Auto merge of #75974 - SkiFire13:peekmut-opt-sift, r=LukasKalbertodt
Avoid useless sift_down when std::collections::binary_heap::PeekMut is never mutably dereferenced If `deref_mut` is never called then it's not possible for the element to be mutated without internal mutability, meaning there's no need to call `sift_down`. This could be a little improvement in cases where you want to mutate the biggest element of the heap only if it satisfies a certain predicate that needs only read access to the element.
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use std::collections::BinaryHeap;
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use rand::{seq::SliceRandom, thread_rng};
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use test::{black_box, Bencher};
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#[bench]
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fn bench_find_smallest_1000(b: &mut Bencher) {
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let mut rng = thread_rng();
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let mut vec: Vec<u32> = (0..100_000).collect();
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vec.shuffle(&mut rng);
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b.iter(|| {
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let mut iter = vec.iter().copied();
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let mut heap: BinaryHeap<_> = iter.by_ref().take(1000).collect();
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for x in iter {
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let mut max = heap.peek_mut().unwrap();
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// This comparison should be true only 1% of the time.
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// Unnecessary `sift_down`s will degrade performance
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if x < *max {
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*max = x;
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}
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}
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heap
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})
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}
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#[bench]
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fn bench_peek_mut_deref_mut(b: &mut Bencher) {
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let mut bheap = BinaryHeap::from(vec![42]);
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let vec: Vec<u32> = (0..1_000_000).collect();
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b.iter(|| {
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let vec = black_box(&vec);
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let mut peek_mut = bheap.peek_mut().unwrap();
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// The compiler shouldn't be able to optimize away the `sift_down`
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// assignment in `PeekMut`'s `DerefMut` implementation since
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// the loop may not run.
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for &i in vec.iter() {
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*peek_mut = i;
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}
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// Remove the already minimal overhead of the sift_down
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std::mem::forget(peek_mut);
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})
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}
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#[bench]
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fn bench_from_vec(b: &mut Bencher) {
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let mut rng = thread_rng();
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let mut vec: Vec<u32> = (0..100_000).collect();
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vec.shuffle(&mut rng);
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b.iter(|| BinaryHeap::from(vec.clone()))
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}
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#[bench]
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fn bench_into_sorted_vec(b: &mut Bencher) {
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let bheap: BinaryHeap<i32> = (0..10_000).collect();
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b.iter(|| bheap.clone().into_sorted_vec())
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}
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#[bench]
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fn bench_push(b: &mut Bencher) {
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let mut bheap = BinaryHeap::with_capacity(50_000);
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let mut rng = thread_rng();
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let mut vec: Vec<u32> = (0..50_000).collect();
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vec.shuffle(&mut rng);
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b.iter(|| {
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for &i in vec.iter() {
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bheap.push(i);
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}
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black_box(&mut bheap);
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bheap.clear();
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})
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}
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#[bench]
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fn bench_pop(b: &mut Bencher) {
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let mut bheap = BinaryHeap::with_capacity(10_000);
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b.iter(|| {
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bheap.extend((0..10_000).rev());
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black_box(&mut bheap);
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while let Some(elem) = bheap.pop() {
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black_box(elem);
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}
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})
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}
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@ -8,6 +8,7 @@
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extern crate test;
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mod binary_heap;
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mod btree;
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mod linked_list;
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mod slice;
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@ -291,6 +291,7 @@ impl<T: Ord> Deref for PeekMut<'_, T> {
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impl<T: Ord> DerefMut for PeekMut<'_, T> {
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fn deref_mut(&mut self) -> &mut T {
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debug_assert!(!self.heap.is_empty());
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self.sift = true;
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// SAFE: PeekMut is only instantiated for non-empty heaps
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unsafe { self.heap.data.get_unchecked_mut(0) }
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}
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@ -396,10 +397,11 @@ impl<T: Ord> BinaryHeap<T> {
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///
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/// # Time complexity
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///
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/// Cost is *O*(1) in the worst case.
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/// If the item is modified then the worst case time complexity is *O*(log(*n*)),
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/// otherwise it's *O*(1).
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#[stable(feature = "binary_heap_peek_mut", since = "1.12.0")]
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pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
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if self.is_empty() { None } else { Some(PeekMut { heap: self, sift: true }) }
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if self.is_empty() { None } else { Some(PeekMut { heap: self, sift: false }) }
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}
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/// Removes the greatest item from the binary heap and returns it, or `None` if it
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