Skip to content

rebase the P3941 wording on P3826r3 #72

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
dietmarkuehl wants to merge 2 commits into
base: main
Choose a base branch
from
Open

rebase the P3941 wording on P3826r3 #72

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
8cd4f2e
rebase the P3941 wording on P3826r3
dietmarkuehl Mar 15, 2026
df3b339
fixed typos noted by the AI (the other changes AI comments on are int…
dietmarkuehl Mar 15, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
142 changes: 64 additions & 78 deletions docs/P3941-affinity.md
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
---
title: Scheduler Affinity
document: P3941R2
date: 2026-02-23
document: D3941R3
date: 2026-03-14
audience:
- Concurrency Working Group (SG1)
- Library Evolution Working Group (LEWG)
Expand Down Expand Up @@ -31,6 +31,10 @@ meet its objective at run-time.

# Change History

## R3

- rebase changes on the customization changes [P3826r3](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3826r3.html)

## R2

- added requirement on `get_scheduler`/`get_start_scheduler`
Expand Down Expand Up @@ -549,6 +553,13 @@ algorithm a better name.

# Wording Changes

::: ednote
This wording is relative to [N5032](https://wg21.link/N5032) with
the changes from
[P3826r3](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3826r3.html)
applied.
:::

::: ednote
If `get_start_scheduler` is introduced add it to the synopsis in
[execution.syn] after `get_scheduler` as follows:
Expand All @@ -565,6 +576,8 @@ namespace std::execution {
struct get_forward_progress_guarantee_t { @_unspecified_@ };
template<class CPO>
struct get_completion_scheduler_t { @_unspecified_@ };
template<class CPO = void>
struct get_completion_domain_t { @_unspecified_@ };
struct get_await_completion_adaptor_t { @_unspecified_@ };

inline constexpr get_domain_t get_domain{};
Expand All @@ -575,6 +588,8 @@ namespace std::execution {
inline constexpr get_forward_progress_guarantee_t get_forward_progress_guarantee{};
template<class CPO>
constexpr get_completion_scheduler_t<CPO> get_completion_scheduler{};
template<class CPO = void>
constexpr get_completion_domain_t<CPO> get_completion_domain{};
inline constexpr get_await_completion_adaptor_t get_await_completion_adaptor{};
...
}
Expand Down Expand Up @@ -609,55 +624,62 @@ expression `get_start_scheduler(get_env(rcvr))` is well-formed, an operation
state that is the result of calling `connect(sndr, rcvr)` shall, if
it is started, be started on an execution agent associated with the
scheduler `get_start_scheduler(get_env(rcvr))`.
:::

::: ednote

If `get_start_scheduler` is introduced change
[[exec.snd.expos](https://wg21.link/exec.snd.expos)] paragraph 8 to have <code><i>SCHED-ENV</i></code>
use `get_start_scheduler` instead of `get_scheduler`:

:::

[8]{.pnum} <code><i>SCHED-ENV</i>(sch)</code> is an expression `o2` whose type
satisfies <code><i>queryable</i></code> such that `o2.query(@[get_scheduler]{.rm}@@[get_start_scheduler]{.add}@)`
is a prvalue with the same type and value as `sch`, and such that
`o2.query(get_domain)` is expression-equivalent to `sch.query(get_domain)`.


::: ednote
If `get_start_scheduler` is introduced change how `on` gets its
scheduler in [exec.on], i.e., change the use from `get_scheduler`
to use `get_start_scheduler`:
The specification of `on` [[exec.on](https://wg21.link/exec.on)] shouldn't use `write_env` as it does,
i.e., this change removes these (not removing them was an oversight
in
[P3826](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3826r3.html)).
In addition, if `get_start_scheduler` is introduced change how `on`
gets its scheduler in [[exec.on](https://wg21.link/exec.on)], i.e., change the use from
`get_scheduler` to use `get_start_scheduler`:
:::

<p>
...
</p>

<p>
The expression `on.transform_sender(out_sndr, env)` has effects equivalent to:
[8]{.pnum} Otherwise, the expression `on.transform_sender(set_value, out_sndr, env)` has effects equivalent to:
</p>

```
auto&& [_, data, child] = out_sndr;
if constexpr (scheduler<decltype(data)>) {
auto orig_sch =
@_query-with-default_@(get_@[start_]{.add}@scheduler, env, @_not-a-scheduler_@());

if constexpr (same_as<decltype(orig_sch), @_not-a-scheduler_@>) {
return @_not-a-sender_@{};
} else {
return continues_on(
starts_on(std::forward_like<OutSndr>(data), std::forward_like<OutSndr>(child)),
std::move(orig_sch));
}
@_call-with-default_@(@[get_scheduler]{.rm}@@[get_start_scheduler]{.add}@, @_not-a-scheduler_@(), env);

return continues_on(
starts_on(std::forward_like<OutSndr>(data), std::forward_like<OutSndr>(child)),
std::move(orig_sch));
} else {
auto& [sch, closure] = data;
auto orig_sch = @_query-with-default_@(
get_completion_scheduler<set_value_t>,
get_env(child),
@_query-with-default_@(get_@[start_]{.add}@scheduler, env, @_not-a-scheduler_@()));

if constexpr (same_as<decltype(orig_sch), @_not-a-scheduler_@>) {
return @_not-a-sender_@{};
} else {
return write_env(
continues_on(
std::forward_like<OutSndr>(closure)(
continues_on(
write_env(std::forward_like<OutSndr>(child), @_SCHED-ENV_@(orig_sch)),
sch)),
orig_sch),
@_SCHED-ENV_@(sch));
}
auto orig_sch = @_call-with-default_@(
get_completion_scheduler<set_value_t>, @_not-a-scheduler_@(), get_env(child), env);

return continues_on(
@[write_env(]{.rm}@
std::forward_like<OutSndr>(closure)(
continues_on(
@[write_env(]{.rm}@std::forward_like<OutSndr>(child)@[,]{.rm}@ @[_SCHED-ENV_(orig_sch))]{.rm}@,
sch)),
@[_SCHED-ENV_(sch)),]{.rm}@
orig_sch);
}
```
<p>
Expand Down Expand Up @@ -770,26 +792,8 @@ satisfy sender, <code>affine_on(sndr[, sch]{.rm})</code> is ill-formed.

