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Claude
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Attempt 4 (phase0-tokei-002, class 3, ACCEPTED — first auto-accept): every file was read through encoding_rs_io's DecodeReaderBytes via generic read_to_end, which zero-fills every grown buffer chunk (~221MB memset/run, 8.5% of Ir) and grows without a size hint. Patch: fs::read with a BOM-gated fallback to the identical decode path (a UTF-8-BOM file transcodes invalid sequences, so the fallback is required for equivalence). Gates: golden byte-identical; differential fuzz old-vs- new binaries 4,332 iterations (BOM grid, invalid UTF-8, binary junk, randomized) zero divergence; 250 upstream tests green; ASan+LSan clean with ASan-run output hashing to golden. Bench: +9.9% [8.0%, 12.7%]. Attempt 5 (phase0-tokei-003, ship tree, ACCEPTED): stacks fat LTO+cgu=1 (real +1.6% standalone but under threshold, phase0-tokei-001) on the accepted patch; benched as a whole vs pristine: +10.4% [8.5%, 12.0%]. Median clears the Phase 0 exit bar; CI lower bound 8.5%. Case study written (results/phase0/case-study-tokei.md), ledger at 5 attempts, CLAUDE.md phase line moved to Phase 1, next hypotheses logged (tokei class-5 byte-skip table; comrak class-3 arena). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2
80 lines
3.7 KiB
Markdown
80 lines
3.7 KiB
Markdown
# Phase 0 case study — tokei (source-code statistics)
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**Result: +10.4% median wall-time speedup (95% CI [+8.5%, +12.0%]) vs
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pristine, byte-identical output, 4,332-iteration differential fuzz
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clean, sanitizers clean, auto-accepted. Phase 0 exit criterion
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(≥1 verified win ≥10%) met by the median; the CI lower bound is 8.5%.**
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## Target & workload
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- tokei v14.0.0, commit `fa44e519`, MIT OR Apache-2.0.
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- Workload: 8 replicas of two pinned trees (progit @ `61833a52`,
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comrak src @ `45c1995f`), 221 MB / 4,360 files, `--sort code`,
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`RAYON_NUM_THREADS=1` (measurement first; multithreaded scaling is a
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class-7 concern). Corpus pinned: `corpora/tokei/MANIFEST.sha256`.
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- Environment: as case-study-comrak.md (pinned core, interleaved ABAB,
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30+3 runs/side, bootstrap 95% CI; A/A null [0.9891, 1.0118]).
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## The find (profile → hypothesis)
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callgrind put **8.5% of instructions in `__memset_avx2`** with heavy
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malloc/memcpy alongside — odd for a tool that just reads and scans
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files. Reading `LanguageType::parse` explained it: every file is read
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through `encoding_rs_io::DecodeReaderBytes` via generic `read_to_end`.
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That reader has no `read_buf` specialization, so std **zero-fills every
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grown buffer chunk before the read lands** (~221 MB of memset per run),
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and the Vec grows without a file-size hint (realloc + memcpy churn).
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The decoding wrapper only *does* anything when the file starts with a
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BOM — for BOM-less input it is byte-for-byte passthrough.
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## The patch (results/phase0/tokei-readpath.patch, ~20 lines)
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`fs::read` (pre-sized from metadata, `read_buf`-specialized, no
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zeroing) for every file; files starting with a UTF-8/UTF-16 BOM fall
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back to the *original* `DecodeReaderBytes` machinery over the same
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bytes. Equivalence argument: encoding_rs_io BOM-sniffs the stream head;
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identical bytes in → identical bytes out on both paths. The subtle case
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that forbids skipping the fallback: a UTF-8-BOM file is actually
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transcoded (invalid sequences → U+FFFD), unlike BOM-less input.
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## Gates
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- Golden replay: stdout sha256 identical to pinned golden.
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- **Differential fuzz, old binary vs new binary**: crafted grid (7 BOM
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prefixes × 12 bodies × 2 extensions — including invalid UTF-8,
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UTF-16LE/BE, truncated BOMs, binary junk, 70 KB single lines,
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unterminated strings/comments) plus randomized cases; **4,332
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iterations / >90 s, zero divergence** in either default or JSON
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output.
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- Upstream test suite: 250 tests green on the patched tree.
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- ASan + LSan (nightly) over the full 221 MB corpus: clean; the
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ASan-instrumented binary's output hashes to golden.
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## Bench (ledger rows phase0-tokei-002 / -003)
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| candidate | median speedup | 95% CI | verdict |
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|---|---|---|---|
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| read-path patch alone | 1.0990 | [1.0797, 1.1271] | accepted |
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| + fat LTO, cgu=1 (ship tree) | **1.1036** | **[1.0852, 1.1201]** | accepted |
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Fat LTO alone had measured a real but sub-threshold +1.6% [1.0026,
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1.0262] (phase0-tokei-001, rejected standalone). The ship tree stacks
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it on the accepted patch and is benched as a whole against the pristine
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baseline — the bench measures exactly what ships.
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## ROI (illustrative, config/pricing.toml rates)
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Hypothetical 100-core 24/7 fleet on this workload shape at $0.04/core-hr:
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cores returned median **9.4**, 95% CI [7.9, 10.7]; dollars/yr median
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**$3,290**, 95% CI [$2,750, $3,760]. Caveats: single workload shape
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(cold-ish batch scan of a large mixed tree, warm page cache, single
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thread); Ir-profile prioritization; container hardware, no governor
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control; results include the codegen (LTO) layer which is
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toolchain-version-sensitive (rustc 1.94.1).
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## Upstreamability
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The read-path change is a candidate for an upstream tokei PR
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(BOM-gated fast path, no behavior change, 250 tests + fuzz evidence).
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Not filed from this session; listed as follow-up.
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