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Improve BinaryHeap performance #78857

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Merged
merged 4 commits into from
Nov 13, 2020
+19 −13
Merged

Improve BinaryHeap performance #78857

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6dfcf9a
Remove branches from sift_down_to_bottom loop
SkiFire13 Nov 7, 2020
25b3f61
Remove useless branches from sift_down_range loop
SkiFire13 Nov 7, 2020
8d15753
Remove useless bound checks from into_sorted_vec
SkiFire13 Nov 7, 2020
387568c
Added SAFETY comment as request
SkiFire13 Nov 9, 2020
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32 changes: 19 additions & 13 deletions library/alloc/src/collections/binary_heap.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -495,7 +495,14 @@ impl<T: Ord> BinaryHeap<T> {
let mut end = self.len();
while end > 1 {
end -= 1;
self.data.swap(0, end);
// SAFETY: `end` goes from `self.len() - 1` to 1 (both included),
// so it's always a valid index to access.
// It is safe to access index 0 (i.e. `ptr`), because
// 1 <= end < self.len(), which means self.len() >= 2.
unsafe {
let ptr = self.data.as_mut_ptr();
ptr::swap(ptr, ptr.add(end));
}
self.sift_down_range(0, end);
}
self.into_vec()
Expand Down Expand Up @@ -531,19 +538,19 @@ impl<T: Ord> BinaryHeap<T> {
unsafe {
let mut hole = Hole::new(&mut self.data, pos);
let mut child = 2 * pos + 1;
while child < end {
let right = child + 1;
while child < end - 1 {
// compare with the greater of the two children
if right < end && hole.get(child) <= hole.get(right) {
child = right;
}
child += (hole.get(child) <= hole.get(child + 1)) as usize;
// if we are already in order, stop.
if hole.element() >= hole.get(child) {
break;
return;
}
hole.move_to(child);
child = 2 * hole.pos() + 1;
}
if child == end - 1 && hole.element() < hole.get(child) {
hole.move_to(child);
}
}
}

Expand All @@ -563,15 +570,14 @@ impl<T: Ord> BinaryHeap<T> {
unsafe {
let mut hole = Hole::new(&mut self.data, pos);
let mut child = 2 * pos + 1;
while child < end {
let right = child + 1;
// compare with the greater of the two children
if right < end && hole.get(child) <= hole.get(right) {
child = right;
}
while child < end - 1 {
child += (hole.get(child) <= hole.get(child + 1)) as usize;
hole.move_to(child);
child = 2 * hole.pos() + 1;
}
if child == end - 1 {
hole.move_to(child);
}
pos = hole.pos;
}
self.sift_up(start, pos);
Expand Down