Add shortcircuit in iteration if we yielded all elements

[AngelicosPhosphoros]

Current implementation works little slower than set.iter().take(set.len()).
See my comment here.

So why not avoid extra integer which added by Iterator::take if we can add limiting logic into our iterator itself?

I don't really know how this change affects reflect_toogle_full and implementation of FusedIterator. Maybe I should make inner iterator "jump" to the end of its memory block?

[Amanieu]

I do have some concerns:

1. The comment just below your change becomes incorrect: items can no longer be eliminated by the compiler since it is now used to check for termination.
2. Since the item count is now being checked, the check against end can be eliminated from RawIterRange, perhaps via a new next_unchecked method.

[AngelicosPhosphoros]

OK, I would fix improve my PR tomorrow.

[AngelicosPhosphoros]

@Amanieu I have a question about this lines:
https://github.com/rust-lang/hashbrown/blob/3dbcdcce09028b39e1640f8a6e4fc8e69a372844/src/raw/mod.rs#L1945..L1952

What this comment means and what should be here in version without end pointer check?

[Amanieu]

You should leave this code and comment as it is. The only change needed is removing the check against self.end above.

[AngelicosPhosphoros]

Updated.

I decided to remove check using constant flag, compiler should remove checks during compilation.

[Amanieu]

Nice work! Have you tried running the benchmarks to compare performance before/after?

[AngelicosPhosphoros]

I forgot :D

Reminder: this is the bench for almost empty hash sets.

HashTableIteration/HashTableIterationFull/64
                        time:   [136.94 ns 138.14 ns 139.48 ns]
                        change: [-31.564% -29.632% -27.330%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  4 (4.00%) high mild
  5 (5.00%) high severe
Benchmarking HashTableIteration/HashTableIterationFull/256: Collecting 100 samples in estimated 5.0065 s (3.5M iteration                                                                                                                        HashTableIteration/HashTableIterationFull/256
                        time:   [275.24 ns 275.67 ns 276.16 ns]
                        change: [-26.684% -25.640% -24.632%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) high mild
  3 (3.00%) high severe
Benchmarking HashTableIteration/HashTableIterationFull/1024: Collecting 100 samples in estimated 5.0015 s (1.2M iteratio                                                                                                                        HashTableIteration/HashTableIterationFull/1024
                        time:   [516.69 ns 517.95 ns 519.52 ns]
                        change: [-13.304% -12.422% -11.469%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  2 (2.00%) high mild
  4 (4.00%) high severe
Benchmarking HashTableIteration/HashTableIterationFull/4096: Collecting 100 samples in estimated 5.0622 s (394k iteratio                                                                                                                        HashTableIteration/HashTableIterationFull/4096
                        time:   [641.24 ns 642.72 ns 644.54 ns]
                        change: [-32.432% -31.616% -30.706%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  2 (2.00%) high mild
  12 (12.00%) high severe
Benchmarking HashTableIteration/HashTableIterationFull/16384: Collecting 100 samples in estimated 5.0527 s (141k iterati                                                                                                                        HashTableIteration/HashTableIterationFull/16384
                        time:   [1.2190 us 1.2247 us 1.2317 us]
                        change: [-59.855% -59.188% -58.579%] (p = 0.00 < 0.05)
                        Performance has improved.
Benchmarking HashTableIteration/HashTableIterationLimited/64: Collecting 100 samples in estimated 5.0011 s (4.1M iterati                                                                                                                        HashTableIteration/HashTableIterationLimited/64
                        time:   [114.96 ns 115.14 ns 115.34 ns]
                        change: [-40.720% -39.669% -38.076%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
  4 (4.00%) high mild
  4 (4.00%) high severe
Benchmarking HashTableIteration/HashTableIterationLimited/256: Collecting 100 samples in estimated 5.0077 s (2.8M iterat                                                                                                                        HashTableIteration/HashTableIterationLimited/256
                        time:   [261.74 ns 266.07 ns 270.04 ns]
                        change: [-28.749% -27.626% -26.307%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 20 outliers among 100 measurements (20.00%)
  14 (14.00%) high mild
  6 (6.00%) high severe
Benchmarking HashTableIteration/HashTableIterationLimited/1024: Collecting 100 samples in estimated 5.0066 s (1.1M itera                                                                                                                        HashTableIteration/HashTableIterationLimited/1024
                        time:   [508.83 ns 510.89 ns 513.35 ns]
                        change: [-13.444% -12.164% -10.693%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 11 outliers among 100 measurements (11.00%)
  5 (5.00%) high mild
  6 (6.00%) high severe
Benchmarking HashTableIteration/HashTableIterationLimited/4096: Collecting 100 samples in estimated 5.0214 s (667k itera                                                                                                                        HashTableIteration/HashTableIterationLimited/4096
                        time:   [640.38 ns 650.84 ns 663.10 ns]
                        change: [-32.123% -30.388% -28.722%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 11 outliers among 100 measurements (11.00%)
  5 (5.00%) high mild
  6 (6.00%) high severe
Benchmarking HashTableIteration/HashTableIterationLimited/16384: Collecting 100 samples in estimated 5.0954 s (217k iter                                                                                                                        HashTableIteration/HashTableIterationLimited/16384
                        time:   [1.1984 us 1.2041 us 1.2109 us]
                        change: [-60.842% -59.932% -58.933%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  12 (12.00%) high mild
  2 (2.00%) high severe

[Amanieu]

@bors r+

[bors]

📌 Commit ea120c7 has been approved by Amanieu

[bors]

⌛ Testing commit ea120c7 with merge d11a701...

[bors]

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing d11a701 to master...