lib: improve debuglog() performance

[mscdex]

Example benchmark results:

                                                        confidence improvement accuracy (*)   (**)  (***)
 streams/pipe-object-mode.js n=5000000                        ***     27.10 %       ±1.46% ±1.95% ±2.55%
 streams/pipe.js n=5000000                                    ***     23.04 %       ±1.54% ±2.08% ±2.76%
 streams/readable-boundaryread.js type='buffer' n=2000         **      3.22 %       ±2.19% ±2.92% ±3.83%
 streams/readable-readall.js n=5000                           ***      9.04 %       ±2.51% ±3.35% ±4.39%
 streams/readable-unevenread.js n=1000                        ***      6.38 %       ±0.63% ±0.84% ±1.10%

Improvements come from optimizing the existing function returned by debuglog() but also by adding an optional callback that can be used to get a reference to the actual underlying debug output function to avoid calling into the unnecessary wrapper function.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/29830/

[himself65]

What about this?

Suggested change

	function debuglog(set, cb) {
	let debug = (...args) => {
	// Only invokes debuglogImpl() when the debug function is
	// called for the first time.
	debug = debuglogImpl(set);
	if (typeof cb === 'function')
	cb(debug);
	debug(...args);
	};
	return (...args) => {
	debug(...args);
	};
	function debuglog(set) {
	let debug = (...args) => {
	// Only invokes debuglogImpl() when the debug function is
	// called for the first time.
	debug = debuglogImpl(set);
	debug(...args);
	};
	const state = { called: false };
	return ((...args) => {
	if(!this.called) {
	debug(...args);
	this.called = true;
	} else {
	debug(...args);
	}
	}).bind(state);

just like

node/lib/events.js

Lines 426 to 432 in ec204d8

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

[himself65]

because I don't think let debug = fn('xxx', v => debug = v) is a good code style

[mscdex]

The suggested change is basically what the code was doing before. Having a callback is the only way to be able to avoid the wrapper function and utilizing it provides a further, noticeable performance improvement.

[himself65]

Thanks, I understand

[jasnell]

Fairly certain this is because of the way V8 optimizes but that's only a guess. It's interesting that it's such a big perf difference.

[mscdex]

Results again with current master:

                                                        confidence improvement accuracy (*)   (**)  (***)
 streams/pipe-object-mode.js n=5000000                        ***     40.88 %       ±2.68% ±3.57% ±4.65%
 streams/pipe.js n=5000000                                    ***     31.51 %       ±1.83% ±2.44% ±3.18%
 streams/readable-boundaryread.js type='buffer' n=2000        ***      4.53 %       ±0.41% ±0.55% ±0.72%
 streams/readable-readall.js n=5000                           ***     10.17 %       ±2.23% ±2.97% ±3.88%
 streams/readable-unevenread.js n=1000                        ***      4.25 %       ±0.82% ±1.09% ±1.43%

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31662/