rust/src/librustc_data_structures/thin_vec.rs

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use crate::stable_hasher::{StableHasher, HashStable};
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/// A vector type optimized for cases where this size is usually 0 (cf. `SmallVector`).
/// The `Option<Box<..>>` wrapping allows us to represent a zero sized vector with `None`,
/// which uses only a single (null) pointer.
#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
pub struct ThinVec<T>(Option<Box<Vec<T>>>);
impl<T> ThinVec<T> {
pub fn new() -> Self {
ThinVec(None)
}
}
impl<T> From<Vec<T>> for ThinVec<T> {
fn from(vec: Vec<T>) -> Self {
if vec.is_empty() {
ThinVec(None)
} else {
ThinVec(Some(Box::new(vec)))
}
}
}
impl<T> Into<Vec<T>> for ThinVec<T> {
fn into(self) -> Vec<T> {
match self {
ThinVec(None) => Vec::new(),
ThinVec(Some(vec)) => *vec,
}
}
}
impl<T> ::std::ops::Deref for ThinVec<T> {
type Target = [T];
fn deref(&self) -> &[T] {
match *self {
ThinVec(None) => &[],
ThinVec(Some(ref vec)) => vec,
}
}
}
Overhaul `syntax::fold::Folder`. This commit changes `syntax::fold::Folder` from a functional style (where most methods take a `T` and produce a new `T`) to a more imperative style (where most methods take and modify a `&mut T`), and renames it `syntax::mut_visit::MutVisitor`. The first benefit is speed. The functional style does not require any reallocations, due to the use of `P::map` and `MoveMap::move_{,flat_}map`. However, every field in the AST must be overwritten; even those fields that are unchanged are overwritten with the same value. This causes a lot of unnecessary memory writes. The imperative style reduces instruction counts by 1--3% across a wide range of workloads, particularly incremental workloads. The second benefit is conciseness; the imperative style is usually more concise. E.g. compare the old functional style: ``` fn fold_abc(&mut self, abc: ABC) { ABC { a: fold_a(abc.a), b: fold_b(abc.b), c: abc.c, } } ``` with the imperative style: ``` fn visit_abc(&mut self, ABC { a, b, c: _ }: &mut ABC) { visit_a(a); visit_b(b); } ``` (The reductions get larger in more complex examples.) Overall, the patch removes over 200 lines of code -- even though the new code has more comments -- and a lot of the remaining lines have fewer characters. Some notes: - The old style used methods called `fold_*`. The new style mostly uses methods called `visit_*`, but there are a few methods that map a `T` to something other than a `T`, which are called `flat_map_*` (`T` maps to multiple `T`s) or `filter_map_*` (`T` maps to 0 or 1 `T`s). - `move_map.rs`/`MoveMap`/`move_map`/`move_flat_map` are renamed `map_in_place.rs`/`MapInPlace`/`map_in_place`/`flat_map_in_place` to reflect their slightly changed signatures. - Although this commit renames the `fold` module as `mut_visit`, it keeps it in the `fold.rs` file, so as not to confuse git. The next commit will rename the file.
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impl<T> ::std::ops::DerefMut for ThinVec<T> {
fn deref_mut(&mut self) -> &mut [T] {
match *self {
ThinVec(None) => &mut [],
ThinVec(Some(ref mut vec)) => vec,
}
}
}
impl<T> Extend<T> for ThinVec<T> {
fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
match *self {
ThinVec(Some(ref mut vec)) => vec.extend(iter),
ThinVec(None) => *self = iter.into_iter().collect::<Vec<_>>().into(),
}
}
}
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impl<T: HashStable<CTX>, CTX> HashStable<CTX> for ThinVec<T> {
fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
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(**self).hash_stable(hcx, hasher)
}
}
impl<T> Default for ThinVec<T> {
fn default() -> Self {
Self(None)
}
}