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SsoHashMap minor refactoring, SSO_ARRAY_SIZE introduced

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This commit is contained in:
Valerii Lashmanov 2020-09-27 23:48:19 -05:00
parent 41942fac7d
commit 92a0668c20
1 changed files with 29 additions and 12 deletions
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41
compiler/rustc_data_structures/src/sso/map.rs
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@ -7,6 +7,25 @@ use std::hash::Hash;
use std::iter::FromIterator;
use std::ops::Index;
// For pointer-sized arguments arrays
// are faster than set/map for up to 64
// arguments.
//
// On the other hand such a big array
// hurts cache performance, makes passing
// sso structures around very expensive.
//
// Biggest performance benefit is gained
// for reasonably small arrays that stay
// small in vast majority of cases.
//
// '8' is choosen as a sane default, to be
// reevaluated later.
//
// Note: As of now ArrayVec design prevents
// us from making it user-customizable.
const SSO_ARRAY_SIZE: usize = 8;
/// Small-storage-optimized implementation of a map.
///
/// Stores elements in a small array up to a certain length
@ -26,7 +45,7 @@ use std::ops::Index;
/// Vacant/Occupied entries and related
#[derive(Clone)]
pub enum SsoHashMap<K, V> {
Array(ArrayVec<[(K, V); 8]>),
Array(ArrayVec<[(K, V); SSO_ARRAY_SIZE]>),
Map(FxHashMap<K, V>),
}
@ -39,9 +58,8 @@ impl<K, V> SsoHashMap<K, V> {
/// Creates an empty `SsoHashMap` with the specified capacity.
pub fn with_capacity(cap: usize) -> Self {
let array = ArrayVec::new();
if array.capacity() >= cap {
SsoHashMap::Array(array)
if cap <= SSO_ARRAY_SIZE {
Self::new()
} else {
SsoHashMap::Map(FxHashMap::with_capacity_and_hasher(cap, Default::default()))
}
@ -59,7 +77,7 @@ impl<K, V> SsoHashMap<K, V> {
/// Returns the number of elements the map can hold without reallocating.
pub fn capacity(&self) -> usize {
match self {
SsoHashMap::Array(array) => array.capacity(),
SsoHashMap::Array(_) => SSO_ARRAY_SIZE,
SsoHashMap::Map(map) => map.capacity(),
}
}
@ -149,7 +167,7 @@ impl<K: Eq + Hash, V> SsoHashMap<K, V> {
pub fn reserve(&mut self, additional: usize) {
match self {
SsoHashMap::Array(array) => {
if array.capacity() < (array.len() + additional) {
if SSO_ARRAY_SIZE < (array.len() + additional) {
let mut map: FxHashMap<K, V> = array.drain(..).collect();
map.reserve(additional);
*self = SsoHashMap::Map(map);
@ -164,10 +182,8 @@ impl<K: Eq + Hash, V> SsoHashMap<K, V> {
/// and possibly leaving some space in accordance with the resize policy.
pub fn shrink_to_fit(&mut self) {
if let SsoHashMap::Map(map) = self {
let mut array = ArrayVec::new();
if map.len() <= array.capacity() {
array.extend(map.drain());
*self = SsoHashMap::Array(array);
if map.len() <= SSO_ARRAY_SIZE {
*self = SsoHashMap::Array(map.drain().collect());
} else {
map.shrink_to_fit();
}
@ -361,7 +377,7 @@ impl<K: Eq + Hash, V> Extend<(K, V)> for SsoHashMap<K, V> {
fn extend_reserve(&mut self, additional: usize) {
match self {
SsoHashMap::Array(array) => {
if array.capacity() < (array.len() + additional) {
if SSO_ARRAY_SIZE < (array.len() + additional) {
let mut map: FxHashMap<K, V> = array.drain(..).collect();
map.extend_reserve(additional);
*self = SsoHashMap::Map(map);
@ -517,8 +533,9 @@ impl<'a, K: Eq + Hash, V> Entry<'a, K, V> {
let index = if let Some(index) = found_index {
index
} else {
let index = array.len();
array.try_push((self.key, default())).unwrap();
array.len() - 1
index
};
&mut array[index].1
}