Final 200 lines of code

#![feature(llvm_asm, naked_functions)]
​
const DEFAULT_STACK_SIZE: usize = 1024 * 1024 * 2;
const MAX_THREADS: usize = 4;
static mut RUNTIME: usize = 0;
​
pub struct Runtime {
    threads: Vec<Thread>,
    current: usize,
}
​
#[derive(PartialEq, Eq, Debug)]
enum State {
    Available,
    Running,
    Ready,
}
​
struct Thread {
    id: usize,
    stack: Vec<u8>,
    ctx: ThreadContext,
    state: State,
}
​
#[derive(Debug, Default)]
#[repr(C)]
struct ThreadContext {
    rsp: u64,
    r15: u64,
    r14: u64,
    r13: u64,
    r12: u64,
    rbx: u64,
    rbp: u64,
}
​
impl Thread {
    fn new(id: usize) -> Self {
        Thread {
            id,
            stack: vec![0_u8; DEFAULT_STACK_SIZE],
            ctx: ThreadContext::default(),
            state: State::Available,
        }
    }
}
​
impl Runtime {
    pub fn new() -> Self {
        let base_thread = Thread {
            id: 0,
            stack: vec![0_u8; DEFAULT_STACK_SIZE],
            ctx: ThreadContext::default(),
            state: State::Running,
        };
​
        let mut threads = vec![base_thread];
        let mut available_threads: Vec<Thread> = (1..MAX_THREADS).map(|i| Thread::new(i)).collect();
        threads.append(&mut available_threads);
​
        Runtime {
            threads,
            current: 0,
        }
    }
​
    pub fn init(&self) {
        unsafe {
            let r_ptr: *const Runtime = self;
            RUNTIME = r_ptr as usize;
        }
    }
​
    pub fn run(&mut self) -> ! {
        while self.t_yield() {}
        std::process::exit(0);
    }
​
    fn t_return(&mut self) {
        if self.current != 0 {
            self.threads[self.current].state = State::Available;
            self.t_yield();
        }
    }
​
    fn t_yield(&mut self) -> bool {
        let mut pos = self.current;
        while self.threads[pos].state != State::Ready {
            pos += 1;
            if pos == self.threads.len() {
                pos = 0;
            }
            if pos == self.current {
                return false;
            }
        }
​
        if self.threads[self.current].state != State::Available {
            self.threads[self.current].state = State::Ready;
        }
​
        self.threads[pos].state = State::Running;
        let old_pos = self.current;
        self.current = pos;
​
        unsafe {
            let old: *mut ThreadContext = &mut self.threads[old_pos].ctx;
            let new: *const ThreadContext = &self.threads[pos].ctx;
            llvm_asm!(
                "mov $0, %rdi
                 mov $1, %rsi"::"r"(old), "r"(new)
            );
            switch();
        }
        self.threads.len() > 0
    }
​
    pub fn spawn(&mut self, f: fn()) {
        let available = self
            .threads
            .iter_mut()
            .find(|t| t.state == State::Available)
            .expect("no available thread.");
​
        let size = available.stack.len();
        unsafe {
            let s_ptr = available.stack.as_mut_ptr().offset(size as isize);
            let s_ptr = (s_ptr as usize & !15) as *mut u8;
            std::ptr::write(s_ptr.offset(-16) as *mut u64, guard as u64);
            std::ptr::write(s_ptr.offset(-24) as *mut u64, skip as u64);
            std::ptr::write(s_ptr.offset(-32) as *mut u64, f as u64);
            available.ctx.rsp = s_ptr.offset(-32) as u64;
        }
        available.state = State::Ready;
    }
}
​
#[naked]
fn skip() { }
​
fn guard() {
    unsafe {
        let rt_ptr = RUNTIME as *mut Runtime;
        (*rt_ptr).t_return();
    };
}
​
pub fn yield_thread() {
    unsafe {
        let rt_ptr = RUNTIME as *mut Runtime;
        (*rt_ptr).t_yield();
    };
}
​
#[naked]
#[inline(never)]
unsafe fn switch() {
    llvm_asm!("
        mov     %rsp, 0x00(%rdi)
        mov     %r15, 0x08(%rdi)
        mov     %r14, 0x10(%rdi)
        mov     %r13, 0x18(%rdi)
        mov     %r12, 0x20(%rdi)
        mov     %rbx, 0x28(%rdi)
        mov     %rbp, 0x30(%rdi)
​
        mov     0x00(%rsi), %rsp
        mov     0x08(%rsi), %r15
        mov     0x10(%rsi), %r14
        mov     0x18(%rsi), %r13
        mov     0x20(%rsi), %r12
        mov     0x28(%rsi), %rbx
        mov     0x30(%rsi), %rbp
        "
    );
}
fn main() {
    let mut runtime = Runtime::new();
    runtime.init();
    runtime.spawn(|| {
        println!("THREAD 1 STARTING");
        let id = 1;
        for i in 0..10 {
            println!("thread: {} counter: {}", id, i);
            yield_thread();
        }
        println!("THREAD 1 FINISHED");
    });
    runtime.spawn(|| {
        println!("THREAD 2 STARTING");
        let id = 2;
        for i in 0..15 {
            println!("thread: {} counter: {}", id, i);
            yield_thread();
        }
        println!("THREAD 2 FINISHED");
    });
    runtime.run();
}