Optimize unnecessary check in VecDeque::retain

[Xuanwo]

This pr is highly inspired by #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

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

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

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

Please see the contribution instructions for more information.

[TennyZhuang]

I ran the benchmark on my Mac

sysctl -n machdep.cpu.brand_string
Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz

Before optimization:

test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     397,713 ns/iter (+/- 19,098)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     382,615 ns/iter (+/- 17,406)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     386,554 ns/iter (+/- 11,450)

After optimization:

test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     318,491 ns/iter (+/- 34,193)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     354,253 ns/iter (+/- 21,848)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     288,971 ns/iter (+/- 16,016)

[dtolnay]

Nice!

[dtolnay]

@bors r+ rollup=never

[bors]

📌 Commit e32f4c0 has been approved by dtolnay

[bors]

⌛ Testing commit e32f4c0 with merge 9faa714...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 9faa714 to master...