我从来不知道use strict
可以加快运行时间,然而一个简单的use strict
让我的基准测试快了很多,而慢的则慢得多(慢了两倍多)。到底怎么回事?
//
// RUN WITH AND WITHOUT THIS
//
"use strict";
var assert = require('assert');
var slice = [].slice;
function thunkify_fast(fn){
assert('function' == typeof fn, 'function required');
return function(){
var args = new Array(arguments.length);
for(var i = 0; i < args.length; ++i) {
args[i] = arguments[i];
}
var ctx = this;
return function(done){
var called;
args.push(function(){
if (called) return;
called = true;
done.apply(null, arguments);
});
try {
fn.apply(ctx, args);
} catch (err) {
done(err);
}
}
}
};
function thunkify_slow(fn){
assert('function' == typeof fn, 'function required');
return function(){
var args = slice.call(arguments);
var ctx = this;
return function(done){
var called;
args.push(function(){
if (called) return;
called = true;
done.apply(null, arguments);
});
try {
fn.apply(ctx, args);
} catch (err) {
done(err);
}
}
}
};
var fn = function () { };
var Benchmark = require('benchmark');
var suite = new Benchmark.Suite;
//
// Only one wrapper can be sent through the optimized compiler
//
suite.add( 'thunkify#fast', function () { thunkify_fast(fn)(function(){}) } )
.add( 'thunkify#slow', function () { thunkify_slow(fn)(function(){}) } )
.on('cycle', function(event) { console.log(String(event.target)); })
.on('complete', function() {
console.log('Fastest is ' + this.filter('fastest').pluck('name'));
})
.run();
没有顶层的"use strict"
,结果就是这样的,
$ node --allow-natives-syntax test.js
thunkify#fast x 8,511,605 ops/sec ±1.22% (95 runs sampled)
thunkify#slow x 4,579,633 ops/sec ±0.68% (96 runs sampled)
Fastest is thunkify#fast
然而,有了那个"use strict;"
,我得到了这个,
$ node --allow-natives-syntax test.js
thunkify#fast x 9,372,375 ops/sec ±0.45% (100 runs sampled)
thunkify#slow x 1,483,664 ops/sec ±0.93% (96 runs sampled)
Fastest is thunkify#fast
我正在运行nodejs v0.11.13。这些都是我使用this guide对speed up node-thunkify所做的工作的一部分。有趣的是,bluebird优化指南并没有提到use strict;
的有益性能。
如果我把测试用例改成,
var f_fast = thunkify_fast(fn);
var f_slow = thunkify_slow(fn);
suite.add( 'thunkify#fast', function () { f_fast(function(){}) } )
.add( 'thunkify#slow', function () { f_slow(function(){}) } )
.on('cycle', function(event) { console.log(String(event.target)); })
.on('complete', function() {
console.log('Fastest is ' + this.filter('fastest').pluck('name'));
})
.run();
因此,删除调用thunkify
后,我仍然看到相同的情况。use strict的情况在未优化的代码上较慢,在优化的代码上较快,
不严格
thunkify#fast x 18,910,556 ops/sec ±0.61% (100 runs sampled)
thunkify#slow x 5,148,036 ops/sec ±0.40% (100 runs sampled)
“使用严格的;”
thunkify#fast x 19,485,652 ops/sec ±1.27% (99 runs sampled)
thunkify#slow x 1,608,235 ops/sec ±3.37% (93 runs sampled)
发布于 2014-06-05 22:57:37
这种缓慢的原因是在ArraySlice内置的this check中。它测试我们是否正在尝试对参数对象进行切片,如果是,则使用快速代码来实现。然而,它只检查松散的模式参数对象。当您在strict函数中分配arguments对象时,您将获得由native_context()->strict_arguments_boilerplate()
生成的严格模式arguments对象,这意味着上面的检查无法识别它,并落入通用的JavaScript代码,它比专门的手工编码的C++快速路径要慢,因为它是一个草率的参数对象。
https://stackoverflow.com/questions/23997267
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