Rollup of 6 pull requests #70800
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Rollup of 6 pull requests #70800
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The field are always re-ordered to minimize padding, regardless of the alignment of the discriminant
This commit changes some usage of mem::forget into mem::ManuallyDrop in some Vec, VecDeque, BTreeMap and Box methods. Before the commit, the generated IR for some of the methods was longer, and even after optimization, some unwinding artifacts were still present.
Was removed in 51938c6
rustc_target: Some cleanup to `no_default_libraries`
Do not disable field reordering on enums with big discriminant The field are always re-ordered to minimize padding, regardless of the alignment of the discriminant (spinoff from rust-lang#70477)
Add slice::fill Adds the `slice::fill` method to fill a slice with an item. This replaces manual for loops where items are copied one-by-one. This is a counterpart to C++20's [`std::fill`](https://en.cppreference.com/w/cpp/algorithm/fill) function. ## Usage ```rust let mut buf = vec![0; 10]; buf.fill(1); assert_eq!(buf, vec![1; 10]); ``` ## Performance When compiling in release mode, for `[u8]` and `[u16]` this method will optimize to a `memset(3)` call ([godbolt](https://godbolt.org/z/85El_c)). The initial implementation relies on LLVM's optimizer to make it as fast as possible for any given input. But as @jonas-schievink [pointed out](https://twitter.com/sheevink/status/1245756597453885442) this can later be optimized through specialization to guarantee it has a specific performance profile. ## Why now? Conversations about adding `slice::fill` are not new. In fact, rust-lang/rfcs#2067 was opened 3 years ago about this exact topic. However discussion stranded while discussing implementation details, and it's not seen much forward motion since. In ["The Hunt for the Fastest Zero"](https://travisdowns.github.io/blog/2020/01/20/zero.html) Travis Downs provides disects C++'s `std::fill` performance profile on gcc, comparing it among others to `memset(3)`. Even though `memset(3)` outperforms `std::fill` in their tests, the author notes the following: > That the optimization fails, perhaps unexpectedly, in some cases is unfortunate but it’s nice that you can fix it yourself. [...] Do we throw out modern C++ idioms, at least where performance matters, for example by replacing std::fill with memset? I don’t think so. Much of the article focuses on how how to fix the performance of `std::fill` by providing specializations for specific input. In Rust we don't have any dedicated methods to fill slices with values, so it either needs to be optimized at the MIR layer, or more likely rely on LLVM's optimizer. By adding a dedicated method for filling slices with values it opens up the ability for us to in the future guarantee that e.g. `Vec<u8>` will always optimize to `memset` even in debug mode. Or perhaps provide stronger guarantees about memory when zeroing values when a certain flag is passed. But regardless of that, it improves general ergonomics of working with slices by providing a dedicated method with documentation and examples. ## References - [slice-fill prototype on docs.rs](https://docs.rs/slice-fill/1.0.1/slice_fill/) - [The Hunt For The Fastest Zero](https://travisdowns.github.io/blog/2020/01/20/zero.html) - [Safe memset for slices](rust-lang/rfcs#2067) - [C++20 std::fill](https://en.cppreference.com/w/cpp/algorithm/fill) - [ASM output on Godbolt](https://godbolt.org/z/5-XU66)
…Mark-Simulacrum,RalfJung use ManuallyDrop instead of forget inside collections This PR changes some usage of `mem::forget` into `mem::ManuallyDrop` in some `Vec`, `VecDeque`, `BTreeMap` and `Box` methods. Before the commit, the generated IR for some of the methods was longer, and even after optimization, some unwinding artifacts were still present.
…rk-i-m macro_rules: `NtLifetime` cannot start with an identifier Fixes rust-lang#70446
comment refers to removed type Was removed in 51938c6
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@bors r+ rollup=never p=6 |
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📌 Commit 5c42faf has been approved by |
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⌛ Testing commit 5c42faf with merge c0ee793d85cb4c18a9df01307f632a478385a9d7... |
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💔 Test failed - checks-azure |
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@bors retry |
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☀️ Test successful - checks-azure |
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Apr 5, 2020
Tested on commit rust-lang/rust@7b657d3. Direct link to PR: <rust-lang/rust#70800> 💔 rls on linux: test-pass → test-fail (cc @Xanewok).
This was referenced Apr 5, 2020
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Successful merges:
no_default_libraries#70635 (rustc_target: Some cleanup tono_default_libraries)NtLifetimecannot start with an identifier #70768 (macro_rules:NtLifetimecannot start with an identifier)Failed merges:
r? @ghost