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diff --git a/third_party/rust/copyless/README.md b/third_party/rust/copyless/README.md new file mode 100644 index 0000000000..8caf5448b6 --- /dev/null +++ b/third_party/rust/copyless/README.md @@ -0,0 +1,35 @@ +## copyless +[](https://travis-ci.org/kvark/copyless) +[](https://crates.io/crates/copyless) + +Rust abstractions can be zero cost in theory, but often reveal quite a few unnecessary `memcpy` calls in practice. This library provides a number of trait extensions for standard containers that expose API that is more friendly to LLVM optimization passes and doesn't end up with as many copies. + +It aims to accelerate [WebRender](https://github.com/servo/webrender) and [gfx-rs](https://github.com/gfx-rs/gfx). + +## Background + +The `memcpy` instructions showed in profiles of WebRender running in Gecko. @jrmuizel built a tool called [memcpy-find](https://github.com/jrmuizel/memcpy-find) that analyzes LLVM IR and spews out the call stacks that end up producing `memcpy` instructions. We figured out a way to convince the compiler to eliminate the copies. This library attempts to make these ways available to Rust ecosystem, at least until the compiler gets smart enough ;) + +## Here is a small example + +```rust +use copyless::BoxHelper; + +enum Foo { + Small(i8), + Big([f32; 100]), +} + +#[inline(never)] +fn foo() -> Box<Foo> { + Box::new(Foo::Small(4)) // this has 1 memcopy + //Box::alloc().init(Foo::Small(4)) // this has 0 memcopies +} + +fn main() { + let z = foo(); + println!("{:?}", &*z as *const _); +} +``` + +Playground [permalink](https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=579ab13345b1266752b1fa4400194cc7). |