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Auto merge of rust-lang#75105 - ssomers:btree_respect_min_len_hard, r=Mark-Simulacrum
Hard way to respect BTreeMap's minimum node length Resolves rust-lang#74834 the hard way (though not the hardest imaginable). Benchmarks (which are all biased/realistic, inserting keys in ascending order) say: ``` benchcmp r0 r1 --threshold 10 name r0 ns/iter r1 ns/iter diff ns/iter diff % speedup btree::map::clone_slim_100_and_clear 2,183 2,723 540 24.74% x 0.80 btree::map::clone_slim_100_and_drain_all 3,652 4,173 521 14.27% x 0.88 btree::map::clone_slim_100_and_drain_half 3,320 3,940 620 18.67% x 0.84 btree::map::clone_slim_100_and_into_iter 2,154 2,717 563 26.14% x 0.79 btree::map::clone_slim_100_and_pop_all 3,372 3,870 498 14.77% x 0.87 btree::map::clone_slim_100_and_remove_all 5,111 5,647 536 10.49% x 0.91 btree::map::clone_slim_100_and_remove_half 3,259 3,821 562 17.24% x 0.85 btree::map::iter_0 1,733 1,509 -224 -12.93% x 1.15 btree::map::iter_100 2,714 3,739 1,025 37.77% x 0.73 btree::map::iter_10k 3,728 4,269 541 14.51% x 0.87 btree::map::range_unbounded_unbounded 28,426 36,631 8,205 28.86% x 0.78 btree::map::range_unbounded_vs_iter 28,808 34,056 5,248 18.22% x 0.85 ``` This difference is not caused by the `debug_assert`-related code in the function `splitpoint`, it's the same without.
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  • library/alloc/src/collections/btree

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‎library/alloc/src/collections/btree/node.rs

+65-16
Original file line numberDiff line numberDiff line change
@@ -821,6 +821,53 @@ impl<BorrowType, K, V, NodeType> Handle<NodeRef<BorrowType, K, V, NodeType>, mar
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}
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}
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enum InsertionPlace {
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Left(usize),
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Right(usize),
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}
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/// Given an edge index where we want to insert into a node filled to capacity,
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/// computes a sensible KV index of a split point and where to perform the insertion.
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/// The goal of the split point is for its key and value to end up in a parent node;
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/// the keys, values and edges to the left of the split point become the left child;
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/// the keys, values and edges to the right of the split point become the right child.
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fn splitpoint(edge_idx: usize) -> (usize, InsertionPlace) {
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debug_assert!(edge_idx <= CAPACITY);
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// Rust issue #74834 tries to explain these symmetric rules.
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let middle_kv_idx;
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let insertion;
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if edge_idx <= B - 2 {
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middle_kv_idx = B - 2;
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insertion = InsertionPlace::Left(edge_idx);
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} else if edge_idx == B - 1 {
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middle_kv_idx = B - 1;
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insertion = InsertionPlace::Left(edge_idx);
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} else if edge_idx == B {
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middle_kv_idx = B - 1;
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insertion = InsertionPlace::Right(0);
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} else {
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middle_kv_idx = B;
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let new_edge_idx = edge_idx - (B + 1);
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insertion = InsertionPlace::Right(new_edge_idx);
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}
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let mut left_len = middle_kv_idx;
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let mut right_len = CAPACITY - middle_kv_idx - 1;
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match insertion {
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InsertionPlace::Left(edge_idx) => {
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debug_assert!(edge_idx <= left_len);
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left_len += 1;
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}
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InsertionPlace::Right(edge_idx) => {
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debug_assert!(edge_idx <= right_len);
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right_len += 1;
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}
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}
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debug_assert!(left_len >= MIN_LEN);
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debug_assert!(right_len >= MIN_LEN);
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debug_assert!(left_len + right_len == CAPACITY);
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(middle_kv_idx, insertion)
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}
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impl<'a, K, V, NodeType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, marker::Edge> {
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/// Helps implementations of `insert_fit` for a particular `NodeType`,
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/// by taking care of leaf data.
@@ -863,18 +910,20 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge
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let kv = unsafe { Handle::new_kv(self.node, self.idx) };
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(InsertResult::Fit(kv), ptr)
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} else {
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let middle = unsafe { Handle::new_kv(self.node, B) };
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let (middle_kv_idx, insertion) = splitpoint(self.idx);
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let middle = unsafe { Handle::new_kv(self.node, middle_kv_idx) };
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let (mut left, k, v, mut right) = middle.split();
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let ptr = if self.idx <= B {
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unsafe { Handle::new_edge(left.reborrow_mut(), self.idx).insert_fit(key, val) }
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} else {
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unsafe {
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let ptr = match insertion {
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InsertionPlace::Left(insert_idx) => unsafe {
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Handle::new_edge(left.reborrow_mut(), insert_idx).insert_fit(key, val)
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},
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InsertionPlace::Right(insert_idx) => unsafe {
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Handle::new_edge(
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right.node_as_mut().cast_unchecked::<marker::Leaf>(),
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self.idx - (B + 1),
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insert_idx,
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)
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.insert_fit(key, val)
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}
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},
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};
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(InsertResult::Split(SplitResult { left: left.forget_type(), k, v, right }), ptr)
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}
@@ -936,20 +985,20 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
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let kv = unsafe { Handle::new_kv(self.node, self.idx) };
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InsertResult::Fit(kv)
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} else {
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let middle = unsafe { Handle::new_kv(self.node, B) };
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let (middle_kv_idx, insertion) = splitpoint(self.idx);
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let middle = unsafe { Handle::new_kv(self.node, middle_kv_idx) };
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let (mut left, k, v, mut right) = middle.split();
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if self.idx <= B {
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unsafe {
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Handle::new_edge(left.reborrow_mut(), self.idx).insert_fit(key, val, edge);
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}
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} else {
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unsafe {
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match insertion {
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InsertionPlace::Left(insert_idx) => unsafe {
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Handle::new_edge(left.reborrow_mut(), insert_idx).insert_fit(key, val, edge);
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},
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InsertionPlace::Right(insert_idx) => unsafe {
947996
Handle::new_edge(
948997
right.node_as_mut().cast_unchecked::<marker::Internal>(),
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self.idx - (B + 1),
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insert_idx,
950999
)
9511000
.insert_fit(key, val, edge);
952-
}
1001+
},
9531002
}
9541003
InsertResult::Split(SplitResult { left: left.forget_type(), k, v, right })
9551004
}

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