Auto merge of #17142 - kenan-rhoton:LRUCacheArrayVecDeque, r=emilio

Move LRUCache to ArrayDeque crate

We move LRUCache from using VecDeque to ArrayDeque to avoid using heap allocations.

This relies on the fix in andylokandy/arraydeque#4.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17054 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because the use cases are the same, only minimal implementation changes have been made

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I additionally ran test-unit, because I'm paranoid.

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components/style/Cargo.toml

@@ -32,6 +32,7 @@ gecko_debug = ["nsstring_vendor/gecko_debug"]
 [dependencies]
 app_units = "0.4.1"
 arrayvec = "0.3.20"
+arraydeque = "0.2.3"
 atomic_refcell = "0.1"
 bitflags = "0.7"
 bit-vec = "0.4.3"

components/style/cache.rs

@@ -6,29 +6,28 @@
 
 #![deny(missing_docs)]
 
-use std::collections::VecDeque;
-use std::collections::vec_deque;
+extern crate arraydeque;
+use self::arraydeque::Array;
+use self::arraydeque::ArrayDeque;
 
 /// A LRU cache used to store a set of at most `n` elements at the same time.
 ///
 /// The most-recently-used entry is at index zero.
-pub struct LRUCache<K> {
-    entries: VecDeque<K>,
-    cache_size: usize,
+pub struct LRUCache <K: Array>{
+    entries: ArrayDeque<K>,
 }
 
 /// A iterator over the items of the LRU cache.
-pub type LRUCacheIterator<'a, K> = vec_deque::Iter<'a, K>;
+pub type LRUCacheIterator<'a, K> = arraydeque::Iter<'a, K>;
 
 /// A iterator over the mutable items of the LRU cache.
-pub type LRUCacheMutIterator<'a, K> = vec_deque::IterMut<'a, K>;
+pub type LRUCacheMutIterator<'a, K> = arraydeque::IterMut<'a, K>;
 
-impl<K: PartialEq> LRUCache<K> {
+impl<K: Array> LRUCache<K> {
     /// Create a new LRU cache with `size` elements at most.
-    pub fn new(size: usize) -> Self {
+    pub fn new() -> Self {
         LRUCache {
-          entries: VecDeque::with_capacity(size),
-          cache_size: size,
+          entries: ArrayDeque::new(),
         }
     }
 
@@ -49,22 +48,22 @@ impl<K: PartialEq> LRUCache<K> {
 
     /// Iterate over the contents of this cache, from more to less recently
     /// used.
-    pub fn iter(&self) -> vec_deque::Iter<K> {
+    pub fn iter(&self) -> arraydeque::Iter<K::Item> {
         self.entries.iter()
     }
 
     /// Iterate mutably over the contents of this cache.
-    pub fn iter_mut(&mut self) -> vec_deque::IterMut<K> {
+    pub fn iter_mut(&mut self) -> arraydeque::IterMut<K::Item> {
         self.entries.iter_mut()
     }
 
     /// Insert a given key in the cache.
-    pub fn insert(&mut self, key: K) {
-        if self.entries.len() == self.cache_size {
+    pub fn insert(&mut self, key: K::Item) {
+        if self.entries.len() == self.entries.capacity() {
             self.entries.pop_back();
         }
         self.entries.push_front(key);
-        debug_assert!(self.entries.len() <= self.cache_size);
+        debug_assert!(self.entries.len() <= self.entries.capacity());
     }
 
     /// Evict all elements from the cache.

components/style/context.rs

@@ -349,7 +349,7 @@ pub struct SelectorFlagsMap<E: TElement> {
     map: FnvHashMap<SendElement<E>, ElementSelectorFlags>,
     /// An LRU cache to avoid hashmap lookups, which can be slow if the map
     /// gets big.
-    cache: LRUCache<(SendElement<E>, ElementSelectorFlags)>,
+    cache: LRUCache<[(SendElement<E>, ElementSelectorFlags); 4 + 1]>,
 }
 
 #[cfg(debug_assertions)]
@@ -364,7 +364,7 @@ impl<E: TElement> SelectorFlagsMap<E> {
     pub fn new() -> Self {
         SelectorFlagsMap {
             map: FnvHashMap::default(),
-            cache: LRUCache::new(4),
+            cache: LRUCache::new(),
         }
     }
 

components/style/sharing/mod.rs

@@ -309,14 +309,14 @@ pub enum StyleSharingResult {
 /// Note that this cache is flushed every time we steal work from the queue, so
 /// storing nodes here temporarily is safe.
 pub struct StyleSharingCandidateCache<E: TElement> {
-    cache: LRUCache<StyleSharingCandidate<E>>,
+    cache: LRUCache<[StyleSharingCandidate<E>; STYLE_SHARING_CANDIDATE_CACHE_SIZE + 1]>,
 }
 
 impl<E: TElement> StyleSharingCandidateCache<E> {
     /// Create a new style sharing candidate cache.
     pub fn new() -> Self {
         StyleSharingCandidateCache {
-            cache: LRUCache::new(STYLE_SHARING_CANDIDATE_CACHE_SIZE),
+            cache: LRUCache::new(),
         }
     }