FWIW the warning is going away. https://bugzilla.mozilla.org/show_bug.cgi?id=1049041
On Sat, 14 May 2016, 20:25 Boris Zbarsky, <[email protected]> wrote: > On 5/14/16 3:11 AM, Andrea Giammarchi wrote: > > 1. why is that? > > Why does mutating the proto after an object has been exposed to script > end up deoptimizing things? Because it invalidates assumptions JITs > otherwise make. So the options are to make the > proto-hasn't-been-mutated case slower by not making those assumptions or > to make the proto-has-been-mutated case slower. Guess which one is a > better choice? > > > There is a > > spec'd method that is not even on Annex B and Firefox deliberately > > discourage its usage. > > Sure. Just because something specced doesn't mean it's a good idea to > actually do it. > > This is why in the HTML spec there's all sorts of stuff that's marked as > "not valid HTML" for authoring purposes even though the spec then goes > ahead and defines what a browser should do with that stuff if authors do > it anyway. > > > Why I don't see warnings every time I > > `[].slice.call(arguments)`? > > Because that's not as big a performance hit? > > > I understand it might de-optimize but I > > wonder if that's really always necessary (maybe it doesn't have to > > deopt if it's a well known operation with a predictable result). > > I'm not an expert on the type inference setup (which is what ends up > deoptimizing on proto mutation, iirc), so I can't usefully answer this. > > > On the other side, I also wish Firefox woudn't show warnings about > > modern and recent specifications. Deprecated stuff is OK, > > There can totally be things that are both recently added to the spec > (for UA implementation purposes, because everyone has to do it for web > compat) and deprecated for authoring purposes (because they're a bad > idea). Dynamic proto mutation is one of those. ;) > > > 2. where were you when the `__proto__` landed on specs? :P > > You mean when every browser on the market implemented it, which was the > relevant bit? The addition to the spec was just acknowledging ugly > reality. > > Where was I when browsers implemented __proto__? We're talking 20ish > years ago, so probably high school or a few years into college. > > -Boris > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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