The Promise constructor spends quite a lot of time under JS::dbg::onNewPromise. Fortunately this no longer has to be a public API and we can now simplify/optimize this code. In particular the fast path can just check cx->compartment()->isDebuggee(). This improves the micro-benchmark below from 360-390 ms to 340-350 ms with --no-async-stacks so it's a pretty easy win. function f() { var count = 1000000; var start = Date.now(); for (var i = 0; i < count; ++i) { new Promise(r => void 0); } var stop = Date.now(); print(stop - start); } f();

Comment on attachment 8914760 [details] [diff] [review] Patch Review of attachment 8914760 [details] [diff] [review]: ----------------------------------------------------------------- Nice easy win indeed, thanks!

Pushed by jandemooij@gmail.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/0b08540034c0 Clean up and optimize Promise debugger hooks. r=till

Backed out for failures like https://treeherder.mozilla.org/logviewer.html#?job_id=134744425&repo=mozilla-inbound https://hg.mozilla.org/integration/mozilla-inbound/rev/810131b10010bc4f2be43d63a7dc1abe31d327cb

Pushed by jandemooij@gmail.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/df08c7645b1a Clean up and optimize Promise debugger hooks. r=till