x86_64 x86_64 x86_64 GNU/Linux

# 1.13
go test -bench="Benchmark.*" -run=^$ -timeout 10m -benchtime 30s ./_dev/bench_defer/ | tee defer_1.13.out

goos: linux
goarch: amd64
pkg: github.com/thanos-io/thanos/_dev/bench_defer
BenchmarkNotDefered-12    	1000000000	         1.32 ns/op
--- BENCH: BenchmarkNotDefered-12
    main_test.go:60: 1
    main_test.go:60: 101
    main_test.go:60: 10101
    main_test.go:60: 1010101
    main_test.go:60: 101010101
    main_test.go:60: 1101010101
BenchmarkDefered-12       	1000000000	        31.1 ns/op
--- BENCH: BenchmarkDefered-12
    main_test.go:75: 1
    main_test.go:75: 101
    main_test.go:75: 10101
    main_test.go:75: 1010101
    main_test.go:75: 101010101
    main_test.go:75: 1101010101
PASS
ok  	github.com/thanos-io/thanos/_dev/bench_defer	35.679s

## 1.14beta1
go test -bench="Benchmark.*" -run=^$ -timeout 10m -benchtime 30s ./_dev/bench_defer/ | tee defer_1.14.out

goos: linux
goarch: amd64
pkg: github.com/thanos-io/thanos/_dev/bench_defer
BenchmarkNotDefered-12    	1000000000	         1.32 ns/op
--- BENCH: BenchmarkNotDefered-12
    main_test.go:60: 1
    main_test.go:60: 101
    main_test.go:60: 10101
    main_test.go:60: 1010101
    main_test.go:60: 101010101
    main_test.go:60: 1101010101
BenchmarkDefered-12       	1000000000	         2.76 ns/op
--- BENCH: BenchmarkDefered-12
    main_test.go:75: 1
    main_test.go:75: 101
    main_test.go:75: 10101
    main_test.go:75: 1010101
    main_test.go:75: 101010101
    main_test.go:75: 1101010101
PASS
ok  	github.com/thanos-io/thanos/_dev/bench_defer	4.690s

# cmp
benchcmp defer_1.13.out defer_1.14.out 
benchmark                  old ns/op     new ns/op     delta
BenchmarkNotDefered-12     1.32          1.32          +0.00%
BenchmarkDefered-12        31.1          2.76          -91.13%

defer_real_example.go

package main

import "sync"

var mtx sync.Mutex

func performanceCriticalMethod() {
	mtx.Lock()
	defer mtx.Unlock()

	// ...do something FAST!
}

defer_real_example2.go

// Excerpt from: https://github.com/golang/go/blob/go1.11.4/src/crypto/tls/conn.go#L1036
package tls

// ...

import "sync/atomic"

// Write writes data to the connection.
func (c *Conn) Write(b []byte) (int, error) {
	// interlock with Close below
	for {
		x := atomic.LoadInt32(&c.activeCall)
		if x&1 != 0 {
			return 0, errClosed
		}
		if atomic.CompareAndSwapInt32(&c.activeCall, x, x+2) {
			defer atomic.AddInt32(&c.activeCall, -2)
			break
		}
	}
	// Rest...
}

defer_real_example3.go

package main

import (
	"net/http"
	"time"

	"github.com/prometheus/client_golang/prometheus"
)

type handler struct {
	latency prometheus.Histogram
}

// Performance critical HTTP handler.
func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	start := time.Now()
	defer func() {
		// Probably this is not great example, as this `Observe` methods is much slower that defer improvement.
		h.latency.Observe(time.Since(start).Seconds())
	}()

	// Handling...

	w.WriteHeader(http.StatusOK)
}