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Add safety comments in private core::slice::rotate::ptr_rotate function

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This commit is contained in:
Alexis Bourget 2021-07-14 15:31:12 +02:00
parent a08f25a7ef
commit 4541aa971f
1 changed files with 54 additions and 2 deletions
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56
library/core/src/slice/rotate.rs
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@ -1,5 +1,3 @@
// ignore-tidy-undocumented-unsafe
use crate::cmp;
use crate::mem::{self, MaybeUninit};
use crate::ptr;
@ -79,8 +77,10 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
// 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)`,
@ -92,6 +92,21 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
// 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
@ -100,6 +115,8 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
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;
}
@ -113,13 +130,24 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
}
// 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;
}
@ -135,14 +163,30 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
// 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);
@ -156,6 +200,10 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
// 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);
@ -168,6 +216,10 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
} 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);