| use core::num::NonZeroUsize; |
| |
| /// Returns the amount of padding we must insert after `len` bytes to ensure |
| /// that the following address will satisfy `align` (measured in bytes). |
| /// |
| /// e.g., if `len` is 9, then `padding_needed_for(len, 4)` returns 3, because |
| /// that is the minimum number of bytes of padding required to get a 4-aligned |
| /// address (assuming that the corresponding memory block starts at a 4-aligned |
| /// address). |
| /// |
| /// The return value of this function has no meaning if `align` is not a |
| /// power-of-two. |
| /// |
| /// # Panics |
| /// |
| /// May panic if `align` is not a power of two. |
| // |
| // TODO(#419): Replace `len` with a witness type for region size. |
| #[allow(unused)] |
| #[inline(always)] |
| pub(crate) const fn padding_needed_for(len: usize, align: NonZeroUsize) -> usize { |
| // Rounded up value is: |
| // len_rounded_up = (len + align - 1) & !(align - 1); |
| // and then we return the padding difference: `len_rounded_up - len`. |
| // |
| // We use modular arithmetic throughout: |
| // |
| // 1. align is guaranteed to be > 0, so align - 1 is always |
| // valid. |
| // |
| // 2. `len + align - 1` can overflow by at most `align - 1`, |
| // so the &-mask with `!(align - 1)` will ensure that in the |
| // case of overflow, `len_rounded_up` will itself be 0. |
| // Thus the returned padding, when added to `len`, yields 0, |
| // which trivially satisfies the alignment `align`. |
| // |
| // (Of course, attempts to allocate blocks of memory whose |
| // size and padding overflow in the above manner should cause |
| // the allocator to yield an error anyway.) |
| |
| let align = align.get(); |
| debug_assert!(align.is_power_of_two()); |
| let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1); |
| len_rounded_up.wrapping_sub(len) |
| } |
