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bors 9faa714154 Auto merge of #88075 - Xuanwo:vec_deque_retain, r=dtolnay 
Optimize unnecessary check in VecDeque::retain

This pr is highly inspired by https://github.com/rust-lang/rust/pull/88060 which shared the same idea: we can split the `for` loop into stages so that we can remove unnecessary checks like `del > 0`.

## Benchmarks

Before

```rust
test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     290,125 ns/iter (+/- 8,717)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     291,588 ns/iter (+/- 9,621)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     287,426 ns/iter (+/- 9,009)
```

After

```rust
test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     243,940 ns/iter (+/- 8,563)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     242,768 ns/iter (+/- 3,903)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     202,926 ns/iter (+/- 6,332)
```

Based on the current benchmark, this PR will improve the perf of `VecDeque::retain` by around 16%. For special cases, the improvement will be up to 30%.

Signed-off-by: Xuanwo <github@xuanwo.io>
2021-08-21 23:35:54 +00:00
..
alloc Rename AllocRef to Allocator and (de)alloc to (de)allocate 2020-12-04 14:47:15 +01:00
collections Auto merge of #88075 - Xuanwo:vec_deque_retain, r=dtolnay 2021-08-21 23:35:54 +00:00
prelude mv std libs to library/ 2020-07-27 19:51:13 -05:00
raw_vec Rename AllocRef to Allocator and (de)alloc to (de)allocate 2020-12-04 14:47:15 +01:00
rc Re-stabilize Weak::as_ptr &friends for unsized T 2021-01-06 19:30:22 -05:00
sync Rollup merge of #80764 - CAD97:weak-unsized-as-ptr-again, r=RalfJung 2021-01-16 17:29:56 +00:00
vec Constified Default implementations 2021-08-17 07:15:54 +00:00
alloc.rs alloc: Add unstable Cfg feature no-global_oom_handling 2021-05-05 16:49:04 -04:00
borrow.rs Added diagnostic items to structs and traits for Clippy 2021-07-15 23:57:02 +02:00
boxed.rs add Box::try_new_uninit_slice for symmetry 2021-08-05 21:21:52 +02:00
fmt.rs Fix may not to appropriate might not or must not 2021-07-29 01:15:20 -04:00
lib.rs Constified Default implementations 2021-08-17 07:15:54 +00:00
macros.rs Remove alloc/malloc/calloc/realloc doc aliases 2021-06-30 19:59:39 +01:00
raw_vec.rs Hide allocator details from TryReserveError 2021-07-24 22:25:08 +01:00
rc.rs Relocate Arc and Rc UnwindSafe impls 2021-07-31 03:57:49 -07:00
slice.rs Use HTTPS links where possible 2021-06-23 16:26:46 -04:00
str.rs Use HTTPS links where possible 2021-06-23 16:26:46 -04:00
string.rs Constified Default implementations 2021-08-17 07:15:54 +00:00
sync.rs Relocate Arc and Rc UnwindSafe impls 2021-07-31 03:57:49 -07:00
task.rs Document From implementations for Waker and RawWaker 2021-04-22 14:16:33 -07:00
tests.rs Fix alloc::test::test_show 2021-04-21 15:45:41 +02:00