Save an instruction in EntityHasher

crates/bevy_ecs/src/entity/mod.rs

@@ -998,4 +998,36 @@ mod tests {
  assert!(Entity::new(2, 2) > Entity::new(1, 2));
  assert!(Entity::new(2, 2) >= Entity::new(1, 2));
  }
+
+ // Feel free to change this test if needed, but it seemed like an important
+ // part of the best-case performance changes in PR#9903.
+ #[test]
+ fn entity_hash_keeps_similar_ids_together() {
+ use std::hash::BuildHasher;
+ let hash = bevy_utils::EntityHash;
+
+ let first_id = 0xC0FFEE << 8;
+ let first_hash = hash.hash_one(Entity::from_raw(first_id));
+
+ for i in 1..=255 {
+ let id = first_id + i;
+ let hash = hash.hash_one(Entity::from_raw(id));
+ assert_eq!(hash.wrapping_sub(first_hash) as u32, i);
+ }
+ }
+
+ #[test]
+ fn entity_hash_id_bitflip_affects_high_7_bits() {
+ use std::hash::BuildHasher;
+ let hash = bevy_utils::EntityHash;
+
+ let first_id = 0xC0FFEE;
+ let first_hash = hash.hash_one(Entity::from_raw(first_id)) >> 57;
+
+ for bit in 0..u32::BITS {
+ let id = first_id ^ (1 << bit);
+ let hash = hash.hash_one(Entity::from_raw(id)) >> 57;
+ assert_ne!(hash, first_hash);
+ }
+ }
 }

crates/bevy_utils/src/lib.rs

@@ -267,29 +267,54 @@ impl BuildHasher for EntityHash {
 /// A very fast hash that is only designed to work on generational indices
 /// like `Entity`. It will panic if attempting to hash a type containing
 /// non-u64 fields.
+///
+/// This is heavily optimized for typical cases, where you have mostly live
+/// entities, and works particularly well for contiguous indices.
+///
+/// If you have an unusual case -- say all your indices are multiples of 256
+/// or most of the entities are dead generations -- then you might want also to
+/// try [`AHasher`] for a slower hash computation but fewer lookup conflicts.
 #[derive(Debug, Default)]
 pub struct EntityHasher {
  hash: u64,
 }
 
-// This value comes from rustc-hash (also known as FxHasher) which in turn got
-// it from Firefox. It is something like `u64::MAX / N` for an N that gives a
-// value close to π and works well for distributing bits for hashing when using
-// with a wrapping multiplication.
-const FRAC_U64MAX_PI: u64 = 0x517cc1b727220a95;
-
 impl Hasher for EntityHasher {
  fn write(&mut self, _bytes: &[u8]) {
  panic!("can only hash u64 using EntityHasher");
  }
 
  #[inline]
- fn write_u64(&mut self, i: u64) {
- // Apparently hashbrown's hashmap uses the upper 7 bits for some SIMD
- // optimisation that uses those bits for binning. This hash function
- // was faster than i | (i << (64 - 7)) in the worst cases, and was
- // faster than PassHasher for all cases tested.
- self.hash = i | (i.wrapping_mul(FRAC_U64MAX_PI) << 32);
+ fn write_u64(&mut self, bits: u64) {
+ // SwissTable (and thus `hashbrown`) cares about two things from the hash:
+ // - H1: low bits (masked by `2ⁿ-1`) to pick the slot in which to store the item
+ // - H2: high 7 bits are used to SIMD optimize hash collision probing
+ // For more see <https://abseil.io/about/design/swisstables#metadata-layout>
+
+ // This hash function assumes that the entity ids are still well-distributed,
+ // so for H1 leaves the entity id alone in the low bits so that id locality
+ // will also give memory locality for things spawned together.
+ // For H2, take advantage of the fact that while multiplication doesn't
+ // spread entropy to the low bits, it's incredibly good at spreading it
+ // upward, which is exactly where we need it the most.
+
+ // While this does include the generation in the output, it doesn't do so
+ // *usefully*. H1 won't care until you have over 3 billion entities in
+ // the table, and H2 won't care until something hits generation 33 million.
+ // Thus the comment suggesting that this is best for live entities,
+ // where there won't be generation conflicts where it would matter.
+
+ // The high 32 bits of this are ⅟φ for Fibonacci hashing. That works
+ // particularly well for hashing for the same reason as described in
+ // <https://extremelearning.com.au/unreasonable-effectiveness-of-quasirandom-sequences/>
+ // It loses no information because it has a modular inverse.
+ // (Specifically, `0x144c_bc89_u32 * 0x9e37_79b9_u32 == 1`.)
+ //
+ // The low 32 bits make that part of the just product a pass-through.
+ const UPPER_PHI: u64 = 0x9e37_79b9_0000_0001;
+
+ // This is `(MAGIC * index + generation) << 32 + index`, in a single instruction.
+ self.hash = bits.wrapping_mul(UPPER_PHI);
  }
 
  #[inline]