stable_heap/
lib.rs

1use std::mem;
2
3/// An arbitrary non-null address to represent zero-size allocations.
4///
5/// This preserves the non-null invariant for types like `Box<T>`. The address
6/// may overlap with non-zero-size memory allocations.
7pub const EMPTY: *mut () = 0x1 as *mut ();
8
9/// Return a pointer to `size` bytes of memory aligned to `align`.
10///
11/// On failure, return a null pointer.
12///
13/// Behavior is undefined if the requested size is 0 or the alignment is not a
14/// power of 2. The alignment must be no larger than the largest supported page
15/// size on the platform.
16#[inline]
17pub unsafe fn allocate(size: usize, align: usize) -> *mut u8 {
18    assert!(size & align == 0, "invalid allocate arguments; size={}; align={}", size, align);
19
20    match align {
21        1 => do_allocate::<u8>(size),
22        2 => do_allocate::<u16>(size >> 1),
23        4 => do_allocate::<u32>(size >> 2),
24        8 => do_allocate::<u64>(size >> 3),
25        _ => panic!("unsupported alignment {}", align),
26    }
27}
28
29unsafe fn do_allocate<T>(capacity: usize) -> *mut u8 {
30    let vec = Vec::<T>::with_capacity(capacity);
31    let ptr = vec.as_ptr();
32
33    mem::forget(vec);
34
35    ptr as *mut u8
36}
37
38/// Deallocates the memory referenced by `ptr`.
39///
40/// The `ptr` parameter must not be null.
41///
42/// The `old_size` and `align` parameters are the parameters that were used to
43/// create the allocation referenced by `ptr`. The `old_size` parameter may be
44/// any value in range_inclusive(requested_size, usable_size).
45#[inline]
46pub unsafe fn deallocate(ptr: *mut u8, old_size: usize, align: usize) {
47    match align {
48        1 => do_deallocate::<u8>(ptr, old_size),
49        2 => do_deallocate::<u16>(ptr, old_size >> 1),
50        4 => do_deallocate::<u32>(ptr, old_size >> 2),
51        8 => do_deallocate::<u64>(ptr, old_size >> 3),
52        _ => panic!("unsupported alignment {}", align),
53    }
54}
55
56unsafe fn do_deallocate<T>(ptr: *mut u8, capacity: usize) {
57    let _ = Vec::from_raw_parts(ptr as *mut T, 0, capacity);
58}
59
60#[cfg(test)]
61mod test {
62    use std::mem;
63
64    #[test]
65    fn test_align() {
66        assert_eq!(1, mem::align_of::<u8>());
67        assert_eq!(2, mem::align_of::<u16>());
68        assert_eq!(4, mem::align_of::<u32>());
69        assert_eq!(8, mem::align_of::<u64>());
70    }
71
72    #[test]
73    fn test_allocate_deallocate() {
74        unsafe {
75            ::deallocate(::allocate(2048, 4), 2048, 4);
76        }
77    }
78
79    #[test]
80    fn test_empty_constant() {
81        let mut v = Vec::<()>::with_capacity(0);
82        assert_eq!(::EMPTY, v.as_mut_ptr());
83    }
84}
stable_heap/lib.rs

stable_heap/
lib.rs
