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events: improve once() performance

This commit takes advantage of the performance improvements V8 has
made to function.bind() in V8 5.4 and uses it to avoid constant
recompilation/reoptimization of the wrapper closure used in once().
This change results in ~27% performance increase for once().

PR-URL: #10445
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
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mscdex committed Dec 25, 2016
1 parent 627fa93 commit 4c9dd6822eb9520588e4d06d251ce8e32469d4bc
Showing with 33 additions and 10 deletions.
  1. +20 −0 benchmark/events/ee-once.js
  2. +13 −10 lib/events.js
@@ -0,0 +1,20 @@
'use strict';
var common = require('../common.js');
var EventEmitter = require('events').EventEmitter;

var bench = common.createBenchmark(main, {n: [2e7]});

function main(conf) {
var n = conf.n | 0;

var ee = new EventEmitter();

function listener() {}

bench.start();
for (var i = 0; i < n; i += 1) {
ee.once('dummy', listener);
ee.emit('dummy');
}
bench.end(n);
}
@@ -283,17 +283,20 @@ EventEmitter.prototype.prependListener =
return _addListener(this, type, listener, true);
};

function _onceWrap(target, type, listener) {
var fired = false;
function g() {
target.removeListener(type, g);
if (!fired) {
fired = true;
listener.apply(target, arguments);
}
function onceWrapper() {
this.target.removeListener(this.type, this.wrapFn);
if (!this.fired) {
this.fired = true;
this.listener.apply(this.target, arguments);
}
g.listener = listener;
return g;
}

function _onceWrap(target, type, listener) {
var state = { fired: false, wrapFn: undefined, target, type, listener };
var wrapped = onceWrapper.bind(state);
wrapped.listener = listener;
state.wrapFn = wrapped;
return wrapped;
}

EventEmitter.prototype.once = function once(type, listener) {

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