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address review comments

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This commit is contained in:
Lukas Markeffsky 2024-02-11 17:13:23 +01:00
parent 8f259ade66
commit 5d989770f2
1 changed files with 99 additions and 44 deletions
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143
library/alloc/src/collections/vec_deque/drain.rs
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@ -95,6 +95,22 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
fn drop(&mut self) {
struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
let guard = DropGuard(self);
if mem::needs_drop::<T>() && guard.0.remaining != 0 {
unsafe {
// SAFETY: We just checked that `self.remaining != 0`.
let (front, back) = guard.0.as_slices();
// since idx is a logical index, we don't need to worry about wrapping.
guard.0.idx += front.len();
guard.0.remaining -= front.len();
ptr::drop_in_place(front);
guard.0.remaining = 0;
ptr::drop_in_place(back);
}
}
// Dropping `guard` handles moving the remaining elements into place.
impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
#[inline]
fn drop(&mut self) {
@ -118,63 +134,102 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
return;
}
let head_len = source_deque.len();
let tail_len = new_len - head_len;
let head_len = source_deque.len; // #elements in front of the drain
let tail_len = new_len - head_len; // #elements behind the drain
// Next, we will fill the hole left by the drain with as few writes as possible.
// The code below handles the following control flow and reduces the amount of
// branches under the assumption that `head_len == 0 || tail_len == 0`, i.e.
// draining at the front or at the back of the dequeue is especially common.
//
// H = "head index" = `deque.head`
// h = elements in front of the drain
// d = elements in the drain
// t = elements behind the drain
//
// Note that the buffer may wrap at any point and the wrapping is handled by
// `wrap_copy` and `to_physical_idx`.
//
// Case 1: if `head_len == 0 && tail_len == 0`
// Everything was drained, reset the head index back to 0.
// H
// [ . . . . . d d d d . . . . . ]
// H
// [ . . . . . . . . . . . . . . ]
//
// Case 2: else if `tail_len == 0`
// Don't move data or the head index.
// H
// [ . . . h h h h d d d d . . . ]
// H
// [ . . . h h h h . . . . . . . ]
//
// Case 3: else if `head_len == 0`
// Don't move data, but move the head index.
// H
// [ . . . d d d d t t t t . . . ]
// H
// [ . . . . . . . t t t t . . . ]
//
// Case 4: else if `tail_len <= head_len`
// Move data, but not the head index.
// H
// [ . . h h h h d d d d t t . . ]
// H
// [ . . h h h h t t . . . . . . ]
//
// Case 5: else
// Move data and the head index.
// H
// [ . . h h d d d d t t t t . . ]
// H
// [ . . . . . . h h t t t t . . ]
// When draining at the front (`.drain(..n)`) or at the back (`.drain(n..)`),
// we don't need to copy any data.
// Outlining this function helps LLVM to eliminate the copies in these cases.
// we don't need to copy any data. The number of elements copied would be 0.
if head_len != 0 && tail_len != 0 {
copy_data(source_deque, drain_len, head_len, tail_len);
join_head_and_tail_wrapping(source_deque, drain_len, head_len, tail_len);
// Marking this function as cold helps LLVM to eliminate it entirely if
// this branch is never taken.
// We use `#[cold]` instead of `#[inline(never)]`, because inlining this
// function into the general case (`.drain(n..m)`) is fine.
// See `tests/codegen/vecdeque-drain.rs` for a test.
#[cold]
fn join_head_and_tail_wrapping<T, A: Allocator>(
source_deque: &mut VecDeque<T, A>,
drain_len: usize,
head_len: usize,
tail_len: usize,
) {
// Pick whether to move the head or the tail here.
let (src, dst, len);
if head_len < tail_len {
src = source_deque.head;
dst = source_deque.to_physical_idx(drain_len);
len = head_len;
} else {
src = source_deque.to_physical_idx(head_len + drain_len);
dst = source_deque.to_physical_idx(head_len);
len = tail_len;
};
unsafe {
source_deque.wrap_copy(src, dst, len);
}
}
}
if new_len == 0 {
// Special case: If the entire dequeue was drained, reset the head back to 0,
// like `.clear()` does.
source_deque.head = 0;
} else if head_len < tail_len {
// If we moved the head above, then we need to adjust the head index here.
source_deque.head = source_deque.to_physical_idx(drain_len);
}
source_deque.len = new_len;
#[cold]
fn copy_data<T, A: Allocator>(
source_deque: &mut VecDeque<T, A>,
drain_len: usize,
head_len: usize,
tail_len: usize,
) {
let (src, dst, len);
if head_len < tail_len {
src = source_deque.head;
dst = source_deque.to_physical_idx(drain_len);
len = head_len;
} else {
src = source_deque.to_physical_idx(head_len + drain_len);
dst = source_deque.to_physical_idx(head_len);
len = tail_len;
};
unsafe {
source_deque.wrap_copy(src, dst, len);
}
}
}
}
let guard = DropGuard(self);
if mem::needs_drop::<T>() && guard.0.remaining != 0 {
unsafe {
// SAFETY: We just checked that `self.remaining != 0`.
let (front, back) = guard.0.as_slices();
// since idx is a logical index, we don't need to worry about wrapping.
guard.0.idx += front.len();
guard.0.remaining -= front.len();
ptr::drop_in_place(front);
guard.0.remaining = 0;
ptr::drop_in_place(back);
}
}
// Dropping `guard` handles moving the remaining elements into place.
}
}