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Tweak nearest_common_ancestor(). #50649

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merged 1 commit into from
May 18, 2018
Merged

Tweak nearest_common_ancestor(). #50649

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78 changes: 19 additions & 59 deletions src/librustc/middle/region.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -542,18 +542,6 @@ impl<'tcx> ScopeTree {
assert!(previous.is_none());
}

fn closure_is_enclosed_by(&self,
mut sub_closure: hir::ItemLocalId,
sup_closure: hir::ItemLocalId) -> bool {
loop {
if sub_closure == sup_closure { return true; }
match self.closure_tree.get(&sub_closure) {
Some(&s) => { sub_closure = s; }
None => { return false; }
}
}
}

fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) {
debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime);
assert!(var != lifetime.item_local_id());
Expand Down Expand Up @@ -688,65 +676,37 @@ impl<'tcx> ScopeTree {
// requires a hash table lookup, and we often have very long scope
// chains (10s or 100s of scopes) that only differ by a few elements at
// the start. So this algorithm is faster.
let mut ma = Some(scope_a);
let mut mb = Some(scope_b);
let mut seen_a: SmallVec<[Scope; 32]> = SmallVec::new();
let mut seen_b: SmallVec<[Scope; 32]> = SmallVec::new();

let mut ma = Some(&scope_a);
let mut mb = Some(&scope_b);

// A HashSet<Scope> is a more obvious choice for these, but SmallVec is
// faster because the set size is normally small so linear search is
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Did you verify this? It sounds like it should be true but I've run into some surprises because FxHashSet is so fast.

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I did verify it. The difference was small but noticeable.

// as good or better than a hash table lookup, plus the size is usually
// small enough to avoid a heap allocation.
let mut seen_a: SmallVec<[&Scope; 32]> = SmallVec::new();
let mut seen_b: SmallVec<[&Scope; 32]> = SmallVec::new();

loop {
if let Some(a) = ma {
if seen_b.iter().position(|s| *s == a).is_some() {
return a;
if seen_b.iter().any(|s| *s == a) {
return *a;
}
seen_a.push(a);
ma = self.parent_map.get(&a).map(|s| *s);
ma = self.parent_map.get(&a);
}

if let Some(b) = mb {
if seen_a.iter().position(|s| *s == b).is_some() {
return b;
if seen_a.iter().any(|s| *s == b) {
return *b;
}
seen_b.push(b);
mb = self.parent_map.get(&b).map(|s| *s);
mb = self.parent_map.get(&b);
}

if ma.is_none() && mb.is_none() {
break;
}
};

fn outermost_scope(parent_map: &FxHashMap<Scope, Scope>, scope: Scope) -> Scope {
let mut scope = scope;
loop {
match parent_map.get(&scope) {
Some(&superscope) => scope = superscope,
None => break scope,
}
}
}

// In this (rare) case, the two regions belong to completely different
// functions. Compare those fn for lexical nesting. The reasoning
// behind this is subtle. See the "Modeling closures" section of the
// README in infer::region_constraints for more details.
let a_root_scope = outermost_scope(&self.parent_map, scope_a);
let b_root_scope = outermost_scope(&self.parent_map, scope_b);
match (a_root_scope.data(), b_root_scope.data()) {
(ScopeData::Destruction(a_root_id),
ScopeData::Destruction(b_root_id)) => {
if self.closure_is_enclosed_by(a_root_id, b_root_id) {
// `a` is enclosed by `b`, hence `b` is the ancestor of everything in `a`
scope_b
} else if self.closure_is_enclosed_by(b_root_id, a_root_id) {
// `b` is enclosed by `a`, hence `a` is the ancestor of everything in `b`
scope_a
} else {
// neither fn encloses the other
bug!()
}
}
_ => {
// root ids are always Node right now
bug!()
// No nearest common ancestor found.
bug!();
}
}
}
Expand Down