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Claude
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phase3-cjson-002 (class 5+3): stack-buffer number parsing + strtod round-trip replacing sscanf. +8.85% median, 95% CI [+7.5%, +10.0%] on the C target, banked. 100% audit: byte-identical golden replay, independent ASan+UBSan clean, 10k-input differential fuzz 0 divergences, patch bounds/free-gating verified by eye. - verdict: an infrastructure sanitizer-BUILD failure (missing runtime, broken toolchain) now caps the accept at needs-human-review and logs it, instead of crashing the verdict and losing a measured bench. - results/phase3/: four public case studies (tokei class-5 wins; build flags + allocators; the comrak caught false-accept; the cJSON cross-language proof) + README. Compound-speedup claim stated as an approximation, not a measured figure (CIs don't multiply). - .gitignore: cjson build-artifact dirs. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2
87 lines
4.2 KiB
Markdown
87 lines
4.2 KiB
Markdown
# Case study 4 — cJSON: the cross-language proof (C, classes 3 + 5)
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**Target:** [cJSON](https://github.com/DaveGamble/cJSON) v1.7.19 — an
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ultralightweight C JSON parser/printer, MIT. **Workload:** a 5.6 MB
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deterministic JSON corpus (20k records: nested objects/arrays,
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escaped/unicode strings, integers, dense floats), parsed and serialized,
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single-threaded, CPU-pinned. **Result:** the first verified win on a
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C/C++ target — proof the trust layer, the gates, and the ROI story
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carry across languages, not just across Rust crates.
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## Getting C through the same pipeline
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Nothing in the trust layer assumes cargo anymore. cJSON is "just a
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`target.toml`" pointing at `clang` build commands, with:
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- a **hermetic Unity/CTest suite** (19 tests: `parse_number`,
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`parse_string`, `print_*`, `misc_tests`, …) as the upstream-test gate;
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- a **trust-layer harness driver** (`targets/cjson/harness.c`) compiled
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*with* the patched `cJSON.c` — the agent can patch the parser but not
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the measurement rig (it lives outside the workspace allowlist);
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- **ASan + UBSan** as the sanitizer gate (the C analogue of the Rust
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nightly ASan path);
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- its own **A/A calibration**: 0/20 false positives, 20/20
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injected-slowdown detections — the C workload is a trusted anchor
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before any verdict runs.
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## The profile
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callgrind cache-sim put **~22% of instructions in glibc float I/O** —
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`__printf_fp` (number serialization) and `strtod`/`str_to_mpn` (number
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parsing) — plus `parse_value`, `parse_string`, and heavy malloc/free
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churn. A number-dense JSON document spends most of its time turning
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numbers into strings and back.
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## Attempt 1 — the honest rejection (`phase3-cjson-001`, class 3)
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`parse_number` heap-allocated a temporary NUL-terminated buffer
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(`malloc` + `memcpy` + `free`) for **every** number, just to hand a
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clean string to `strtod`. With ~100k+ numbers in the corpus that is a
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large share of the allocation churn. The fix: a 64-byte **stack buffer**
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for the common short-number case, heap only for pathologically long
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number strings — byte-identical (strtod sees the same bytes).
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- **Speedup: 1.012 median, 95% CI [1.004, 1.022]** → **rejected-bench.**
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A real mechanism, a real +1.2%, but the CI lower bound (+0.4%) does not
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clear the 2% bar — because the corpus's wall-clock is dominated by float
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*formatting*, not by malloc. The statistical bar rejected a genuine win,
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exactly as designed. It is in the ledger.
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## Attempt 2 — the accepted win (`phase3-cjson-002`, classes 5 + 3)
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Adding the class-5 lever: `print_number` verified every fractional
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number's `"%1.15g"` round-trip by calling **`sscanf("%lg")`** — a full
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`strtod` *plus* format-string interpretation, per number. Replacing it
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with a direct `strtod` is the identical parse with none of the
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format-string overhead. Combined with the stack buffer:
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- **Speedup: 1.089 median, 95% CI [1.075, 1.100]** → **accepted.**
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- Upstream Unity suite: **PASS**. Golden replay: **byte-identical**.
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- **ASan + UBSan: clean** — through the machine-enforced sanitizer gate.
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- **10,000-input differential fuzz** (`scripts/diff-fuzz-cjson.py`),
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pristine vs candidate, **0 divergences**.
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A **+8.9%** verified win on a C target, banked.
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## A finding worth keeping
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The first cJSON accept nearly wasn't recorded: the ASan+UBSan build
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failed to link because the clang sanitizer runtime (`libclang_rt.asan`)
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was not installed, and `sanitizer_check` treated that as a hard error —
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crashing the verdict and losing the measured bench. The fix hardened the
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gate: an **infrastructure** build failure (missing runtime, broken
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toolchain) now caps the accept at `needs-human-review` and logs the
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failure for the operator, rather than discarding a real result. "Cannot
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verify" is a review tier, not a crash.
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## What it demonstrates
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The progression — a real win *rejected* for being too small, then a
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larger win *accepted* only after golden replay, fuzzing, and sanitizers
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all held — is the whole method in miniature, now shown on C. Nothing
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about the trust story is Rust-specific. The cheapest safe lever (skip a
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per-number allocation) wasn't enough; the algorithmic lever (stop doing a
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redundant heavyweight parse) was — and the pipeline proved it the same
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way it proves everything else.
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