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Mono item collection microoptimizations #162480

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Kobzol wants to merge 2 commits into from
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Mono item collection microoptimizations #162480

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25b34de
Reuse `reachable_inlined_items`
Kobzol Sep 8, 2026
430c6f4
Reserve capacity for used and mentioned items
Kobzol Sep 8, 2026
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6 changes: 6 additions & 0 deletions compiler/rustc_monomorphize/src/collector.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -326,6 +326,10 @@ impl<'tcx> MonoItems<'tcx> {
self.items.entry(item.node).or_insert(item.span);
}

fn reserve(&mut self, additional: usize) {
self.items.reserve(additional);
}

fn items(&self) -> impl Iterator<Item = MonoItem<'tcx>> {
self.items.keys().cloned()
}
Expand Down Expand Up @@ -487,7 +491,9 @@ fn collect_items_rec<'tcx>(
def_path_str,
});
};
used_items.reserve(used.len());

@JonathanBrouwer JonathanBrouwer Sep 8, 2026 •
edited by rustbot
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Instead of reserving outside of the extend call which is easily forgotten, how about this implementation:
#162495

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used_items.extend(used.into_iter().copied());
mentioned_items.reserve(mentioned.len());
mentioned_items.extend(mentioned.into_iter().copied());
}
MonoItem::GlobalAsm(item_id) => {
Expand Down
7 changes: 4 additions & 3 deletions compiler/rustc_monomorphize/src/partitioning.rs
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Expand Up @@ -216,6 +216,7 @@ where

let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx);
let cgu_name_cache = &mut UnordMap::default();
let mut reachable_inlined_items = FxIndexSet::default();

for mono_item in mono_items {
// Handle only root (GloballyShared) items directly here. Inlined (LocalCopy) items
Expand Down Expand Up @@ -264,15 +265,15 @@ where
// going via another root item. This includes drop-glue, functions from
// external crates, and local functions the definition of which is
// marked with `#[inline]`.
let mut reachable_inlined_items = FxIndexSet::default();
reachable_inlined_items.clear();
get_reachable_inlined_items(cx.tcx, mono_item, cx.usage_map, &mut reachable_inlined_items);

// Add those inlined items. It's possible an inlined item is reachable
// from multiple root items within a CGU, which is fine, it just means
// the `insert` will be a no-op.
for inlined_item in reachable_inlined_items {
for inlined_item in &reachable_inlined_items {

@JonathanBrouwer JonathanBrouwer Sep 8, 2026 •
edited by rustbot
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An alternative which might be slightly nicer as it doesn't need a copy:
reachable_inlined_items.drain(..)
and remove the .clear() above

The current implementation is also fine by me tho, if you prefer it. I think both of our options should be identical for performance

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@Kobzol Kobzol Sep 8, 2026

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I think that iter + clear is generally faster than drain, because the implementation of drain tends to be complicated.

// This is a CGU-private copy.
cgu.items_mut().entry(inlined_item).or_insert_with(|| MonoItemData {
cgu.items_mut().entry(*inlined_item).or_insert_with(|| MonoItemData {
inlined: true,
linkage: Linkage::Internal,
visibility: Visibility::Default,
Expand Down
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