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std.hash_map: use 7 bits of metadata instead of 6

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we only effectively need 1 control bit to represent 2 special states for
the metadata (free and tombstone)
this should reduce the number of actual element equality tests, but since
it's very low already, the impact is negligible
This commit is contained in:
Sahnvour 2021-05-12 19:39:36 +02:00 committed by Andrew Kelley
parent 8b4e91e18c
commit 2d4d4baa42
1 changed files with 13 additions and 12 deletions
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25
lib/std/hash_map.zig
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@ -304,29 +304,32 @@ pub fn HashMapUnmanaged(
/// Metadata for a slot. It can be in three states: empty, used or
/// tombstone. Tombstones indicate that an entry was previously used,
/// they are a simple way to handle removal.
/// To this state, we add 6 bits from the slot's key hash. These are
/// To this state, we add 7 bits from the slot's key hash. These are
/// used as a fast way to disambiguate between entries without
/// having to use the equality function. If two fingerprints are
/// different, we know that we don't have to compare the keys at all.
/// The 6 bits are the highest ones from a 64 bit hash. This way, not
/// The 7 bits are the highest ones from a 64 bit hash. This way, not
/// only we use the `log2(capacity)` lowest bits from the hash to determine
/// a slot index, but we use 6 more bits to quickly resolve collisions
/// when multiple elements with different hashes end up wanting to be in / the same slot.
/// a slot index, but we use 7 more bits to quickly resolve collisions
/// when multiple elements with different hashes end up wanting to be in the same slot.
/// Not using the equality function means we don't have to read into
/// the entries array, avoiding a likely cache miss.
/// the entries array, likely avoiding a cache miss and a potentially
/// costly function call.
const Metadata = packed struct {
const FingerPrint = u6;
const FingerPrint = u7;
const free: FingerPrint = 0;
const tombstone: FingerPrint = 1;
fingerprint: FingerPrint = free,
used: u1 = 0,
tombstone: u1 = 0,
fingerprint: FingerPrint = 0,
pub fn isUsed(self: Metadata) bool {
return self.used == 1;
}
pub fn isTombstone(self: Metadata) bool {
return self.tombstone == 1;
return !self.isUsed() and self.fingerprint == tombstone;
}
pub fn takeFingerprint(hash: Hash) FingerPrint {
@ -337,14 +340,12 @@ pub fn HashMapUnmanaged(
pub fn fill(self: *Metadata, fp: FingerPrint) void {
self.used = 1;
self.tombstone = 0;
self.fingerprint = fp;
}
pub fn remove(self: *Metadata) void {
self.used = 0;
self.tombstone = 1;
self.fingerprint = 0;
self.fingerprint = tombstone;
}
};