From 80972f1e0e6b6d409cc4d86202608c22e5ee3e5a Mon Sep 17 00:00:00 2001
From: Dario Nieuwenhuis <dirbaio@dirbaio.net>
Date: Thu, 30 Mar 2023 17:55:55 +0200
Subject: executor,sync: add support for turbo-wakers.

This is a `core` patch to make wakers 1 word (the task pointer) instead of 2 (task pointer + vtable). It allows having the "waker optimization" we had a while back on `WakerRegistration/AtomicWaker`, but EVERYWHERE, without patching all crates.

Advantages:
- Less memory usage.
- Faster.
- `AtomicWaker` can actually use atomics to load/store the waker, No critical section needed.
- No `dyn` call, which means `cargo-call-stack` can now see through wakes.

Disadvantages:
- You have to patch `core`...
- Breaks all executors and other things that create wakers, unless they opt in to using the new `from_ptr` API.

How to use:

- Run this shell script to patch `core`. https://gist.github.com/Dirbaio/c67da7cf318515181539122c9d32b395
- Enable `build-std`
- Enable `build-std-features = core/turbowakers`
- Enable feature `turbowakers` in `embassy-executor`, `embassy-sync`.
- Make sure you have no other crate creating wakers other than `embassy-executor`. These will panic at runtime.

Note that the patched `core` is equivalent to the unpached one when the `turbowakers` feature is not enabled, so it should be fine to leave it there.
---
 embassy-sync/src/waitqueue/waker.rs | 92 -------------------------------------
 1 file changed, 92 deletions(-)
 delete mode 100644 embassy-sync/src/waitqueue/waker.rs

(limited to 'embassy-sync/src/waitqueue/waker.rs')

diff --git a/embassy-sync/src/waitqueue/waker.rs b/embassy-sync/src/waitqueue/waker.rs
deleted file mode 100644
index 9ce94a089..000000000
--- a/embassy-sync/src/waitqueue/waker.rs
+++ /dev/null
@@ -1,92 +0,0 @@
-use core::cell::Cell;
-use core::mem;
-use core::task::Waker;
-
-use crate::blocking_mutex::raw::CriticalSectionRawMutex;
-use crate::blocking_mutex::Mutex;
-
-/// Utility struct to register and wake a waker.
-#[derive(Debug, Default)]
-pub struct WakerRegistration {
-    waker: Option<Waker>,
-}
-
-impl WakerRegistration {
-    /// Create a new `WakerRegistration`.
-    pub const fn new() -> Self {
-        Self { waker: None }
-    }
-
-    /// Register a waker. Overwrites the previous waker, if any.
-    pub fn register(&mut self, w: &Waker) {
-        match self.waker {
-            // Optimization: If both the old and new Wakers wake the same task, we can simply
-            // keep the old waker, skipping the clone. (In most executor implementations,
-            // cloning a waker is somewhat expensive, comparable to cloning an Arc).
-            Some(ref w2) if (w2.will_wake(w)) => {}
-            _ => {
-                // clone the new waker and store it
-                if let Some(old_waker) = mem::replace(&mut self.waker, Some(w.clone())) {
-                    // We had a waker registered for another task. Wake it, so the other task can
-                    // reregister itself if it's still interested.
-                    //
-                    // If two tasks are waiting on the same thing concurrently, this will cause them
-                    // to wake each other in a loop fighting over this WakerRegistration. This wastes
-                    // CPU but things will still work.
-                    //
-                    // If the user wants to have two tasks waiting on the same thing they should use
-                    // a more appropriate primitive that can store multiple wakers.
-                    old_waker.wake()
-                }
-            }
-        }
-    }
-
-    /// Wake the registered waker, if any.
-    pub fn wake(&mut self) {
-        if let Some(w) = self.waker.take() {
-            w.wake()
-        }
-    }
-
-    /// Returns true if a waker is currently registered
-    pub fn occupied(&self) -> bool {
-        self.waker.is_some()
-    }
-}
-
-/// Utility struct to register and wake a waker.
-pub struct AtomicWaker {
-    waker: Mutex<CriticalSectionRawMutex, Cell<Option<Waker>>>,
-}
-
-impl AtomicWaker {
-    /// Create a new `AtomicWaker`.
-    pub const fn new() -> Self {
-        Self {
-            waker: Mutex::const_new(CriticalSectionRawMutex::new(), Cell::new(None)),
-        }
-    }
-
-    /// Register a waker. Overwrites the previous waker, if any.
-    pub fn register(&self, w: &Waker) {
-        critical_section::with(|cs| {
-            let cell = self.waker.borrow(cs);
-            cell.set(match cell.replace(None) {
-                Some(w2) if (w2.will_wake(w)) => Some(w2),
-                _ => Some(w.clone()),
-            })
-        })
-    }
-
-    /// Wake the registered waker, if any.
-    pub fn wake(&self) {
-        critical_section::with(|cs| {
-            let cell = self.waker.borrow(cs);
-            if let Some(w) = cell.replace(None) {
-                w.wake_by_ref();
-                cell.set(Some(w));
-            }
-        })
-    }
-}
-- 
cgit