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38880 don't compute hash when searching an empty HashMap

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This addresses issue #38880
This commit is contained in:
Shaun Steenkamp 2018-02-06 08:56:27 +00:00
parent ca7d839088
commit 9bc59865f1
1 changed files with 36 additions and 6 deletions
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42
src/libstd/collections/hash/map.rs
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@ -397,9 +397,21 @@ pub struct HashMap<K, V, S = RandomState> {
resize_policy: DefaultResizePolicy,
}
/// Search for a pre-hashed key.
/// Search for a pre-hashed key when the hash map is known to be non-empty.
#[inline]
fn search_hashed<K, V, M, F>(table: M, hash: SafeHash, mut is_match: F) -> InternalEntry<K, V, M>
fn search_hashed_nonempty<K, V, M, F>(table: M, hash: SafeHash, is_match: F)
-> InternalEntry<K, V, M>
where M: Deref<Target = RawTable<K, V>>,
F: FnMut(&K) -> bool
{
// Do not check the capacity as an extra branch could slow the lookup.
search_hashed_body(table, hash, is_match)
}
/// Search for a pre-hashed key.
/// If you don't already know the hash, use search or search_mut instead
#[inline]
fn search_hashed<K, V, M, F>(table: M, hash: SafeHash, is_match: F) -> InternalEntry<K, V, M>
where M: Deref<Target = RawTable<K, V>>,
F: FnMut(&K) -> bool
{
@ -410,6 +422,16 @@ fn search_hashed<K, V, M, F>(table: M, hash: SafeHash, mut is_match: F) -> Inter
return InternalEntry::TableIsEmpty;
}
search_hashed_body(table, hash, is_match)
}
/// The body of the search_hashed[_nonempty] functions
#[inline]
fn search_hashed_body<K, V, M, F>(table: M, hash: SafeHash, mut is_match: F)
-> InternalEntry<K, V, M>
where M: Deref<Target = RawTable<K, V>>,
F: FnMut(&K) -> bool
{
let size = table.size();
let mut probe = Bucket::new(table, hash);
let mut displacement = 0;
@ -550,8 +572,12 @@ impl<K, V, S> HashMap<K, V, S>
where K: Borrow<Q>,
Q: Eq + Hash
{
let hash = self.make_hash(q);
search_hashed(&self.table, hash, |k| q.eq(k.borrow()))
if self.table.capacity() != 0 {
let hash = self.make_hash(q);
search_hashed_nonempty(&self.table, hash, |k| q.eq(k.borrow()))
} else {
InternalEntry::TableIsEmpty
}
}
#[inline]
@ -559,8 +585,12 @@ impl<K, V, S> HashMap<K, V, S>
where K: Borrow<Q>,
Q: Eq + Hash
{
let hash = self.make_hash(q);
search_hashed(&mut self.table, hash, |k| q.eq(k.borrow()))
if self.table.capacity() != 0 {
let hash = self.make_hash(q);
search_hashed_nonempty(&mut self.table, hash, |k| q.eq(k.borrow()))
} else {
InternalEntry::TableIsEmpty
}
}
// The caller should ensure that invariants by Robin Hood Hashing hold