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 >>>>> >>>>> 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-...@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-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. 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