perf(common): optimize BitmapIter

[kwannoel]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

• Before this PR, bitmap iter is shown to be expensive: Tracking: optimize HashAggExecutor #7792 (comment)
• In this PR, we call it once for each usize, copy it into the iterator, and reference that instead.
• Save division calls by a factor of usize.
• Save calls to get_unchecked by a factor of usize.
• Incur cost of copy usize on to stack for caching reads from memory
• Incur cost of conditional check of whether to fetch usize from memory or use the one cached.
• Also fix benchmark inaccuracies.

Overall maybe save about 1% 🤣
Saves about 17% for bitmap iter itself. See below: #8848 (comment).

Checklist For Contributors

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• I have demonstrated that backward compatibility is not broken by breaking changes and created issues to track deprecated features to be removed in the future. (Please refer to the issue)
• All checks passed in ./risedev check (or alias, ./risedev c)

Checklist For Reviewers

• I have requested macro/micro-benchmarks as this PR can affect performance substantially, and the results are shown.

Documentation

• My PR DOES NOT contain user-facing changes.

Click here for Documentation

Types of user-facing changes

Please keep the types that apply to your changes, and remove the others.

• Installation and deployment
• Connector (sources & sinks)
• SQL commands, functions, and operators
• RisingWave cluster configuration changes
• Other (please specify in the release note below)

Release note

[kwannoel]

Before:

noelkwan@Noels-MacBook-Pro benches % cargo bench --bench bitmap -- zeros_iter
   Compiling risingwave_common v0.2.0-alpha (/Users/noelkwan/projects/risingwave/src/common)
    Finished bench [optimized] target(s) in 22.16s
     Running benches/bitmap.rs (/Users/noelkwan/projects/risingwave/target/release/deps/bitmap-1b7d7d1feebf7e97)
zeros_iter              time:   [5.9332 ms 5.9379 ms 5.9430 ms]
                        change: [-0.5865% -0.4578% -0.3226%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 15 outliers among 100 measurements (15.00%)
  11 (11.00%) high mild
  4 (4.00%) high severe

After:

noelkwan@Noels-MacBook-Pro benches % cargo bench --bench bitmap -- zeros_iter
   Compiling risingwave_common v0.2.0-alpha (/Users/noelkwan/projects/risingwave/src/common)
    Finished bench [optimized] target(s) in 22.14s
     Running benches/bitmap.rs (/Users/noelkwan/projects/risingwave/target/release/deps/bitmap-1b7d7d1feebf7e97)
zeros_iter              time:   [4.9172 ms 4.9247 ms 4.9327 ms]
                        change: [-18.040% -17.891% -17.733%] (p = 0.00 < 0.05)
                        Performance has improved.

[wangrunji0408]

LGTM!

[wangrunji0408]

The code looks correct to me. I can't tell why the test failed. 🥵

[wangrunji0408]

We should reload current_usize here, otherwise the cache would be stale.

    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        self.idx += n;
        self.next()
    }

[codecov]

Codecov Report

Merging #8848 (c1bf0a1) into main (a4a46fd) will decrease coverage by 0.01%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##             main    #8848      +/-   ##
==========================================
- Coverage   70.80%   70.80%   -0.01%     
==========================================
  Files        1172     1172              
  Lines      194525   194554      +29     
==========================================
+ Hits       137733   137747      +14     
- Misses      56792    56807      +15     

Flag	Coverage Δ
rust	70.80% <96.87%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/common/src/buffer/bitmap.rs	96.66% <96.87%> (-0.01%)	⬇️

... and 4 files with indirect coverage changes

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