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Merge pull request #2495 from dtolnay/cautious
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Allow larger preallocated capacity for smaller elements
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dtolnay authored Jul 9, 2023
2 parents 3686277 + 399ef08 commit a622b8a
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Showing 3 changed files with 31 additions and 15 deletions.
19 changes: 10 additions & 9 deletions serde/src/de/impls.rs
Original file line number Diff line number Diff line change
Expand Up @@ -681,8 +681,8 @@ impl<'de> Visitor<'de> for CStringVisitor {
where
A: SeqAccess<'de>,
{
let len = size_hint::cautious(seq.size_hint());
let mut values = Vec::with_capacity(len);
let capacity = size_hint::cautious::<u8>(seq.size_hint());
let mut values = Vec::<u8>::with_capacity(capacity);

while let Some(value) = try!(seq.next_element()) {
values.push(value);
Expand Down Expand Up @@ -936,7 +936,7 @@ macro_rules! seq_impl {
A: SeqAccess<'de>,
{
$clear(&mut self.0);
$reserve(&mut self.0, size_hint::cautious($access.size_hint()));
$reserve(&mut self.0, size_hint::cautious::<T>($access.size_hint()));

// FIXME: try to overwrite old values here? (Vec, VecDeque, LinkedList)
while let Some(value) = try!($access.next_element()) {
Expand All @@ -962,7 +962,7 @@ seq_impl!(
BinaryHeap<T: Ord>,
seq,
BinaryHeap::clear,
BinaryHeap::with_capacity(size_hint::cautious(seq.size_hint())),
BinaryHeap::with_capacity(size_hint::cautious::<T>(seq.size_hint())),
BinaryHeap::reserve,
BinaryHeap::push
);
Expand Down Expand Up @@ -992,7 +992,7 @@ seq_impl!(
HashSet<T: Eq + Hash, S: BuildHasher + Default>,
seq,
HashSet::clear,
HashSet::with_capacity_and_hasher(size_hint::cautious(seq.size_hint()), S::default()),
HashSet::with_capacity_and_hasher(size_hint::cautious::<T>(seq.size_hint()), S::default()),
HashSet::reserve,
HashSet::insert
);
Expand All @@ -1002,7 +1002,7 @@ seq_impl!(
VecDeque<T>,
seq,
VecDeque::clear,
VecDeque::with_capacity(size_hint::cautious(seq.size_hint())),
VecDeque::with_capacity(size_hint::cautious::<T>(seq.size_hint())),
VecDeque::reserve,
VecDeque::push_back
);
Expand Down Expand Up @@ -1036,7 +1036,8 @@ where
where
A: SeqAccess<'de>,
{
let mut values = Vec::with_capacity(size_hint::cautious(seq.size_hint()));
let capacity = size_hint::cautious::<T>(seq.size_hint());
let mut values = Vec::<T>::with_capacity(capacity);

while let Some(value) = try!(seq.next_element()) {
values.push(value);
Expand Down Expand Up @@ -1072,7 +1073,7 @@ where
where
A: SeqAccess<'de>,
{
let hint = size_hint::cautious(seq.size_hint());
let hint = size_hint::cautious::<T>(seq.size_hint());
if let Some(additional) = hint.checked_sub(self.0.len()) {
self.0.reserve(additional);
}
Expand Down Expand Up @@ -1416,7 +1417,7 @@ map_impl!(BTreeMap<K: Ord, V>, map, BTreeMap::new());
map_impl!(
HashMap<K: Eq + Hash, V, S: BuildHasher + Default>,
map,
HashMap::with_capacity_and_hasher(size_hint::cautious(map.size_hint()), S::default())
HashMap::with_capacity_and_hasher(size_hint::cautious::<(K, V)>(map.size_hint()), S::default())
);

////////////////////////////////////////////////////////////////////////////////
Expand Down
13 changes: 10 additions & 3 deletions serde/src/private/de.rs
Original file line number Diff line number Diff line change
Expand Up @@ -474,7 +474,8 @@ mod content {
where
V: SeqAccess<'de>,
{
let mut vec = Vec::with_capacity(size_hint::cautious(visitor.size_hint()));
let mut vec =
Vec::<Content>::with_capacity(size_hint::cautious::<Content>(visitor.size_hint()));
while let Some(e) = try!(visitor.next_element()) {
vec.push(e);
}
Expand All @@ -485,7 +486,10 @@ mod content {
where
V: MapAccess<'de>,
{
let mut vec = Vec::with_capacity(size_hint::cautious(visitor.size_hint()));
let mut vec =
Vec::<(Content, Content)>::with_capacity(
size_hint::cautious::<(Content, Content)>(visitor.size_hint()),
);
while let Some(kv) = try!(visitor.next_entry()) {
vec.push(kv);
}
Expand Down Expand Up @@ -844,7 +848,10 @@ mod content {
M: MapAccess<'de>,
{
let mut tag = None;
let mut vec = Vec::with_capacity(size_hint::cautious(map.size_hint()));
let mut vec = Vec::<(Content, Content)>::with_capacity(size_hint::cautious::<(
Content,
Content,
)>(map.size_hint()));
while let Some(k) = try!(map.next_key_seed(TagOrContentVisitor::new(self.tag_name))) {
match k {
TagOrContent::Tag => {
Expand Down
14 changes: 11 additions & 3 deletions serde/src/private/size_hint.rs
Original file line number Diff line number Diff line change
Expand Up @@ -8,9 +8,17 @@ where
}

#[cfg(any(feature = "std", feature = "alloc"))]
#[inline]
pub fn cautious(hint: Option<usize>) -> usize {
cmp::min(hint.unwrap_or(0), 4096)
pub fn cautious<Element>(hint: Option<usize>) -> usize {
const MAX_PREALLOC_BYTES: usize = 1024 * 1024;

if mem::size_of::<Element>() == 0 {
0
} else {
cmp::min(
hint.unwrap_or(0),
MAX_PREALLOC_BYTES / mem::size_of::<Element>(),
)
}
}

fn helper(bounds: (usize, Option<usize>)) -> Option<usize> {
Expand Down

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