maciejhirsz / beef

Faster, more compact implementation of std::borrow::Cow

beef

Faster, more compact implementation of Cow.

Changelog - Documentation - Cargo - Repository

use beef::Cow;

let borrowed: Cow<str> = Cow::borrowed("Hello");
let owned: Cow<str> = Cow::owned(String::from("World"));

assert_eq!(
    format!("{} {}!", borrowed, owned),
    "Hello World!",
);

There are two versions of Cow exposed by this crate:

• beef::Cow is 3 words wide: pointer, length, and capacity. It stores the ownership tag in capacity.
• beef::lean::Cow is 2 words wide, storing length, capacity, and the ownership tag all in one word.

Both versions are leaner than the std::borrow::Cow:

use std::mem::size_of;

const WORD: usize = size_of::<usize>();

assert_eq!(size_of::<std::borrow::Cow<str>>(), 4 * WORD);
assert_eq!(size_of::<beef::Cow<str>>(), 3 * WORD);
assert_eq!(size_of::<beef::lean::Cow<str>>(), 2 * WORD);

How does it work?

The standard library Cow is an enum with two variants:

pub enum Cow<'a, B> where
    B: 'a + ToOwned + ?Sized,
{
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}

For the most common pairs of values - &str and String, or &[u8] and Vec<u8> - this means that the entire enum is 4 words wide:

                                                 Padding
                                                    |
                                                    v
          +-----------+-----------+-----------+-----------+
Borrowed: | Tag       | Pointer   | Length    | XXXXXXXXX |
          +-----------+-----------+-----------+-----------+

          +-----------+-----------+-----------+-----------+
Owned:    | Tag       | Pointer   | Length    | Capacity  |
          +-----------+-----------+-----------+-----------+

Instead of being an enum with a tag, beef::Cow uses capacity to determine whether the value it's holding is owned (capacity is greater than 0), or borrowed (capacity is 0).

beef::lean::Cow goes even further and puts length and capacity on a single 64 word.

                 +-----------+-----------+-----------+
beef::Cow        | Pointer   | Length    | Capacity? |
                 +-----------+-----------+-----------+

                 +-----------+-----------+
beef::lean::Cow  | Pointer   | Cap | Len |
                 +-----------+-----------+

Any owned Vec or String that has 0 capacity is effectively treated as a borrowed value. Since having no capacity means there is no actual allocation behind the pointer, this is safe.

Benchmarks

cargo +nightly bench

Microbenchmarking obtaining a &str reference is rather flaky and you can have widely different results. In general the following seems to hold true:

• beef::Cow and beef::lean::Cow are faster than std::borrow::Cow at obtaining a reference &T. This makes sense since we avoid the enum tag branching.
• The 3-word beef::Cow is faster at creating borrowed variants, but slower at creating owned variants than std::borrow::Cow.
• The 2-word beef::lean::Cow is faster at both.

running 9 tests
test beef_as_ref            ... bench:          57 ns/iter (+/- 15)
test beef_create            ... bench:         135 ns/iter (+/- 5)
test beef_create_mixed      ... bench:         659 ns/iter (+/- 52)
test lean_beef_as_ref       ... bench:          50 ns/iter (+/- 2)
test lean_beef_create       ... bench:          77 ns/iter (+/- 3)
test lean_beef_create_mixed ... bench:         594 ns/iter (+/- 52)
test std_as_ref             ... bench:          70 ns/iter (+/- 6)
test std_create             ... bench:         142 ns/iter (+/- 7)
test std_create_mixed       ... bench:         663 ns/iter (+/- 32)

License

This crate is distributed under the terms of both the MIT license and the Apache License (Version 2.0). Choose whichever one works best for you.

See LICENSE-APACHE and LICENSE-MIT for details.