block scope optimization test

test-block-scope-performance.js

'use strict'

const Benchmark = require('benchmark')


function MASK() {
	return 100
}

function MAX() {
	return 10000
}


function es2015() {
	let sum = 0
	const mask = MASK()
	const n = MAX()
	for (let i = 0; i < n; ++i) {
		let t = i ^ mask
		sum += t
	}
	return sum
}

function es5() {
	var sum = 0
	var mask = MASK()
	var n = MAX()
	for (var i = 0; i < n; ++i) {
		var t = i ^ mask
		sum += t
	}
	return sum
}

function es5const() {
	var sum = 0
	const mask = MASK()
	const n = MAX()
	for (var i = 0; i < n; ++i) {
		var t = i ^ mask
		sum += t
	}
	return sum
}

function simpleLet() {
	var sum = 0
	const mask = MASK()
	const n = MAX()
	for (var i = 0; i < n; ++i) {
		let t = i ^ mask
		sum += t
	}
	return sum
}

function compoundLet1() {
	let sum = 0
	const mask = MASK()
	const n = MAX()
	for (var i = 0; i < n; ++i) {
		var t = i ^ mask
		sum += t
	}
	return sum
}

function compoundLet2() {
	var sum = 0
	const mask = MASK()
	const n = MAX()
	for (let i = 0; i < n; ++i) {
		var t = i ^ mask
		sum += t
	}
	return sum
}


var result = 49996536
console.assert(es2015(), result)
console.assert(es5(), result)
console.assert(es5const(), result)
console.assert(simpleLet(), result)
console.assert(compoundLet1(), result)
console.assert(compoundLet2(), result)


new Benchmark.Suite()
.add('es2015', es2015)
.add('es5', es5)
.add('es5const', es5const)
.add('simpleLet', simpleLet)
.add('compoundLet1', compoundLet1)
.add('compoundLet2', compoundLet2)
// add listeners
.on('cycle', function(event) {
  console.log(String(event.target));
})
.on('complete', function() {
  console.log('Fastest is ' + this.filter('fastest').map('name'));
})
// run async
.run({ 'async': true })

I revisit this performance tests, and find that it's V8 special optimization which will utilize multi-core for loops in some cases (like apply SIMD automatically), so the profiling result differ by hardware (how many core you have). It seems let will forbidden such optimization.

Consider the programmers could use Worker, and SIMD, Atomics to explicitly utilize multi-core, I guess we'd better drop such optimization.