falloc/
lib.rs

1#![no_std]
2
3extern crate alloc;
4
5use core::alloc::{GlobalAlloc, Layout};
6
7use core::cell::UnsafeCell;
8use core::ptr::null_mut;
9use core::sync::atomic::{AtomicUsize, Ordering::SeqCst};
10const ARENA_SIZE: usize = 8192 * 8192;
11const MAX_SUPPORTED_ALIGN: usize = 4096;
12#[repr(C, align(4096))] // 4096 == MAX_SUPPORTED_ALIGN
13struct FAlloc {
14	arena: UnsafeCell<[u8; ARENA_SIZE]>,
15	remaining: AtomicUsize, // we allocate from the top, counting down
16}
17
18#[global_allocator]
19static ALLOCATOR: FAlloc = FAlloc {
20	arena: UnsafeCell::new([0x55; ARENA_SIZE]),
21	remaining: AtomicUsize::new(ARENA_SIZE),
22};
23unsafe impl Sync for FAlloc {}
24unsafe impl GlobalAlloc for FAlloc {
25	unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
26		let size = layout.size();
27		let align = layout.align();
28		// `Layout` contract forbids making a `Layout` with align=0, or align not power of 2.
29		// So we can safely use a mask to ensure alignment without worrying about UB.
30		let align_mask_to_round_down = !(align - 1);
31		if align > MAX_SUPPORTED_ALIGN {
32			return null_mut();
33		}
34		let mut allocated = 0;
35		if self
36			.remaining
37			.fetch_update(SeqCst, SeqCst, |mut remaining| {
38				if size > remaining {
39					return None;
40				}
41				remaining -= size;
42				remaining &= align_mask_to_round_down;
43				allocated = remaining;
44				Some(remaining)
45			})
46			.is_err()
47		{
48			return null_mut();
49		};
50		self.arena.get().cast::<u8>().add(allocated)
51	}
52	unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
53}
54#[cfg(test)]
55mod tests {
56	use super::*;
57	use alloc::vec;
58
59	#[test]
60	fn it_works() {
61		let _ = unsafe { ALLOCATOR.alloc(Layout::from_size_align(1024, 4).unwrap()) };
62		assert!(true);
63	}
64
65	#[test]
66	fn vec_test() {
67		let _ = vec![1, 2, 3];
68		assert!(true);
69	}
70	#[test]
71	fn vec_test_2() {
72		let mut v = vec![1, 2, 3];
73		for _ in 0..10000 {
74			v.push(1)
75		}
76	}
77}
falloc/lib.rs

falloc/
lib.rs
