I've created a separate discussion around a regression caused by a v8 fast api path on node.js - https://groups.google.com/g/v8-dev/c/x2Fs6do0jDA.
On Thursday, May 1, 2025 at 2:00:17 PM UTC-4 ah...@google.com wrote: > There is no roadmap at the moment for the fast API to expand the return > type values. > > When I think about it, there is a fundamental problem with returns of type > `Local<Value>`. A `Local<Value>` is allocated in a `HandleScope`. The > `HandleScope` would be opened at the beginning of the C++ function, and > closed in its epilogue. After the `HandleScope` is closed, all > `Local<Value>`s are deallocated again and can therefore not be returned > anymore. Within V8 the function would just return the `Value` instead, but > for the embedder it is not possible to extract the `Value` from a > `Local<Value>`. > > There exist workarounds for this problem, but these add overhead, and with > that overhead, fast API calls may already not be faster anymore than fast > API calls. > > Cheers, Andreas > > On Tue, Apr 29, 2025 at 3:32 PM 'James Snell' via v8-dev < > v8-...@googlegroups.com> wrote: > >> In workers we don't have a lot of use for Externals but we do extensively >> use c++ API objects that are wrapped by JS objects (defined by Function >> templates). Is there a roadmap for fast API to expand the return value >> types? Could their potentially be a path for returning pointers to API >> objects that are wrapped by their associated function templates rather than >> External? >> >> - James M Snell >> he/him >> https://bsky.app/profile/jasnell.me >> >> On Tue, Apr 29, 2025, 06:21 Aapo Alasuutari <aapo.al...@gmail.com> wrote: >> >>> It becomes a v8::External which is an entirely immutable object with no >>> properties and no prototype; from JS point of view it's a object that >>> cannot be interacted with aside from doing equality comparisons. >>> >>> The engine side of course can use that object to track down the actual >>> pointer value (which I believe is stored either in the object directly or >>> in the External Pointer Table, depending on build flags). >>> >>> -Aapo >>> >>> On Tuesday, 29 April 2025 at 15:40:11 UTC+3 erik...@chromium.org wrote: >>> >>>> Since JS doesn't have 64 ints, what does the pointer turn into on the >>>> JS side? >>>> >>>> On Tue, Apr 29, 2025 at 1:37 PM Aapo Alasuutari <aapo.al...@gmail.com> >>>> wrote: >>>> >>>>> It's a highly useful feature for doing FFI in JavaScript. Deno's FFI >>>>> API <https://docs.deno.com/runtime/fundamentals/ffi/> relies on it >>>>> heavily, as it enables passing opaque pointers through JavaScript without >>>>> resorting to weird zero-sized ArrayBuffer workarounds, which I believe is >>>>> the go-to method for doing this in the Node Addon / N-API world. >>>>> >>>>> TypedArrays used to be fully supported in Fast API, at which time >>>>> zero-sized ArrayBuffers could've been used at mostly equal performance as >>>>> the External pointer objects, but there were some unfortunate bugs around >>>>> that (like zero-sized AB/TAs always giving a null pointer in Fast API, or >>>>> inline-created zero-size TAs giving a bogus pointer in Fast API; the >>>>> first >>>>> has been fixed, the second hasn't IIRC) and nowadays the TA support has >>>>> been deprecated entirely. As a result, the only way to pass opaque >>>>> pointers >>>>> through Fast API without needing dedicated wrap and unwrap code on entry >>>>> and exit from V8 is using the External pointers. >>>>> >>>>> Hope this answers your question. >>>>> -Aapo >>>>> On Monday, 28 April 2025 at 20:41:03 UTC+3 ya...@cloudflare.com wrote: >>>>> >>>>>> What's the use case of returning a pointer from a v8 fast api? >>>>>> >>>>>> On Monday, April 28, 2025 at 4:50:32 AM UTC-4 aapo.al...@gmail.com >>>>>> wrote: >>>>>> >>>>>>> Don't forget about pointers! Those can be returned as well :) >>>>>>> >>>>>>> On Monday, 28 April 2025 at 09:48:28 UTC+3 ah...