andygrove opened a new pull request, #4903:
URL: https://github.com/apache/datafusion-comet/pull/4903
> This PR was created by an LLM as a draft PR. I will mark it as ready for
review after human review.
## Which issue does this PR close?
N/A — autonomous exploratory PR.
## Rationale for this change
Replaced Arrow's substring_by_char with an implementation that skips the
per-element chars().count() (unused for non-negative starts) and adds a bulk
ASCII fast path that slices windows by byte offset without decoding UTF-8.
## What changes are included in this PR?
Replaced Arrow's substring_by_char with an implementation that skips the
per-element chars().count() (unused for non-negative starts) and adds a bulk
ASCII fast path that slices windows by byte offset without decoding UTF-8.
## How are these changes tested?
Correctness: unit tests + seeded differential fuzz (bit-identical Arrow
output vs `main`).
Benchmark (criterion):
- substring_ long ascii tail: 90.315% faster (base 1049866ns -> cand
101681ns)
- substring_ long ascii prefix: 45.268% faster (base 161701ns -> cand
88502ns)
- substring_ short ascii: 69.169% faster (base 89039ns -> cand 27451ns)
- substring_ long utf8 tail: 34.768% faster (base 3226354ns -> cand
2104613ns)
- substring_ long utf8 prefix: 63.96% faster (base 365291ns -> cand 131650ns)
Full criterion output:
```text
substring: short ascii time: [27.371 µs 27.413 µs 27.457 µs]
change: [−69.325% −69.169% −69.016%] (p = 0.00 <
0.05)
Performance has improved.
Found 10 outliers among 100 measurements (10.00%)
6 (6.00%) low mild
2 (2.00%) high mild
2 (2.00%) high severe
substring: long ascii prefix
time: [88.107 µs 88.511 µs 88.914 µs]
change: [−45.520% −45.268% −45.006%] (p = 0.00 <
0.05)
Performance has improved.
substring: long ascii tail
time: [101.42 µs 101.97 µs 102.50 µs]
change: [−90.371% −90.315% −90.260%] (p = 0.00 <
0.05)
Performance has improved.
substring: long utf8 prefix
time: [131.49 µs 131.61 µs 131.77 µs]
change: [−64.048% −63.960% −63.868%] (p = 0.00 <
0.05)
Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
3 (3.00%) high mild
6 (6.00%) high severe
substring: long utf8 tail
time: [2.1015 ms 2.1046 ms 2.1087 ms]
change: [−34.887% −34.768% −34.620%] (p = 0.00 <
0.05)
Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
1 (1.00%) low mild
3 (3.00%) high mild
9 (9.00%) high severe
```
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