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Faster insert for the index == len case #282

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merged 2 commits into from Jun 24, 2022
Merged

Faster insert for the index == len case #282

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016e71d
Optimize `insert` for the case where `index == len`.
nnethercote Jun 23, 2022
0a6c208
Add a benchmark for the insert case where `index == len`.
nnethercote Jun 23, 2022
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19 changes: 19 additions & 0 deletions benches/bench.rs
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Expand Up @@ -96,6 +96,8 @@ make_benches! {
SmallVec<[u64; VEC_SIZE]> {
bench_push => gen_push(SPILLED_SIZE as _),
bench_push_small => gen_push(VEC_SIZE as _),
bench_insert_push => gen_insert_push(SPILLED_SIZE as _),
bench_insert_push_small => gen_insert_push(VEC_SIZE as _),
bench_insert => gen_insert(SPILLED_SIZE as _),
bench_insert_small => gen_insert(VEC_SIZE as _),
bench_remove => gen_remove(SPILLED_SIZE as _),
Expand All @@ -118,6 +120,8 @@ make_benches! {
Vec<u64> {
bench_push_vec => gen_push(SPILLED_SIZE as _),
bench_push_vec_small => gen_push(VEC_SIZE as _),
bench_insert_push_vec => gen_insert_push(SPILLED_SIZE as _),
bench_insert_push_vec_small => gen_insert_push(VEC_SIZE as _),
bench_insert_vec => gen_insert(SPILLED_SIZE as _),
bench_insert_vec_small => gen_insert(VEC_SIZE as _),
bench_remove_vec => gen_remove(SPILLED_SIZE as _),
Expand Down Expand Up @@ -151,6 +155,21 @@ fn gen_push<V: Vector<u64>>(n: u64, b: &mut Bencher) {
});
}

fn gen_insert_push<V: Vector<u64>>(n: u64, b: &mut Bencher) {
#[inline(never)]
fn insert_push_noinline<V: Vector<u64>>(vec: &mut V, x: u64) {
vec.insert(x as usize, x);
}

b.iter(|| {
let mut vec = V::new();
for x in 0..n {
insert_push_noinline(&mut vec, x);
}
vec
});
}

fn gen_insert<V: Vector<u64>>(n: u64, b: &mut Bencher) {
#[inline(never)]
fn insert_noinline<V: Vector<u64>>(vec: &mut V, p: usize, x: u64) {
Expand Down
13 changes: 9 additions & 4 deletions src/lib.rs
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Expand Up @@ -1068,17 +1068,22 @@ impl<A: Array> SmallVec<A> {

/// Insert an element at position `index`, shifting all elements after it to the right.
///
/// Panics if `index` is out of bounds.
/// Panics if `index > len`.
pub fn insert(&mut self, index: usize, element: A::Item) {
self.reserve(1);

unsafe {
let (mut ptr, len_ptr, _) = self.triple_mut();
let len = *len_ptr;
assert!(index <= len);
*len_ptr = len + 1;
ptr = ptr.add(index);
ptr::copy(ptr, ptr.add(1), len - index);
if index < len {
ptr::copy(ptr, ptr.add(1), len - index);
} else if index == len {
// No elements need shifting.
} else {
panic!("index exceeds length");
}
*len_ptr = len + 1;
ptr::write(ptr, element);
}
}
Expand Down