@google.com wrote: >>>>>>> >>>>>>>> Hi, >>>>>>>> >>>>>>>> I would say the V8 Fast API is still considered unstable. Since >>>>>>>> many limitations were removed, not only the documentation but also the >>>>>>>> APIs >>>>>>>> are out-dated and would benefit from cleanup. However, since old APIs >>>>>>>> have >>>>>>>> to be deprecated before they can be changed, this takes some time. >>>>>>>> >>>>>>>> When it comes to arguments, there are some subtle limitations left, >>>>>>>> see [1], but those are platform-specific. Mostly all limitations >>>>>>>> should be >>>>>>>> gone. I will go over the documentation and update it. When it comes to >>>>>>>> return values, then indeed, as you noted, objects and strings cannot >>>>>>>> be >>>>>>>> returned, only integers, floats, and void. >>>>>>>> >>>>>>>> The heuristics of CFunction overloads depends now only on the >>>>>>>> number of arguments. If an API function is called in JavaScript with n >>>>>>>> parameters, then it will call the C++ function which expects n >>>>>>>> parameters. >>>>>>>> If no such C++ exists, then no fast call is happening. If the API has >>>>>>>> overloads or optional parameters, then this has to be resolved by the >>>>>>>> C++ >>>>>>>> functions. There should not be two CFunctions with the same number of >>>>>>>> arguments, although I'm not sure if we are checking for that. >>>>>>>> >>>>>>>> Unfortunately, `FastOneByteString` is faster than what you can >>>>>>>> achieve with a `v8::Local<v8::Value>` parameter, when you use >>>>>>>> `FastOneByteString`, you should not trigger a GC. >>>>>>>> >>>>>>>> Cheers, Andreas >>>>>>>> >>>>>>>> [1] >>>>>>>> https://source.chromium.org/chromium/chromium/src/+/main:v8/src/compiler/fast-api-calls.cc;l=65;drc=2bf826180ca85d007fb39024ff18322fa4635efb >>>>>>>> >>>>>>>> On Thu, Apr 24, 2025 at 10:17 PM 'Yagiz Nizipli' via v8-dev < >>>>>>>> v8-...@googlegroups.com> wrote: >>>>>>>> >>>>>>>>> We are actively investigating/working on adding V8 Fast API >>>>>>>>> support for Cloudflare Workers. While looking at the >>>>>>>>> v8-fast-api-calls.h >>>>>>>>> file (ref: >>>>>>>>> https://github.com/v8/v8/blob/main/include/v8-fast-api-calls.h) , >>>>>>>>> I realized that some of the documentation referencing the limitations >>>>>>>>> are >>>>>>>>> not correct and up to date. >>>>>>>>> >>>>>>>>> I'm more than happy to update the header file to reflect the >>>>>>>>> current state of the implementation, but first, I'd like to ask >>>>>>>>> couple of >>>>>>>>> questions to understand the implementation better. >>>>>>>>> >>>>>>>>> - Is V8 Fast API considered unstable? >>>>>>>>> - Since V8 fast api can now allocate, trigger JS and GC, which >>>>>>>>> limitations still apply? Looking at line 24, I believe these comments >>>>>>>>> are >>>>>>>>> not valid. >>>>>>>>> - What is the heuristics of CFunction overloads? In an example >>>>>>>>> where a function has a required and an optional parameter, is it >>>>>>>>> sufficient >>>>>>>>> to add 2 functions, method(a) and method(a, b)? What is the behavior >>>>>>>>> if >>>>>>>>> the user calls this method with 3 arguments? Would it trigger >>>>>>>>> method(a,b)? >>>>>>>>> Is the order MemorySpan CFunctions make a difference? Since >>>>>>>>> v8::Local<Value> is supported, it seems we can get away with a >>>>>>>>> CFunction >>>>>>>>> that has a second parameter of v8::Local<Value> for the optional >>>>>>>>> parameter. >>>>>>>>> - Since v8 fast api supports v8::Local<v8::Value> now, is there >>>>>>>>> any particular reason for using FastOneByteString over v8 local value >>>>>>>>> for >>>>>>>>> one byte strings? >>>>>>>>> - By looking at the documentation, the only limitations I could >>>>>>>>> think of are the ones related to the return type of CFunction, which >>>>>>>>> doesn't allow returning objects/values/strings. What are the actual >>>>>>>>> limitations of V8 fast api? >>>>>>>>> >>>>>>>>> Thank you for your help. >>>>>>>>> >>>>>>>>> -- >>>>>>>>> -- >>>>>>>>> v8-dev mailing list >>>>>>>>> v8-...