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avoid processing mentioned items that are also still used

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This commit is contained in:
Ralf Jung 2024-03-17 09:37:52 +01:00
parent 91b35a1b40
commit ee4b758161
2 changed files with 28 additions and 4 deletions
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2
compiler/rustc_middle/src/mir/mod.rs
View File

@ -314,7 +314,7 @@ pub fn initial(
}
/// Some item that needs to monomorphize successfully for a MIR body to be considered well-formed.
#[derive(Copy, Clone, PartialEq, Debug, HashStable, TyEncodable, TyDecodable)]
#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash, HashStable, TyEncodable, TyDecodable)]
#[derive(TypeFoldable, TypeVisitable)]
pub enum MentionedItem<'tcx> {
Fn(DefId, GenericArgsRef<'tcx>),

30
compiler/rustc_monomorphize/src/collector.rs
View File

@ -720,6 +720,9 @@ struct MirUsedCollector<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
body: &'a mir::Body<'tcx>,
used_items: &'a mut MonoItems<'tcx>,
/// See the comment in `collect_items_of_instance` for the purpose of this set.
/// Note that this contains *not-monomorphized* items!
used_mentioned_items: &'a mut FxHashSet<MentionedItem<'tcx>>,
instance: Instance<'tcx>,
/// Spans for move size lints already emitted. Helps avoid duplicate lints.
move_size_spans: Vec<Span>,
@ -990,12 +993,18 @@ fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Loc
match terminator.kind {
mir::TerminatorKind::Call { ref func, ref args, ref fn_span, .. } => {
let callee_ty = func.ty(self.body, tcx);
// *Before* monomorphizing, record that we already handled this mention.
if let ty::FnDef(def_id, args) = callee_ty.kind() {
self.used_mentioned_items.insert(MentionedItem::Fn(*def_id, args));
}
let callee_ty = self.monomorphize(callee_ty);
self.check_fn_args_move_size(callee_ty, args, *fn_span, location);
visit_fn_use(self.tcx, callee_ty, true, source, &mut self.used_items)
}
mir::TerminatorKind::Drop { ref place, .. } => {
let ty = place.ty(self.body, self.tcx).ty;
// *Before* monomorphizing, record that we already handled this mention.
self.used_mentioned_items.insert(MentionedItem::Drop(ty));
let ty = self.monomorphize(ty);
visit_drop_use(self.tcx, ty, true, source, self.used_items);
}
@ -1639,10 +1648,22 @@ fn collect_items_of_instance<'tcx>(
mode: CollectionMode,
) {
let body = tcx.instance_mir(instance.def);
// Naively, in "used" collection mode, all functions get added to *both* `used_items` and
// `mentioned_items`. Mentioned items processing will then notice that they have already been
// visited, but at that point each mentioned item has been monomorphized, added to the
// `mentioned_items` worklist, and checked in the global set of visited items. To removes that
// overhead, we have a special optimization that avoids adding items to `mentioned_items` when
// they are already added in `used_items`. We could just scan `used_items`, but that's a linear
// scan and not very efficient. Furthermore we can only do that *after* monomorphizing the
// mentioned item. So instead we collect all pre-monomorphized `MentionedItem` that were already
// added to `used_items` in a hash set, which can efficiently query in the
// `body.mentioned_items` loop below.
let mut used_mentioned_items = FxHashSet::<MentionedItem<'tcx>>::default();
let mut collector = MirUsedCollector {
tcx,
body,
used_items,
used_mentioned_items: &mut used_mentioned_items,
instance,
move_size_spans: vec![],
visiting_call_terminator: false,
@ -1662,10 +1683,13 @@ fn collect_items_of_instance<'tcx>(
}
}
// Always gather mentioned items.
// Always gather mentioned items. We try to avoid processing items that we have already added to
// `used_items` above.
for item in &body.mentioned_items {
let item_mono = collector.monomorphize(item.node);
visit_mentioned_item(tcx, &item_mono, item.span, mentioned_items);
if !collector.used_mentioned_items.contains(&item.node) {
let item_mono = collector.monomorphize(item.node);
visit_mentioned_item(tcx, &item_mono, item.span, mentioned_items);
}
}
}