-// ignore-tidy-undocumented-unsafe
-
use crate::cmp;
use crate::mem::{self, MaybeUninit};
use crate::ptr;
// the way until about `left + right == 32`, but the worst case performance breaks even
// around 16. 24 was chosen as middle ground. If the size of `T` is larger than 4
// `usize`s, this algorithm also outperforms other algorithms.
+ // SAFETY: callers must ensure `mid - left` is valid for reading and writing.
let x = unsafe { mid.sub(left) };
// beginning of first round
+ // SAFETY: see previous comment.
let mut tmp: T = unsafe { x.read() };
let mut i = right;
// `gcd` can be found before hand by calculating `gcd(left + right, right)`,
// the very end. This is possibly due to the fact that swapping or replacing temporaries
// uses only one memory address in the loop instead of needing to manage two.
loop {
+ // [long-safety-expl]
+ // SAFETY: callers must ensure `[left, left+mid+right)` are all valid for reading and
+ // writing.
+ //
+ // - `i` start with `right` so `mid-left <= x+i = x+right = mid-left+right < mid+right`
+ // - `i <= left+right-1` is always true
+ // - if `i < left`, `right` is added so `i < left+right` and on the next
+ // iteration `left` is removed from `i` so it doesn't go further
+ // - if `i >= left`, `left` is removed immediately and so it doesn't go further.
+ // - overflows cannot happen for `i` since the function's safety contract ask for
+ // `mid+right-1 = x+left+right` to be valid for writing
+ // - underflows cannot happen because `i` must be bigger or equal to `left` for
+ // a substraction of `left` to happen.
+ //
+ // So `x+i` is valid for reading and writing if the caller respected the contract
tmp = unsafe { x.add(i).replace(tmp) };
// instead of incrementing `i` and then checking if it is outside the bounds, we
// check if `i` will go outside the bounds on the next increment. This prevents
i -= left;
if i == 0 {
// end of first round
+ // SAFETY: tmp has been read from a valid source and x is valid for writing
+ // according to the caller.
unsafe { x.write(tmp) };
break;
}
}
// finish the chunk with more rounds
for start in 1..gcd {
+ // SAFETY: `gcd` is at most equal to `right` so all values in `1..gcd` are valid for
+ // reading and writing as per the function's safety contract, see [long-safety-expl]
+ // above
tmp = unsafe { x.add(start).read() };
+ // [safety-expl-addition]
+ //
+ // Here `start < gcd` so `start < right` so `i < right+right`: `right` being the
+ // greatest common divisor of `(left+right, right)` means that `left = right` so
+ // `i < left+right` so `x+i = mid-left+i` is always valid for reading and writing
+ // according to the function's safety contract.
i = start + right;
loop {
+ // SAFETY: see [long-safety-expl] and [safety-expl-addition]
tmp = unsafe { x.add(i).replace(tmp) };
if i >= left {
i -= left;
if i == start {
+ // SAFETY: see [long-safety-expl] and [safety-expl-addition]
unsafe { x.add(start).write(tmp) };
break;
}
// The `[T; 0]` here is to ensure this is appropriately aligned for T
let mut rawarray = MaybeUninit::<(BufType, [T; 0])>::uninit();
let buf = rawarray.as_mut_ptr() as *mut T;
+ // SAFETY: `mid-left <= mid-left+right < mid+right`
let dim = unsafe { mid.sub(left).add(right) };
if left <= right {
+ // SAFETY:
+ //
+ // 1) The `else if` condition about the sizes ensures `[mid-left; left]` will fit in
+ // `buf` without overflow and `buf` was created just above and so cannot be
+ // overlapped with any value of `[mid-left; left]`
+ // 2) [mid-left, mid+right) are all valid for reading and writing and we don't care
+ // about overlaps here.
+ // 3) The `if` condition about `left <= right` ensures writing `left` elements to
+ // `dim = mid-left+right` is valid because:
+ // - `buf` is valid and `left` elements were written in it in 1)
+ // - `dim+left = mid-left+right+left = mid+right` and we write `[dim, dim+left)`
unsafe {
+ // 1)
ptr::copy_nonoverlapping(mid.sub(left), buf, left);
+ // 2)
ptr::copy(mid, mid.sub(left), right);
+ // 3)
ptr::copy_nonoverlapping(buf, dim, left);
}
} else {
+ // SAFETY: same reasoning as above but with `left` and `right` reversed
unsafe {
ptr::copy_nonoverlapping(mid, buf, right);
ptr::copy(mid.sub(left), dim, left);
// of this algorithm would be, and swapping using that last chunk instead of swapping
// adjacent chunks like this algorithm is doing, but this way is still faster.
loop {
+ // SAFETY:
+ // `left >= right` so `[mid-right, mid+right)` is valid for reading and writing
+ // Substracting `right` from `mid` each turn is counterbalanced by the addition and
+ // check after it.
unsafe {
ptr::swap_nonoverlapping(mid.sub(right), mid, right);
mid = mid.sub(right);
} else {
// Algorithm 3, `left < right`
loop {
+ // SAFETY: `[mid-left, mid+left)` is valid for reading and writing because
+ // `left < right` so `mid+left < mid+right`.
+ // Adding `left` to `mid` each turn is counterbalanced by the substraction and check
+ // after it.
unsafe {
ptr::swap_nonoverlapping(mid.sub(left), mid, left);
mid = mid.add(left);
projects / rust.git / commitdiff