diff options
Diffstat (limited to 'compiler/rustc_data_structures/src/sharded.rs')
-rw-r--r-- | compiler/rustc_data_structures/src/sharded.rs | 48 |
1 files changed, 38 insertions, 10 deletions
diff --git a/compiler/rustc_data_structures/src/sharded.rs b/compiler/rustc_data_structures/src/sharded.rs index 7ed70ba1e..40cbf1495 100644 --- a/compiler/rustc_data_structures/src/sharded.rs +++ b/compiler/rustc_data_structures/src/sharded.rs @@ -1,4 +1,6 @@ use crate::fx::{FxHashMap, FxHasher}; +#[cfg(parallel_compiler)] +use crate::sync::is_dyn_thread_safe; use crate::sync::{CacheAligned, Lock, LockGuard}; use std::borrow::Borrow; use std::collections::hash_map::RawEntryMut; @@ -18,6 +20,11 @@ pub const SHARDS: usize = 1 << SHARD_BITS; /// An array of cache-line aligned inner locked structures with convenience methods. pub struct Sharded<T> { + /// This mask is used to ensure that accesses are inbounds of `shards`. + /// When dynamic thread safety is off, this field is set to 0 causing only + /// a single shard to be used for greater cache efficiency. + #[cfg(parallel_compiler)] + mask: usize, shards: [CacheAligned<Lock<T>>; SHARDS], } @@ -31,31 +38,54 @@ impl<T: Default> Default for Sharded<T> { impl<T> Sharded<T> { #[inline] pub fn new(mut value: impl FnMut() -> T) -> Self { - Sharded { shards: [(); SHARDS].map(|()| CacheAligned(Lock::new(value()))) } + Sharded { + #[cfg(parallel_compiler)] + mask: if is_dyn_thread_safe() { SHARDS - 1 } else { 0 }, + shards: [(); SHARDS].map(|()| CacheAligned(Lock::new(value()))), + } + } + + #[inline(always)] + fn mask(&self) -> usize { + #[cfg(parallel_compiler)] + { + if SHARDS == 1 { 0 } else { self.mask } + } + #[cfg(not(parallel_compiler))] + { + 0 + } + } + + #[inline(always)] + fn count(&self) -> usize { + // `self.mask` is always one below the used shard count + self.mask() + 1 } /// The shard is selected by hashing `val` with `FxHasher`. #[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)) } + self.get_shard_by_hash(if SHARDS == 1 { 0 } else { make_hash(val) }) } #[inline] pub fn get_shard_by_hash(&self, hash: u64) -> &Lock<T> { - &self.shards[get_shard_index_by_hash(hash)].0 + self.get_shard_by_index(get_shard_hash(hash)) } #[inline] pub fn get_shard_by_index(&self, i: usize) -> &Lock<T> { - &self.shards[i].0 + // SAFETY: The index get ANDed with the mask, ensuring it is always inbounds. + unsafe { &self.shards.get_unchecked(i & self.mask()).0 } } pub fn lock_shards(&self) -> Vec<LockGuard<'_, T>> { - (0..SHARDS).map(|i| self.shards[i].0.lock()).collect() + (0..self.count()).map(|i| self.get_shard_by_index(i).lock()).collect() } pub fn try_lock_shards(&self) -> Option<Vec<LockGuard<'_, T>>> { - (0..SHARDS).map(|i| self.shards[i].0.try_lock()).collect() + (0..self.count()).map(|i| self.get_shard_by_index(i).try_lock()).collect() } } @@ -136,11 +166,9 @@ pub fn make_hash<K: Hash + ?Sized>(val: &K) -> u64 { /// `hash` can be computed with any hasher, so long as that hasher is used /// consistently for each `Sharded` instance. #[inline] -#[allow(clippy::modulo_one)] -pub fn get_shard_index_by_hash(hash: u64) -> usize { +fn get_shard_hash(hash: u64) -> usize { 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; - bits % SHARDS + (hash >> (hash_len * 8 - 7 - SHARD_BITS)) as usize } |