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Auto merge of #50240 - nnethercote:LazyBTreeMap, r=cramertj
Implement LazyBTreeMap and use it in a few places. This is a thin wrapper around BTreeMap that avoids allocating upon creation. I would prefer to change BTreeMap directly to make it lazy (like I did with HashSet in #36734) and I initially attempted that by making BTreeMap::root an Option<>. But then I also had to change Iter and Range to handle trees with no root, and those types have stability markers on them and I wasn't sure if that was acceptable. Also, BTreeMap has a lot of complex code and changing it all was challenging, and I didn't have high confidence about my general approach. So I prototyped this wrapper instead and used it in the hottest locations to get some measurements about the effect. The measurements are pretty good! - Doing a debug build of serde, it reduces the total number of heap allocations from 17,728,709 to 13,359,384, a 25% reduction. The number of bytes allocated drops from 7,474,672,966 to 5,482,308,388, a 27% reduction. - It gives speedups of up to 3.6% on some rustc-perf benchmark jobs. crates.io, futures, and serde benefit most. ``` futures-check avg: -1.9% min: -3.6% max: -0.5% serde-check avg: -2.1% min: -3.5% max: -0.7% crates.io-check avg: -1.7% min: -3.5% max: -0.3% serde avg: -2.0% min: -3.0% max: -0.9% serde-opt avg: -1.8% min: -2.9% max: -0.3% futures avg: -1.5% min: -2.8% max: -0.4% tokio-webpush-simple-check avg: -1.1% min: -2.2% max: -0.1% futures-opt avg: -1.2% min: -2.1% max: -0.4% piston-image-check avg: -0.8% min: -1.1% max: -0.3% crates.io avg: -0.6% min: -1.0% max: -0.3% ``` @gankro, how do you think I should proceed here? Is leaving this as a wrapper reasonable? Or should I try to make BTreeMap itself lazy? If so, can I change the representation of Iter and Range? Thanks!
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| // Copyright 2018 The Rust Project Developers. See the COPYRIGHT | ||
| // file at the top-level directory of this distribution and at | ||
| // http://rust-lang.org/COPYRIGHT. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
| // option. This file may not be copied, modified, or distributed | ||
| // except according to those terms. | ||
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| use std::collections::btree_map; | ||
| use std::collections::BTreeMap; | ||
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| /// A thin wrapper around BTreeMap that avoids allocating upon creation. | ||
| /// | ||
| /// Vec, HashSet and HashMap all have the nice feature that they don't do any | ||
| /// heap allocation when creating a new structure of the default size. In | ||
| /// contrast, BTreeMap *does* allocate in that situation. The compiler uses | ||
| /// B-Tree maps in some places such that many maps are created but few are | ||
| /// inserted into, so having a BTreeMap alternative that avoids allocating on | ||
| /// creation is a performance win. | ||
| /// | ||
| /// Only a fraction of BTreeMap's functionality is currently supported. | ||
| /// Additional functionality should be added on demand. | ||
| #[derive(Debug)] | ||
| pub struct LazyBTreeMap<K, V>(Option<BTreeMap<K, V>>); | ||
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| impl<K, V> LazyBTreeMap<K, V> { | ||
| pub fn new() -> LazyBTreeMap<K, V> { | ||
| LazyBTreeMap(None) | ||
| } | ||
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| pub fn iter(&self) -> Iter<K, V> { | ||
| Iter(self.0.as_ref().map(|btm| btm.iter())) | ||
| } | ||
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| pub fn is_empty(&self) -> bool { | ||
| self.0.as_ref().map_or(true, |btm| btm.is_empty()) | ||
| } | ||
| } | ||
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| impl<K: Ord, V> LazyBTreeMap<K, V> { | ||
| fn instantiate(&mut self) -> &mut BTreeMap<K, V> { | ||
| if let Some(ref mut btm) = self.0 { | ||
| btm | ||
| } else { | ||
| let btm = BTreeMap::new(); | ||
| self.0 = Some(btm); | ||
| self.0.as_mut().unwrap() | ||
| } | ||
| } | ||
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| pub fn insert(&mut self, key: K, value: V) -> Option<V> { | ||
| self.instantiate().insert(key, value) | ||
| } | ||
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| pub fn entry(&mut self, key: K) -> btree_map::Entry<K, V> { | ||
| self.instantiate().entry(key) | ||
| } | ||
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| pub fn values<'a>(&'a self) -> Values<'a, K, V> { | ||
| Values(self.0.as_ref().map(|btm| btm.values())) | ||
| } | ||
| } | ||
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| impl<K: Ord, V> Default for LazyBTreeMap<K, V> { | ||
| fn default() -> LazyBTreeMap<K, V> { | ||
| LazyBTreeMap::new() | ||
| } | ||
| } | ||
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| impl<'a, K: 'a, V: 'a> IntoIterator for &'a LazyBTreeMap<K, V> { | ||
| type Item = (&'a K, &'a V); | ||
| type IntoIter = Iter<'a, K, V>; | ||
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| fn into_iter(self) -> Iter<'a, K, V> { | ||
| self.iter() | ||
| } | ||
| } | ||
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| pub struct Iter<'a, K: 'a, V: 'a>(Option<btree_map::Iter<'a, K, V>>); | ||
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| impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> { | ||
| type Item = (&'a K, &'a V); | ||
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| fn next(&mut self) -> Option<(&'a K, &'a V)> { | ||
| self.0.as_mut().and_then(|iter| iter.next()) | ||
| } | ||
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| fn size_hint(&self) -> (usize, Option<usize>) { | ||
| self.0.as_ref().map_or_else(|| (0, Some(0)), |iter| iter.size_hint()) | ||
| } | ||
| } | ||
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| pub struct Values<'a, K: 'a, V: 'a>(Option<btree_map::Values<'a, K, V>>); | ||
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| impl<'a, K, V> Iterator for Values<'a, K, V> { | ||
| type Item = &'a V; | ||
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| fn next(&mut self) -> Option<&'a V> { | ||
| self.0.as_mut().and_then(|values| values.next()) | ||
| } | ||
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| fn size_hint(&self) -> (usize, Option<usize>) { | ||
| self.0.as_ref().map_or_else(|| (0, Some(0)), |values| values.size_hint()) | ||
| } | ||
| } | ||
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