I was thinking of making the change to Array.prototype.fill, and that if correctness issues arose I could try introducing a new array state just for empty arrays to recover element kind monotonicity, something like:
%DebugPrint(Array(100)) DebugPrint: 0000015DF1C8CF29: [JSArray] - map: 0x00280f5cf721 <Map[32](EMPTY)> [FastProperties] However I get the impression this isn't #good-first-issue material On Wed, Feb 12, 2025 at 7:08 AM Leszek Swirski <lesz...@chromium.org> wrote: > If it's only polymorphism we're worried about, there's scenarios we could > come up with where this actually reduces polymorphism (e.g. if there is an > array access site which gets data sometimes from Array.fill, sometimes from > array literals, right now it would be polymorphic and if we made this > change it would stay monomorphic). I'd be more worried about correctness > issues stemming from us assuming that elements kinds manifold are > monotonic, e.g. due to HOLEY_ELEMENTS being a stable map. > > On Wednesday, February 12, 2025 at 12:52:07 PM UTC+1 Jakob Kummerow wrote: > >> The latter increases polymorphism. As a general principle, V8 tries to >> avoid flip-flopping of types/maps/etc; instead it prefers to settle into a >> stable state as quickly as possible. That why so far, the rule has been >> "once an array is holey, it'll stay holey forever". >> >> The former avoids the flip-flopping but is more difficult to accomplish. >> If done partially (e.g. only in optimizing compilers with code dependencies >> on pristine Array.prototype), it also increases polymorphism, as well as >> risk of deopts. >> >> So the cost of increased polymorphism (and perhaps deopts) has to be >> weighed against the (fairly minor) benefit of a few avoided hole checks. >> I'd say some very careful benchmarking is in order to demonstrate that the >> wins justify the additional complexity. >> >> >> On Wed, Feb 12, 2025 at 11:22 AM Marja Hölttä <ma...@chromium.org> wrote: >> >>> Hi, >>> >>> I was wondering whether breaking the invariant "elements transitions >>> always go from specific to generic" will break something. I don't know. But >>> given that Vyacheslav proposed doing this, and nobody came up with >>> objections detailing what will break, maybe it won't break anything? >>> >>> Is the goal to just optimize "Array(n).fill(something)" (that exact >>> syntactic pattern) or more generally, make Array.prototype.fill transition >>> to a more specific ElementsKind if certain conditions are met? >>> >>> >>> On Mon, Feb 10, 2025 at 6:53 PM Benjamin Kietzman <beng...@gmail.com> >>> wrote: >>> >>>> Currently, calling Array.fill will produce an array with HOLEY elements. >>>> >>>> > %DebugPrint(Array(1).fill(0)) >>>> DebugPrint: 0000015DF1C8CF29: [JSArray] >>>> - map: 0x00280f5cf721 <Map[32](HOLEY_SMI_ELEMENTS)> [FastProperties] >>>> - prototype: 0x02f93035a851 <JSArray[0]> >>>> - elements: 0x015df1c8cf49 <FixedArray[1]> [HOLEY_SMI_ELEMENTS] >>>> - length: 1 >>>> >>>> This seems like a missed opportunity for optimization since the new >>>> array could be upgraded to PACKED_SMI_ELEMENTS. There's a very old issue >>>> about this https://issues.chromium.org/issues/42210138 which hasn't >>>> collected any objections. >>>> >>>> I'd be a first time contributor to V8 but I'd be happy to try patching >>>> this. All advice would be welcome! >>>> >>>> Ben Kietzman >>>> >>>> -- >>>> >>>> -- > -- > v8-dev mailing list > v8-dev@googlegroups.com > http://groups.google.com/group/v8-dev > --- > You received this message because you are subscribed to a topic in the > Google Groups "v8-dev" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/v8-dev/AmK_zmyHvFU/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > v8-dev+unsubscr...@googlegroups.com. > To view this discussion visit > https://groups.google.com/d/msgid/v8-dev/4d2d29ac-e241-46eb-9eb0-f60ab164b7a6n%40googlegroups.com > <https://groups.google.com/d/msgid/v8-dev/4d2d29ac-e241-46eb-9eb0-f60ab164b7a6n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- -- v8-dev mailing list v8-dev@googlegroups.com http://groups.google.com/group/v8-dev --- You received this message because you are subscribed to the Google Groups "v8-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to v8-dev+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/v8-dev/CAHLCUTAFXMinqPi%3DGVQpA0gdU7eY3UAnECqzRLnfUioROvha2w%40mail.gmail.com.