[3]{.pnum}
Otherwise, the expression <code>affine_on(sndr[, sch]{.rm})</code>
is expression-equivalent to:
<code>transform_sender(_get-domain-early_(sndr), _make-sender_(affine_on,
[sch]{.rm}[env&lt;&gt;()]{.add}, sndr))</code> except that `sndr`
is evaluated only once.

[4]{.pnum}
The exposition-only class template <code>_impls-for_</code>
([exec.snd.expos]) is specialized for `affine_on_t` as follows:

```c++
namespace std::execution {
template<>
struct impls-for<affine_on_t> : default-impls {
static constexpr auto get-attrs =
[](const auto&@[ data]{.rm}]@, const auto& child) noexcept -> decltype(auto) {
return @[_JOIN-ENV_(_SCHED-ATTRS_(data),_FWD-ENV_(]{.rm}@get_env(child)@[))]{.rm}@;
};
};
}
```
is expression-equivalent to
`@_make-sender_@(affine_on, @[sch]{.rm}@@[env&lt;&gt;()]{.add}@, sndr)`.

:::{.add}
[?]{.pnum}
Expand All @@ -806,13 +810,13 @@ if constexpr (requires(const child_tag_t& t){ t.affine_on(child, ev); })
return t.affine_on(child, ev);
else
return write_env(
schedule_from(get_start_scheduler(get_env(ev)), write_env(std::move(child), ev)),
continues_on(write_env(std::move(child), ev), get_start_scheduler(get_env(ev))),
JOIN-ENV(env{prop{get_stop_token, never_stop_token()}}, ev)
);
```

[Note 1: This causes the `affine_on(sndr)` sender to become
`schedule_from(sch, sndr)` when it is connected with a receiver
`continues_on(sndr, sch)` when it is connected with a receiver
`rcvr` whose execution domain does not customize `affine_on`,
for which `get_start_scheduler(get_env(rcvr))` is `sch`, and `affine_on`
isn't specialized for the child sender.
Expand Down Expand Up @@ -875,26 +879,8 @@ satisfy sender, <code>affine_on(sndr[, sch]{.rm})</code> is ill-formed.

[3]{.pnum}
Otherwise, the expression <code>affine_on(sndr[, sch]{.rm})</code>
is expression-equivalent to:
<code>transform_sender(_get-domain-early_(sndr), _make-sender_(affine_on,
[sch]{.rm}[env&lt;&gt;()]{.add}, sndr))</code> except that `sndr`
is evaluated only once.

[4]{.pnum}
The exposition-only class template <code>_impls-for_</code>
([exec.snd.expos]) is specialized for `affine_on_t` as follows:

```c++
namespace std::execution {
template<>
struct impls-for<affine_on_t> : default-impls {
static constexpr auto get-attrs =
[](const auto&@[ data]{.rm}]@, const auto& child) noexcept -> decltype(auto) {
return @[_JOIN-ENV_(_SCHED-ATTRS_(data),_FWD-ENV_(]{.rm}@get_env(child)@[))]{.rm}@;
};
};
}
```
is expression-equivalent to
`@_make-sender_@(affine_on, @[sch]{.rm}@@[env&lt;&gt;()]{.add}@, sndr)`.

:::{.add}
[?]{.pnum}
Expand All @@ -911,13 +897,13 @@ if constexpr (requires(const child_tag_t& t){ t.affine_on(child, ev); })
return t.affine_on(child, ev);
else
return write_env(
schedule_from(get_scheduler(get_env(ev)), write_env(std::move(child), ev)),
continues_on(write_env(std::move(child), ev), get_scheduler(get_env(ev))),
JOIN-ENV(env{prop{get_stop_token, never_stop_token()}}, ev)
);
```

[Note 1: This causes the `affine_on(sndr)` sender to become
`schedule_from(sch, sndr)` when it is connected with a receiver
`continues_on(sndr, sch)` when it is connected with a receiver
`rcvr` whose execution domain does not customize `affine_on`,
for which `get_scheduler(get_env(rcvr))` is `sch`, and `affine_on`
isn't specialized for the child sender.
Expand Down Expand Up @@ -1128,7 +1114,7 @@ In [exec.run.loop.types] change the paragraph defining the completion signatures
class run-loop-sender;
```

[5]{.pnum}
[6]{.pnum}
<code><i>run-loop-sender</i></code> is an exposition-only type that satisfies `sender`.
[Let `E` be the type of an environment. If `unstoppable_token<decltype(get_stop_token(declval<E>()))>` is `true`,
then ]{.add} <code>completion_signatures_of_t&lt;<i>run-loop-sender</i>[, E]{.add}&gt;</code> is
Expand All @@ -1149,7 +1135,7 @@ Otherwise it is
```
:::

[6]{.pnum} An instance of <code><i>run-loop-sender</i></code> remains
[7]{.pnum} An instance of <code><i>run-loop-sender</i></code> remains
valid until the end of the lifetime of its associated `run_loop`
instance.

Expand Down
Loading