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Auto merge of #98755 - nnethercote:faster-vec-insert, r=cuviper

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Optimize `Vec::insert` for the case where `index == len`.

By skipping the call to `copy` with a zero length. This makes it closer
to `push`.

I did this recently for `SmallVec`
(https://github.com/servo/rust-smallvec/pull/282) and it was a big perf win in
one case. Although I don't have a specific use case in mind, it seems
worth doing it for `Vec` as well.

Things to note:
- In the `index < len` case, the number of conditions checked is
  unchanged.
- In the `index == len` case, the number of conditions checked increases
  by one, but the more expensive zero-length copy is avoided.
- In the `index > len` case the code now reserves space for the extra
  element before panicking. This seems like an unimportant change.

r? `@cuviper`
This commit is contained in:
bors 2022-07-03 09:36:37 +00:00
commit f99f9e48ed
1 changed files with 9 additions and 6 deletions
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15
library/alloc/src/vec/mod.rs
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@ -1379,9 +1379,6 @@ impl<T, A: Allocator> Vec<T, A> {
}
let len = self.len();
if index > len {
assert_failed(index, len);
}
// space for the new element
if len == self.buf.capacity() {
@ -1393,9 +1390,15 @@ impl<T, A: Allocator> Vec<T, A> {
// The spot to put the new value
{
let p = self.as_mut_ptr().add(index);
// Shift everything over to make space. (Duplicating the
// `index`th element into two consecutive places.)
ptr::copy(p, p.offset(1), len - index);
if index < len {
// Shift everything over to make space. (Duplicating the
// `index`th element into two consecutive places.)
ptr::copy(p, p.offset(1), len - index);
} else if index == len {
// No elements need shifting.
} else {
assert_failed(index, len);
}
// Write it in, overwriting the first copy of the `index`th
// element.
ptr::write(p, element);