doc: add v8 fast api contribution guidelines #46199

anonrig · 2023-01-13T14:07:12Z

I'm introducing a guideline to define and share the limitations/caveats and experiences I've gained from researching on this topic. This would help new contributors to add fast APIs to existing functions.

Referencing: nodejs/performance#23

nodejs-github-bot · 2023-01-13T14:07:16Z

Review requested:

@nodejs/tsc

doc/contributing/adding-v8-fast-api.md

ronag

Instead of calculateX could you maybe have div(a, b) as an example and have divide by 0 as example for when to switch to slow path?

tniessen

cc @devsnek who has done most of the fast API work in node IIRC

tniessen · 2023-01-13T14:47:47Z

doc/contributing/adding-v8-fast-api.md

+Node.js uses [V8](https://github.com/v8/v8) as the JavaScript engine.
+In order to provide fast paths for functions that are called quite often,
+V8 proposes the usage of `fast api calls` that does not use any of the V8
+internals, but uses internal C functions.


internal C functions is rather ambiguous. Also, V8 internals usually refers to the non-public V8 API, which we try not to use in any case. Lastly, if I remember correctly, V8 API calls are allowed to some degree. Not performing any allocations on the V8 heap is what's critical. (Unless this changed since I last worked on fast API calls.)

I see. Thanks! Can you give an example of V8 API call, without any allocation on the V8 heap?

I might be wrong, but I think @MayaLekova implemented a fast API that relies on GetAlignedPointerFromInternalField in 6cfba9f, for example. @devsnek might know more about the exact conditions for which APIs are allowed.

You are right. You can access pre-existing/pre-allocated data from the fast apis. I thought it was inferred from the documentation (at least I assumed). Is there any specific wording you'd like me to change?

I've updated the documentation.

[...] Not performing any allocations on the V8 heap is what's critical.

Indeed that's still the case, and until we have the current GC architecture, it will stay like this. Possible future relaxation of this rule might come with conservative stack scanning. Please note that this also leads to the limitation for not calling back into JS code (I think this was discussed in this thread too).

Some brief background on the support for various types in V8 could be found in this external doc.

anonrig · 2023-01-13T15:16:09Z

Instead of calculateX could you maybe have div(a, b) as an example and have divide by 0 as example for when to switch to slow path?

I updated the example. Thanks for the hint! 🥇 💯

doc/contributing/adding-v8-fast-api.md

RafaelGSS

I think you should mention this file in the src/README.md too.

tony-go · 2023-01-13T21:28:25Z

Awesome @anonrig 🙌🏼

Learned a lot just by reading the PR ^^

doc/contributing/adding-v8-fast-api.md

nodejs-github-bot · 2023-01-20T03:25:41Z

CI: https://ci.nodejs.org/job/node-test-pull-request/49099/

doc/contributing/adding-v8-fast-api.md

tniessen · 2023-01-20T09:59:27Z

doc/contributing/adding-v8-fast-api.md

+* Throwing errors is not available on fast API, but can be done
+  through the fallback to slow API.


Suggested change

* Throwing errors is not available on fast API, but can be done

through the fallback to slow API.

* Throwing errors is not available from within a fast API call, but can be done

through the fallback to the slow API.

Perhaps it's also better to suggest that since in principal, the fast API and the slow API should behave the same to avoid bugs, it's better not to throw errors in the slow API either and just update some flags and return to the JS land to throw?

I agree in principal with @joyeecheung, but that is not a limitation of a V8 Fast API declaration, but an implementation recommendation. I think it should not be included in this documentation, but in a more style specific documentation like README.md?

doc/contributing/adding-v8-fast-api.md

tniessen · 2023-01-20T10:08:02Z

doc/contributing/adding-v8-fast-api.md

+In V8, the options fallback is defined as `FastApiCallbackOptions` inside
+[`v8-fast-api-calls.h`](../../deps/v8/include/v8-fast-api-calls.h).
+
+* C++ land


Why is this a bullet point?

It aligns with other sections in the same file for mentioning the context of the example it's referring to. Do you have a recommendation for an alternative?

src/README.md

mcollina

lgtm

nodejs-github-bot · 2023-01-24T02:26:19Z

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----------------------------------- PR info ------------------------------------
Title      doc: add v8 fast api contribution guidelines (#46199)
Author     Yagiz Nizipli  (@anonrig)
Branch     anonrig:docs-fast-api -> nodejs:main
Labels     doc, author ready, commit-queue-squash
Commits    7
 - doc: add v8 fast api contribution guidelines
 - fixup! doc: add v8 fast api contribution guidelines
 - fixup! doc: add v8 fast api contribution guidelines
 - fixup! doc: add v8 fast api contribution guidelines
 - fixup! doc: add v8 fast api contribution guidelines
 - fixup! doc: add v8 fast api contribution guidelines
 - fixup! doc: add v8 fast api contribution guidelines
Committers 1
 - Yagiz Nizipli 
PR-URL: https://github.com/nodejs/node/pull/46199
Reviewed-By: Matteo Collina 
Reviewed-By: Robert Nagy 
Reviewed-By: Darshan Sen 
Reviewed-By: Rafael Gonzaga 
Reviewed-By: James M Snell 
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/46199
Reviewed-By: Matteo Collina 
Reviewed-By: Robert Nagy 
Reviewed-By: Darshan Sen 
Reviewed-By: Rafael Gonzaga 
Reviewed-By: James M Snell 
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   ℹ  This PR was created on Fri, 13 Jan 2023 14:07:12 GMT
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   ✔  - Darshan Sen (@RaisinTen) (TSC): https://github.com/nodejs/node/pull/46199#pullrequestreview-1249572133
   ✔  - Rafael Gonzaga (@RafaelGSS) (TSC): https://github.com/nodejs/node/pull/46199#pullrequestreview-1253500941
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CI: https://ci.nodejs.org/job/node-test-pull-request/49145/

nodejs-github-bot · 2023-01-24T03:46:04Z

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doc: add v8 fast api contribution guidelines #46199

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		* Throwing errors is not available on fast API, but can be done
		through the fallback to slow API.

doc: add v8 fast api contribution guidelines #46199

doc: add v8 fast api contribution guidelines #46199

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