Use sharded maps for interning #61779
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,128 @@ | ||
| use std::hash::{Hasher, Hash}; | ||
| use std::mem; | ||
| use std::borrow::Borrow; | ||
| use std::collections::hash_map::RawEntryMut; | ||
| use crate::fx::{FxHasher, FxHashMap}; | ||
| use crate::sync::{Lock, LockGuard}; | ||
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| #[derive(Clone, Default)] | ||
| #[cfg_attr(parallel_compiler, repr(align(64)))] | ||
| struct CacheAligned<T>(T); | ||
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| #[cfg(parallel_compiler)] | ||
| // 32 shards is sufficient to reduce contention on an 8-core Ryzen 7 1700, | ||
| // but this should be tested on higher core count CPUs. How the `Sharded` type gets used | ||
| // may also affect the ideal nunber of shards. | ||
| const SHARD_BITS: usize = 5; | ||
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| #[cfg(not(parallel_compiler))] | ||
| const SHARD_BITS: usize = 0; | ||
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| const SHARDS: usize = 1 << SHARD_BITS; | ||
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| /// An array of cache-line aligned inner locked structures with convenience methods. | ||
| #[derive(Clone)] | ||
| pub struct Sharded<T> { | ||
| shards: [CacheAligned<Lock<T>>; SHARDS], | ||
| } | ||
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| impl<T: Default> Default for Sharded<T> { | ||
| #[inline] | ||
| fn default() -> Self { | ||
| let mut shards: mem::MaybeUninit<[CacheAligned<Lock<T>>; SHARDS]> = | ||
| mem::MaybeUninit::uninit(); | ||
| let first = shards.as_mut_ptr() as *mut CacheAligned<Lock<T>>; | ||
| unsafe { | ||
| for i in 0..SHARDS { | ||
| first.add(i).write(CacheAligned(Lock::new(T::default()))); | ||
| } | ||
| Sharded { | ||
| shards: shards.assume_init(), | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| impl<T> Sharded<T> { | ||
| #[inline] | ||
| pub fn get_shard_by_value<K: Hash + ?Sized>(&self, val: &K) -> &Lock<T> { | ||
| if SHARDS == 1 { | ||
| &self.shards[0].0 | ||
| } else { | ||
| self.get_shard_by_hash(make_hash(val)) | ||
| } | ||
| } | ||
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| #[inline] | ||
| pub fn get_shard_by_hash(&self, hash: u64) -> &Lock<T> { | ||
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| let hash_len = mem::size_of::<usize>(); | ||
| // Ignore the top 7 bits as hashbrown uses these and get the next SHARD_BITS highest bits. | ||
| // hashbrown also uses the lowest bits, so we can't use those | ||
| let bits = (hash >> (hash_len * 8 - 7 - SHARD_BITS)) as usize; | ||
| let i = bits % SHARDS; | ||
| &self.shards[i].0 | ||
| } | ||
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| pub fn lock_shards(&self) -> Vec<LockGuard<'_, T>> { | ||
| (0..SHARDS).map(|i| self.shards[i].0.lock()).collect() | ||
| } | ||
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| pub fn try_lock_shards(&self) -> Option<Vec<LockGuard<'_, T>>> { | ||
| (0..SHARDS).map(|i| self.shards[i].0.try_lock()).collect() | ||
| } | ||
| } | ||
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| pub type ShardedHashMap<K, V> = Sharded<FxHashMap<K, V>>; | ||
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| impl<K: Eq + Hash, V> ShardedHashMap<K, V> { | ||
| pub fn len(&self) -> usize { | ||
| self.lock_shards().iter().map(|shard| shard.len()).sum() | ||
| } | ||
| } | ||
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| impl<K: Eq + Hash + Copy> ShardedHashMap<K, ()> { | ||
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There was a problem hiding this comment. Absent intent to actually use this type as a map, you might as well just make this ShardedHashSet? There was a problem hiding this comment. It doesn't have set operations, so I'd find that misleading. |
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| #[inline] | ||
| pub fn intern_ref<Q: ?Sized>(&self, value: &Q, make: impl FnOnce() -> K) -> K | ||
| where K: Borrow<Q>, | ||
| Q: Hash + Eq | ||
| { | ||
| let hash = make_hash(value); | ||
|
There was a problem hiding this comment. Worth putting a comment here explaining that we can't use the map's hasher because we need the hash to find the map? Also you could arguably do something silly like make a HashMap::default here just so this code is easier to change but... meh? There was a problem hiding this comment. The hash map's hasher is never used, so there isn't really a reason to access it. |
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| let mut shard = self.get_shard_by_hash(hash).lock(); | ||
| let entry = shard.raw_entry_mut().from_key_hashed_nocheck(hash, value); | ||
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| match entry { | ||
| RawEntryMut::Occupied(e) => *e.key(), | ||
| RawEntryMut::Vacant(e) => { | ||
| let v = make(); | ||
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There was a problem hiding this comment. it's a bit off to call this v when it's a key (same for other function) There was a problem hiding this comment. It's also the interned value we return =P |
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| e.insert_hashed_nocheck(hash, v, ()); | ||
| v | ||
| } | ||
| } | ||
| } | ||
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| #[inline] | ||
| pub fn intern<Q>(&self, value: Q, make: impl FnOnce(Q) -> K) -> K | ||
| where K: Borrow<Q>, | ||
| Q: Hash + Eq | ||
| { | ||
| let hash = make_hash(&value); | ||
| let mut shard = self.get_shard_by_hash(hash).lock(); | ||
| let entry = shard.raw_entry_mut().from_key_hashed_nocheck(hash, &value); | ||
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| match entry { | ||
| RawEntryMut::Occupied(e) => *e.key(), | ||
| RawEntryMut::Vacant(e) => { | ||
| let v = make(value); | ||
| e.insert_hashed_nocheck(hash, v, ()); | ||
| v | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| #[inline] | ||
| fn make_hash<K: Hash + ?Sized>(val: &K) -> u64 { | ||
| let mut state = FxHasher::default(); | ||
| val.hash(&mut state); | ||
| state.finish() | ||
| } | ||
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Some comment on the magic number would ne nice
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Added a comment here.