zerocopy/third_party/rust/
layout.rs

1use core::num::NonZeroUsize;
2
3/// Returns the amount of padding we must insert after `len` bytes to ensure
4/// that the following address will satisfy `align` (measured in bytes).
5///
6/// e.g., if `len` is 9, then `padding_needed_for(len, 4)` returns 3, because
7/// that is the minimum number of bytes of padding required to get a 4-aligned
8/// address (assuming that the corresponding memory block starts at a 4-aligned
9/// address).
10///
11/// The return value of this function has no meaning if `align` is not a
12/// power-of-two.
13///
14/// # Panics
15///
16/// May panic if `align` is not a power of two.
17//
18// TODO(#419): Replace `len` with a witness type for region size.
19#[allow(unused)]
20#[inline(always)]
21pub(crate) const fn padding_needed_for(len: usize, align: NonZeroUsize) -> usize {
22    // Rounded up value is:
23    //   len_rounded_up = (len + align - 1) & !(align - 1);
24    // and then we return the padding difference: `len_rounded_up - len`.
25    //
26    // We use modular arithmetic throughout:
27    //
28    // 1. align is guaranteed to be > 0, so align - 1 is always
29    //    valid.
30    //
31    // 2. `len + align - 1` can overflow by at most `align - 1`,
32    //    so the &-mask with `!(align - 1)` will ensure that in the
33    //    case of overflow, `len_rounded_up` will itself be 0.
34    //    Thus the returned padding, when added to `len`, yields 0,
35    //    which trivially satisfies the alignment `align`.
36    //
37    // (Of course, attempts to allocate blocks of memory whose
38    // size and padding overflow in the above manner should cause
39    // the allocator to yield an error anyway.)
40
41    let align = align.get();
42    debug_assert!(align.is_power_of_two());
43    let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
44    len_rounded_up.wrapping_sub(len)
45}