[New-bugs-announce] [issue46965] Enable informing callee it's awaited via vector call flag

[Dino Viehland]

New submission from Dino Viehland <dinoviehland at gmail.com>:

The idea here is to add a new flag to the vectorcall nargs that indicates the call is being awaited: _Py_AWAITED_CALL_MARKER.  This flag will allow the callee to know that it's being eagerly evaluated.  When the call is eagerly evaluated the callee can potentially avoid various amounts of overhead.  For a coroutine the function can avoid creating the coroutine object and instead returns a singleton instance of a wait handle indicating eager execution has occurred:
https://github.com/facebookincubator/cinder/blob/cinder/3.8/Python/ceval.c#L6617

This gives a small win by reducing the overhead of allocating the co-routine object.

For something like gather much more significant wins can be achieved.  If all of the inputs have already been computed the creation of tasks and scheduling of them to the event loop can be elided.  An example implementation of this is available in Cinder: https://github.com/facebookincubator/cinder/blob/cinder/3.8/Modules/_asynciomodule.c#L7103

Again the gather implementation uses the singleton wait handle object to return the value indicating the computation completed synchronously.

We've used this elsewhere in Cinder as well - for example if we have an "AsyncLazyValue" which lazily performs a one-time computation of a value   and caches it.  Therefore the common case becomes that the value is already available, and the await can be performed without allocating any intermediate values.

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assignee: dino.viehland
messages: 414782
nosy: Mark.Shannon, carljm, dino.viehland, gvanrossum, itamaro
priority: normal
severity: normal
stage: needs patch
status: open
title: Enable informing callee it's awaited via vector call flag
type: performance
versions: Python 3.11

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46965>
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