@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+un...@googlegroups.com. >>>>>>>>> To view this discussion visit >>>>>>>>> https://groups.google.com/d/msgid/v8-dev/08d9e03f-9db2-47c9-98ce-72c464c72475n%40googlegroups.com >>>>>>>>> >>>>>>>>> <https://groups.google.com/d/msgid/v8-dev/08d9e03f-9db2-47c9-98ce-72c464c72475n%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>>>>>> . >>>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> >>>>>>>> Andreas Haas >>>>>>>> >>>>>>>> Software Engineer >>>>>>>> >>>>>>>> ah...@google.com >>>>>>>> >>>>>>>> >>>>>>>> Google Germany GmbH >>>>>>>> >>>>>>>> Erika-Mann-Straße 33 >>>>>>>> <https://www.google.com/maps/search/Erika-Mann-Stra%C3%9Fe+33+80636+M%C3%BCnchen?entry=gmail&source=g> >>>>>>>> >>>>>>>> 80636 München >>>>>>>> <https://www.google.com/maps/search/Erika-Mann-Stra%C3%9Fe+33+80636+M%C3%BCnchen?entry=gmail&source=g> >>>>>>>> >>>>>>>> >>>>>>>> Geschäftsführer: Paul Manicle, Liana Sebastian >>>>>>>> >>>>>>>> Registergericht und -nummer: Hamburg, HRB 86891 >>>>>>>> >>>>>>>> Sitz der Gesellschaft: Hamburg >>>>>>>> >>>>>>>> >>>>>>>> Diese E-Mail ist vertraulich. Falls Sie diese fälschlicherweise >>>>>>>> erhalten haben sollten, leiten Sie diese bitte nicht an jemand anderes >>>>>>>> weiter, löschen Sie alle Kopien und Anhänge davon und lassen Sie mich >>>>>>>> bitte >>>>>>>> wissen, dass die E-Mail an die falsche Person gesendet wurde. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> This e-mail is confidential. If you received this communication by >>>>>>>> mistake, please don't forward it to anyone else, please erase all >>>>>>>> copies >>>>>>>> and attachments, and please let me know that it has gone to the wrong >>>>>>>> person. >>>>>>>> >>>>>>>> -- >>>>> -- >>>>> v8-dev mailing list >>>>> v8-...@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+un...@googlegroups.com. >>>>> >>>> To view this discussion visit >>>>> https://groups.google.com/d/msgid/v8-dev/b3efbcb3-8e74-4c39-b9cc-0f14f5ef667an%40googlegroups.com >>>>> >>>>> <https://groups.google.com/d/msgid/v8-dev/b3efbcb3-8e74-4c39-b9cc-0f14f5ef667an%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>> . >>>>> >>>> -- >>> -- >>> v8-dev mailing list >>> v8-...@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+un...@googlegroups.com. >>> To view this discussion visit >>> https://groups.google.com/d/msgid/v8-dev/ef86a176-b450-422b-9ecf-85daf1e635a1n%40googlegroups.com >>> >>> <https://groups.google.com/d/msgid/v8-dev/ef86a176-b450-422b-9ecf-85daf1e635a1n%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> >> -- >> -- >> v8-dev mailing list >> v8-...@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+un...@googlegroups.com. >> > To view this discussion visit >> https://groups.google.com/d/msgid/v8-dev/CACFvHWmbaOi_MRbhsQKzKvc6nqO-Z0%3Daf49Jw02ov_W2A4ocvA%40mail.gmail.com >> >> <https://groups.google.com/d/msgid/v8-dev/CACFvHWmbaOi_MRbhsQKzKvc6nqO-Z0%3Daf49Jw02ov_W2A4ocvA%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> > > > -- > > Andreas Haas > > Software Engineer > > ah...@google.com > > > Google Germany GmbH > > Erika-Mann-Straße 33 > > 80636 München > > > Geschäftsführer: Paul Manicle, Liana Sebastian > > Registergericht und -nummer: Hamburg, HRB 86891 > > Sitz der Gesellschaft: Hamburg > > > Diese E-Mail ist vertraulich. Falls Sie diese fälschlicherweise erhalten > haben sollten, leiten Sie diese bitte nicht an jemand anderes weiter, > löschen Sie alle Kopien und Anhänge davon und lassen Sie mich bitte wissen, > dass die E-Mail an die falsche Person gesendet wurde. > > > > This e-mail is confidential. If you received this communication by > mistake, please don't forward it to anyone else, please erase all copies > and attachments, and please let me know that it has gone to the wrong > person. > > -- -- 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/85e86769-0d99-4ca6-b759-8ae40f853907n%40googlegroups.com.
