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Merged
rust-bors merged 1 commit into from
Feb 15, 2026
Merged

Use scope for par_slice instead of join #152375

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41 changes: 14 additions & 27 deletions compiler/rustc_data_structures/src/sync/parallel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -138,34 +138,21 @@ fn par_slice<I: DynSend>(
guard: &ParallelGuard,
for_each: impl Fn(&mut I) + DynSync + DynSend,
) {
struct State<'a, F> {
for_each: FromDyn<F>,
guard: &'a ParallelGuard,
group: usize,
}

fn par_rec<I: DynSend, F: Fn(&mut I) + DynSync + DynSend>(
items: &mut [I],
state: &State<'_, F>,
) {
if items.len() <= state.group {
for item in items {
state.guard.run(|| (state.for_each)(item));
}
} else {
let (left, right) = items.split_at_mut(items.len() / 2);
let mut left = state.for_each.derive(left);
let mut right = state.for_each.derive(right);
rustc_thread_pool::join(move || par_rec(*left, state), move || par_rec(*right, state));
let for_each = FromDyn::from(for_each);
let mut items = for_each.derive(items);
rustc_thread_pool::scope(|s| {
let proof = items.derive(());
let group_size = std::cmp::max(items.len() / 128, 1);
for group in items.chunks_exact_mut(group_size) {
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@zetanumbers zetanumbers Feb 16, 2026

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Hold on. chunks_exact_mut and not chunks_mut?

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@petrochenkov petrochenkov Feb 16, 2026

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Yes, looks like some items are simply not processed now.

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@Zoxc Zoxc Feb 16, 2026

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Well, that would skew performance numbers too.

let group = proof.derive(group);
s.spawn(|_| {
let mut group = group;
for i in group.iter_mut() {
guard.run(|| for_each(i));
}
});
}
}

let state = State {
for_each: FromDyn::from(for_each),
guard,
group: std::cmp::max(items.len() / 128, 1),
};
par_rec(items, &state)
});
}

pub fn par_for_each_in<I: DynSend, T: IntoIterator<Item = I>>(
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