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Rework collect_and_apply to not rely on size hint for optimization #141652

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Jun 4, 2025
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136 changes: 90 additions & 46 deletions compiler/rustc_type_ir/src/interner.rs
Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,6 @@ use std::ops::Deref;

use rustc_ast_ir::Movability;
use rustc_index::bit_set::DenseBitSet;
use smallvec::SmallVec;

use crate::fold::TypeFoldable;
use crate::inherent::*;
Expand Down Expand Up @@ -382,28 +381,45 @@ impl<T, R> CollectAndApply<T, R> for T {
F: FnOnce(&[T]) -> R,
{
// This code is hot enough that it's worth specializing for the most
// common length lists, to avoid the overhead of `SmallVec` creation.
// Lengths 0, 1, and 2 typically account for ~95% of cases. If
// `size_hint` is incorrect a panic will occur via an `unwrap` or an
// `assert`.
match iter.size_hint() {
(0, Some(0)) => {
assert!(iter.next().is_none());
f(&[])
}
(1, Some(1)) => {
let t0 = iter.next().unwrap();
assert!(iter.next().is_none());
f(&[t0])
}
(2, Some(2)) => {
let t0 = iter.next().unwrap();
let t1 = iter.next().unwrap();
assert!(iter.next().is_none());
f(&[t0, t1])
}
_ => f(&iter.collect::<SmallVec<[_; 8]>>()),
}
// common length lists, to avoid the overhead of `Vec` creation.

let Some(t0) = iter.next() else {
return f(&[]);
};

let Some(t1) = iter.next() else {
return f(&[t0]);
};

let Some(t2) = iter.next() else {
return f(&[t0, t1]);
};

let Some(t3) = iter.next() else {
return f(&[t0, t1, t2]);
};

let Some(t4) = iter.next() else {
return f(&[t0, t1, t2, t3]);
};

let Some(t5) = iter.next() else {
return f(&[t0, t1, t2, t3, t4]);
};

let Some(t6) = iter.next() else {
return f(&[t0, t1, t2, t3, t4, t5]);
};

let Some(t7) = iter.next() else {
return f(&[t0, t1, t2, t3, t4, t5, t6]);
};

let Some(t8) = iter.next() else {
return f(&[t0, t1, t2, t3, t4, t5, t6, t7]);
};

f(&[t0, t1, t2, t3, t4, t5, t6, t7, t8].into_iter().chain(iter).collect::<Vec<_>>())
}
}

Expand All @@ -419,29 +435,57 @@ impl<T, R, E> CollectAndApply<T, R> for Result<T, E> {
F: FnOnce(&[T]) -> R,
{
// This code is hot enough that it's worth specializing for the most
// common length lists, to avoid the overhead of `SmallVec` creation.
// Lengths 0, 1, and 2 typically account for ~95% of cases. If
// `size_hint` is incorrect a panic will occur via an `unwrap` or an
// `assert`, unless a failure happens first, in which case the result
// will be an error anyway.
Ok(match iter.size_hint() {
(0, Some(0)) => {
assert!(iter.next().is_none());
f(&[])
}
(1, Some(1)) => {
let t0 = iter.next().unwrap()?;
assert!(iter.next().is_none());
f(&[t0])
}
(2, Some(2)) => {
let t0 = iter.next().unwrap()?;
let t1 = iter.next().unwrap()?;
assert!(iter.next().is_none());
f(&[t0, t1])
}
_ => f(&iter.collect::<Result<SmallVec<[_; 8]>, _>>()?),
})
// common length lists, to avoid the overhead of `Vec` creation.

let Some(t0) = iter.next() else {
return Ok(f(&[]));
};
let t0 = t0?;

let Some(t1) = iter.next() else {
return Ok(f(&[t0]));
};
let t1 = t1?;

let Some(t2) = iter.next() else {
return Ok(f(&[t0, t1]));
};
let t2 = t2?;

let Some(t3) = iter.next() else {
return Ok(f(&[t0, t1, t2]));
};
let t3 = t3?;

let Some(t4) = iter.next() else {
return Ok(f(&[t0, t1, t2, t3]));
};
let t4 = t4?;

let Some(t5) = iter.next() else {
return Ok(f(&[t0, t1, t2, t3, t4]));
};
let t5 = t5?;

let Some(t6) = iter.next() else {
return Ok(f(&[t0, t1, t2, t3, t4, t5]));
};
let t6 = t6?;

let Some(t7) = iter.next() else {
return Ok(f(&[t0, t1, t2, t3, t4, t5, t6]));
};
let t7 = t7?;

let Some(t8) = iter.next() else {
return Ok(f(&[t0, t1, t2, t3, t4, t5, t6, t7]));
};
let t8 = t8?;

Ok(f(&[Ok(t0), Ok(t1), Ok(t2), Ok(t3), Ok(t4), Ok(t5), Ok(t6), Ok(t7), Ok(t8)]
.into_iter()
.chain(iter)
.collect::<Result<Vec<_>, _>>()?))
}
}

Expand Down
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