Intent to Ship: Promise.{all, race, allSettled} optimization
Sathya Gunasekaran
Contact emails
Explainer
https://github.com/tc39/proposal-promise-allSettled/pull/40
https://github.com/tc39/ecma262/issues/1505
Spec
https://github.com/tc39/ecma262/pull/1506/files
https://github.com/tc39/proposal-promise-allSettled/pull/40/files
Summary
Promise.{all, race, allSettled} lookup the constructor of the receiver on every iteration of the loop. Instead, this change makes the lookup happen only once outside the loop.
Motivation
To optimize away the Get and Call of the resolve method on the constructor, I'd have to check the constructor to see if its resolve method has been modified or not. If it's not been modified, I can directly call (or even inline) the builtin %PromiseResolve%, saving the lookup and call overhead for faster performance.
Without this patch, I would have to check against the constructor for every iteration of the loop before going to the fast path. With this patch, I can do a check against the constructor just once at the beginning.
The change in behavior with this patch is that if you modify the constructor's resolve property in the middle of iterating the iterable argument, then it is not observed.
Risks
Interoperability and Compatibility
There is a very small risk of interop and web compatibility in the case of Promise.all and Promise.race, as we are modifying the behavior of methods that have been shipping for a while. There is no risk of interop or web compat with Promise.allSettled as it is a new proposal.
I expect the risk to be very low considering the surprising side-effecting nature of changing the resolve method *while* iterating over the argument or if there is a user installed ‘resolve’ getter that side effects.
Unfortunately it’s not possible to use counters to determine if this will break or not -- we can’t count the side effects of calling a method. We can only determine if the resolve method has been patched or not, but that’s not fully sufficient to determine the risk.
Given the very low risk of this surprising behavior, I’d like to ship this to determine if there is any breakage.
Ergonomics
N/A
Activation
N/A
Is this feature supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?
Yes.
Debuggability
This doesn’t change any of the debugging functionality of Promises.
Is this feature fully tested by web-platform-tests? Link to test suite results from wpt.fyi.
V8 passes all the test262 tests: https://github.com/tc39/test262/pull/2131
Entry on the feature dashboard
https://www.chromestatus.com/feature/5171235300311040
Requesting approval to ship?
Yes
Sathya Gunasekaran
Adam Klein
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Mathias Bynens
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Alex Russell
Yoav Weiss
LGTM1
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Chris Harrelson
LGTM1
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Chris Harrelson
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Philip Jägenstedt
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Benedikt Meurer
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| Benedikt Meurer | | Software Engineer, V8 | | Google Germany GmbH | | Erika-Mann-Str. 33, 80636 München |