rustos 0.4.3

//! [Linked List] Allocator
//!
//! [linked list]: https://os.phil-opp.com/allocator-designs/#linked-list-allocator
use super::alloc::alloc::{GlobalAlloc, Layout};
use core::{mem, ptr};

type LockedAllocator = super::Locked<Allocator>;

unsafe impl GlobalAlloc for LockedAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        let (size, align) = Allocator::size_align(layout);
        let mut inner = self.lock();
        if let Some((region, alloc_start)) = inner.find_region(size, align) {
            let alloc_end = alloc_start.checked_add(size).expect("overflow");
            let excess_size = region.end_addr() - alloc_end;
            if excess_size > 0 {
                inner.add_free_region(alloc_end, excess_size);
            }
            alloc_start as *mut u8
        } else {
            ptr::null_mut()
        }
    }
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        let (size, _) = Allocator::size_align(layout);
        self.lock().add_free_region(ptr as usize, size)
    }
}

pub(super) struct Allocator {
    head: Node,
}

impl Allocator {
    #[allow(dead_code)]
    pub(super) const fn new() -> Self {
        Self { head: Node::new(0) }
    }
    #[allow(dead_code)]
    pub(super) unsafe fn init(&mut self, heap_start: usize, heap_size: usize) {
        self.add_free_region(heap_start, heap_size);
    }
    unsafe fn add_free_region(&mut self, addr: usize, size: usize) {
        assert!(super::align_up(addr, mem::align_of::<Node>()) == addr);
        assert!(size >= mem::size_of::<Node>());
        let mut node = Node::new(size);
        node.next = self.head.next.take();
        let node_ptr = addr as *mut Node;
        node_ptr.write(node);
        self.head.next = Some(&mut *node_ptr)
    }
    fn find_region(&mut self, size: usize, align: usize) -> Option<(&'static mut Node, usize)> {
        let mut current = &mut self.head;
        while let Some(ref mut region) = current.next {
            if let Ok(alloc_start) = Self::alloc_from_region(&region, size, align) {
                let next = region.next.take();
                let ret = Some((current.next.take().unwrap(), alloc_start));
                current.next = next;
                return ret;
            } else {
                // region not suitable -> continue with the next region
                current = current.next.as_mut().unwrap();
            }
        }
        None
    }
    fn alloc_from_region(region: &Node, size: usize, align: usize) -> Result<usize, ()> {
        let alloc_start = super::align_up(region.start_addr(), align);
        let alloc_end = alloc_start.checked_add(size).ok_or(())?;
        if alloc_end > region.end_addr() {
            return Err(());
        }
        let excess_size = region.end_addr() - alloc_end;
        if excess_size > 0 && excess_size < mem::size_of::<Node>() {
            // rest of region too small to hold a Node (required because the
            // allocation splits the region in a used and a free part)
            return Err(());
        }
        Ok(alloc_start)
    }
    fn size_align(layout: Layout) -> (usize, usize) {
        let layout = layout
            .align_to(mem::align_of::<Node>())
            .expect("adjusting alignment failed")
            .pad_to_align();
        let size = layout.size().max(mem::size_of::<Node>());
        (size, layout.align())
    }
}

struct Node {
    size: usize,
    next: Option<&'static mut Node>,
}

impl Node {
    const fn new(size: usize) -> Self {
        Self { size, next: None }
    }
    fn start_addr(&self) -> usize {
        self as *const Self as usize
    }
    fn end_addr(&self) -> usize {
        self.start_addr() + self.size
    }
}