From 679c5ee244f997a563af45e335681e16fc089b34 Mon Sep 17 00:00:00 2001 From: Nicholas Nethercote Date: Fri, 1 Jul 2022 06:46:30 +1000 Subject: [PATCH] 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. --- library/alloc/src/vec/mod.rs | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs index e25f98d8aa695..b2f72d1d32a8d 100644 --- a/library/alloc/src/vec/mod.rs +++ b/library/alloc/src/vec/mod.rs @@ -1379,9 +1379,6 @@ impl Vec { } 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 Vec { // 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);