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The memcpy optimization PR was merged

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Philipp Oppermann
2020-10-31 17:53:22 +01:00
parent c175d25048
commit 352ba47971
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blog/content/edition-2/posts/02-minimal-rust-kernel/index.md
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@@ -365,14 +365,13 @@ cargo build --target x86_64-blog_os.json -Z build-std=core \
(Support for the `compiler-builtins-mem` feature was only [added very recently](https://github.com/rust-lang/rust/pull/77284), so you need at least Rust nightly `2020-09-30` for it.)
Behind the scenes, the new flag enables the [`mem` feature] of the `compiler_builtins` crate. The effect of this is that the `#[no_mangle]` attribute is applied to the [`memcpy` etc. implementations] of the crate, which makes them available to the linker. It's worth noting that these functions are [not optimized] right now, so their performance might not be the best, but at least they are correct. For `x86_64`, there is an open pull request to [optimize these functions using special assembly instructions][memcpy rep movsb].
Behind the scenes, the new flag enables the [`mem` feature] of the `compiler_builtins` crate. The effect of this is that the `#[no_mangle]` attribute is applied to the [`memcpy` etc. implementations] of the crate, which makes them available to the linker. It's worth noting that these functions are already optimized using [inline assembly] on `x86_64`, so their performance should be much better than a custom loop-based implementation.
[`mem` feature]: https://github.com/rust-lang/compiler-builtins/blob/eff506cd49b637f1ab5931625a33cef7e91fbbf6/Cargo.toml#L54-L55
[`memcpy` etc. implementations]: https://github.com/rust-lang/compiler-builtins/blob/eff506cd49b637f1ab5931625a33cef7e91fbbf6/src/mem.rs#L12-L69
[not optimized]: https://github.com/rust-lang/compiler-builtins/issues/339
[memcpy rep movsb]: https://github.com/rust-lang/compiler-builtins/pull/365
[inline assembly]: https://doc.rust-lang.org/unstable-book/library-features/asm.html
With this additional flag, our kernel has valid implementations for all compiler-required functions, so it will continue to compile even if our code gets more complex.
With the additional `compiler-builtins-mem` flag, our kernel has valid implementations for all compiler-required functions, so it will continue to compile even if our code gets more complex.
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