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perf: Avoid creating a SmallVec if nothing changes during a fold #68031

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Merged
merged 3 commits into from
Jan 26, 2020
Merged

perf: Avoid creating a SmallVec if nothing changes during a fold #68031

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a5657db
perf: Avoid creating a SmallVec if nothing changes during a fold
Marwes Jan 8, 2020
898ed63
Fix copy_from_slice which should be extend_from_slice
Marwes Jan 9, 2020
a1586f1
Explain fold_list
Marwes Jan 13, 2020
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52 changes: 35 additions & 17 deletions src/librustc/ty/structural_impls.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -803,8 +803,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for ty::Binder<T> {

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>> {
fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
folder.tcx().intern_existential_predicates(&v)
fold_list(*self, folder, |tcx, v| tcx.intern_existential_predicates(v))
}

fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
Expand All @@ -814,8 +813,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>>

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
let v = self.iter().map(|t| t.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
folder.tcx().intern_type_list(&v)
fold_list(*self, folder, |tcx, v| tcx.intern_type_list(v))
}

fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
Expand All @@ -825,8 +823,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ProjectionKind> {
fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
let v = self.iter().map(|t| t.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
folder.tcx().intern_projs(&v)
fold_list(*self, folder, |tcx, v| tcx.intern_projs(v))
}

fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
Expand Down Expand Up @@ -990,17 +987,7 @@ impl<'tcx> TypeFoldable<'tcx> for ty::Region<'tcx> {

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Predicate<'tcx>> {
fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
// This code is hot enough that it's worth specializing for a list of
// length 0. (No other length is common enough to be worth singling
// out).
if self.len() == 0 {
self
} else {
// Don't bother interning if nothing changed, which is the common
// case.
let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
if v[..] == self[..] { self } else { folder.tcx().intern_predicates(&v) }
}
fold_list(*self, folder, |tcx, v| tcx.intern_predicates(v))
}

fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
Expand Down Expand Up @@ -1073,3 +1060,34 @@ impl<'tcx> TypeFoldable<'tcx> for InferConst<'tcx> {
false
}
}

// Does the equivalent of
// ```
// let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
// folder.tcx().intern_*(&v)
// ```
fn fold_list<'tcx, F, T>(
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Might be worth adding a comment here that explain that this is an optimization over .iter().map(...).collect()

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Done in a1586f1

list: &'tcx ty::List<T>,
folder: &mut F,
intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> &'tcx ty::List<T>,
) -> &'tcx ty::List<T>
where
F: TypeFolder<'tcx>,
T: TypeFoldable<'tcx> + PartialEq + Copy,
{
let mut iter = list.iter();
// Look for the first element that changed
if let Some((i, new_t)) = iter.by_ref().enumerate().find_map(|(i, t)| {
let new_t = t.fold_with(folder);
if new_t == *t { None } else { Some((i, new_t)) }
}) {
// An element changed, prepare to intern the resulting list
let mut new_list = SmallVec::<[_; 8]>::with_capacity(list.len());
new_list.extend_from_slice(&list[..i]);
new_list.push(new_t);
new_list.extend(iter.map(|t| t.fold_with(folder)));
intern(folder.tcx(), &new_list)
} else {
list
}
}