Using indirection for faster message passing

Message passing always involves copying the payload from the sender into a static variable and then from the static variable into the receiver. Thus sending a large buffer, like a [u8; 128], as a message involves two expensive memcpys.

Indirection can minimize message passing overhead: instead of sending the buffer by value, one can send an owning pointer into the buffer.

One can use a global memory allocator to achieve indirection (alloc::Box, alloc::Rc, etc.), which requires using the nightly channel as of Rust v1.37.0, or one can use a statically allocated memory pool like heapless::Pool.

As this example of approach goes completely outside of RTIC resource model with shared and local the program would rely on the correctness of the memory allocator, in this case heapless::pool.

Here's an example where heapless::Pool is used to "box" buffers of 128 bytes.

//! examples/pool.rs

#![no_main]
#![no_std]
#![deny(warnings)]
#![feature(type_alias_impl_trait)]

use heapless::{
    pool,
    pool::singleton::{Box, Pool},
};
use panic_semihosting as _;
use rtic::app;

// Declare a pool of 128-byte memory blocks
pool!(P: [u8; 128]);

#[app(device = lm3s6965, dispatchers = [SSI0, QEI0])]
mod app {
    use crate::{Box, Pool};
    use cortex_m_semihosting::debug;
    use lm3s6965::Interrupt;

    // Import the memory pool into scope
    use super::P;

    #[shared]
    struct Shared {}

    #[local]
    struct Local {}

    #[init(local = [memory: [u8; 512] = [0; 512]])]
    fn init(cx: init::Context) -> (Shared, Local) {
        // Increase the capacity of the memory pool by ~4
        P::grow(cx.local.memory);

        rtic::pend(Interrupt::I2C0);

        (Shared {}, Local {})
    }

    #[task(binds = I2C0, priority = 2)]
    fn i2c0(_: i2c0::Context) {
        // claim a memory block, initialize it and ..
        let x = P::alloc().unwrap().init([0u8; 128]);

        // .. send it to the `foo` task
        foo::spawn(x).ok().unwrap();

        // send another block to the task `bar`
        bar::spawn(P::alloc().unwrap().init([0u8; 128]))
            .ok()
            .unwrap();
    }

    #[task]
    async fn foo(_: foo::Context, _x: Box<P>) {
        // explicitly return the block to the pool
        drop(_x);

        debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
    }

    #[task(priority = 2)]
    async fn bar(_: bar::Context, _x: Box<P>) {
        // this is done automatically so we can omit the call to `drop`
        // drop(_x);
    }
}

$ cargo run --target thumbv7m-none-eabi --example pool