Optimize VecDeque::append

[tabokie]

Optimize VecDeque::append to do unsafe copy rather than iterating through each element.

On my Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz, the benchmark shows 37% improvements:

Master:
custom-bench vec_deque_append 583164 ns/iter
custom-bench vec_deque_append 550040 ns/iter

Patched:
custom-bench vec_deque_append 349204 ns/iter
custom-bench vec_deque_append 368164 ns/iter

Additional notes on the context: this is the third attempt to implement a non-trivial version of VecDeque::append, the last two are reverted due to unsoundness or regression, see:

• Non-naive implementation of VecDeque.append #52553, reverted in Fix unsoundness for VecDeque #53571
• Reoptimize VecDeque::append #53564, reverted in Fix a regression in 1.30 by reverting #53564 #54851

Both cases are covered by existing tests.

Signed-off-by: tabokie xy.tao@outlook.com

[rust-highfive]

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

Please see the contribution instructions for more information.

[leonardo-m]

Is Miri silent?

[tabokie]

Is Miri silent?

Sorry I'm new here, are you referring to running ./x.py test src/tools/miri? If so the results look good.

[the8472]

I think that only runs the dedicated tests in the miri test directory. You can also run all alloc tests under miri. https://github.com/rust-lang/miri-test-libstd

[tabokie]

I think that only runs the dedicated tests in the miri test directory. You can also run all alloc tests under miri. https://github.com/rust-lang/miri-test-libstd

Passed.

[m-ou-se]

Thanks!

@bors r+

[bors]

📌 Commit a929e60 has been approved by m-ou-se

[tabokie]

Kindly ping @m-ou-se, do you know why bors hasn't attempted to merge this PR yet? 5 days of inactivity seems unusual. Is it because I appended one commit after your review?

[Mark-Simulacrum]

@bors r=m-ou-se

Yes, pushing code will invalidate the approved state.

[bors]

📌 Commit cd773c3 has been approved by m-ou-se

[matthiaskrgr]

@bors rollup=never likely has some perf effects

[bors]

⌛ Testing commit cd773c3 with merge af9b508...

[bors]

☀️ Test successful - checks-actions
Approved by: m-ou-se
Pushing af9b508 to master...

[rust-timer]

Finished benchmarking commit (af9b508): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression