diff: reuse precomputed hash values #4917
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Since patience diff is recursive, it makes some sense to reuse precomputed hash values. This patch migrates Histogram to remembering hashed values. The precomputed values will be cached globally by DiffSource. Technically, Histogram doesn't have to keep a separate copy of hash values, but this appears to give better perf than slicing text and hash value from two Vecs.
… index I'm going to add a Vec of precomputed hashes, and the Vec will be owned by DiffSource.
This isn't always fast because it increases the chance of cache miss, but in
practice, it makes "jj file annotate" faster. It's still slower than
"git blame", though.
Maybe we should also change the hash function.
```
group new old
----- --- ---
bench_diff_git_git_read_tree_c 1.00 45.2±0.38µs 1.29 58.4±0.32µs
bench_diff_lines/modified/10k 1.00 32.7±0.24ms 1.05 34.4±0.17ms
bench_diff_lines/modified/1k 1.00 2.9±0.00ms 1.06 3.1±0.01ms
bench_diff_lines/reversed/10k 1.00 22.7±0.18ms 1.02 23.2±0.29ms
bench_diff_lines/reversed/1k 1.00 439.0±9.46µs 1.19 523.9±6.05µs
bench_diff_lines/unchanged/10k 1.00 2.9±0.06ms 1.20 3.5±0.02ms
bench_diff_lines/unchanged/1k 1.00 240.8±1.03µs 1.30 312.1±1.05µs
```
```
% hyperfine --sort command --warmup 3 --runs 10 -L bin jj-0,jj-1 \
'target/release-with-debug/{bin} --ignore-working-copy file annotate lib/src/revset.rs'
Benchmark 1: target/release-with-debug/jj-0 ..
Time (mean ± σ): 1.604 s ± 0.259 s [User: 1.543 s, System: 0.057 s]
Range (min … max): 1.348 s … 1.917 s 10 runs
Benchmark 2: target/release-with-debug/jj-1 ..
Time (mean ± σ): 1.183 s ± 0.026 s [User: 1.118 s, System: 0.062 s]
Range (min … max): 1.155 s … 1.237 s 10 runs
```
PhilipMetzger
approved these changes
Nov 19, 2024
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CHANGELOG.md