This prevents the std time driver from overflowing when setting the next
wakeup time. If an overflow occurs, default to sleeping up to 1 second.
Fixes#438
428: executor: Use critical sections instead of atomic CAS loops r=lulf a=Dirbaio
Optimize executor wakes.
CAS loops (either `fetch_update`, or manual `load + compare_exchange_weak`) generate surprisingly horrible code: https://godbolt.org/z/zhscnM1cb
This switches to using critical sections, which makes it faster. On thumbv6 (Cortex-M0) it should make it even faster, as it is currently using `atomic-polyfill`, which will make many critical sections for each `compare_exchange_weak` anyway.
```
opt-level=3 opt-level=s
atmics: 105 cycles 101 cycles
CS: 76 cycles 72 cycles
CS+inline: 72 cycles 64 cycles
```
Measured in nrf52 with icache disabled, with this code:
```rust
poll_fn(|cx| {
let task = unsafe { task_from_waker(cx.waker()) };
compiler_fence(Ordering::SeqCst);
let a = cortex_m::peripheral::DWT::get_cycle_count();
compiler_fence(Ordering::SeqCst);
unsafe { wake_task(task) }
compiler_fence(Ordering::SeqCst);
let b = cortex_m::peripheral::DWT::get_cycle_count();
compiler_fence(Ordering::SeqCst);
defmt::info!("cycles: {=u32}", b.wrapping_sub(a));
Poll::Ready(())
})
.await;
````
Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>
That steal method has a TAKEN=true write [here](6b013138b7/src/peripheral/mod.rs (L180)).
This is not zero cost, we don't want it. Transmute instead, which is zero cost.
* Adds an executor for WASM runtimes based on wasm_bindgen.
* Add time driver based on JS time handling.
* Add example that can run in browser locally.
* Update to critical-section version that supports 'std' flag
Portal is very niche, I've only ever used it in `nrf-softdevice` and in a very particular case:
sending event raw-pointers across tasks but "synchronously", because the destination task must process
it now, so it's not deallocated. For most usecases, Signal or channels is enough.
It's unclear to me whether it can be made ub-free. It has problems with reentrancy. It's also not leak-safe, which is quite annoying.
So, remove it for now. We can always add it back later.
