[gunwooterry]

TL;DR

• Reduce predictor-range checking overhead in opt-in HEVC decoding on AVX2 x86-64 CPUs, including Main10.

Background

The scalar predictor-domain pass checks every active sample before combination. This is a distinct follow-up to safe row processing. Refs #22.

What changes

Decoded samples and invalid-input errors remain the same. Default builds still exclude HEVC.

• Covered: Predictor-range validation and differential invalid-input coverage.
• Not covered: Combination arithmetic, interpolation, threading and other architectures. Expanding the existing fuzz fault-side selector is tracked separately; both-predictor rejection is covered by unit and independent differential checks.

After merging: None.

Decisions

• Enter the vector implementation once for both predictor blocks rather than dispatching each row, to avoid per-row call overhead; retain scalar tails and the original scalar fallback loops.
• Use signed comparisons with the existing inclusive bounds for each depth, leaving arithmetic and stores in safe Rust.
• Treat the four-input measurements as reference evidence only. Retain the historical Main10 timing regression and every new cell in the private comparison report rather than making a broad throughput claim.

Evidence

Verification run: Implementation evidence at 3bb68a61d7fe49e6d65b6a0f351b3c5175277ca2: formatting, unsafe-site inventory, strict x86 cross-Clippy in both modes, native x86 kernel tests (49 automatic, 45 scalar), and compilation of every kernel fuzz binary pass. Qualified-oracle pixels match for baseline, automatic and forced-scalar decoding on all four reference inputs. Callgrind proves block-validator entry with no row-validator calls on two-picture prefixes of all four inputs. Combination sanitizer fuzzing passes 4,587,778 inputs over 601 seconds. The final commit corrects one design sentence; every other tracked file is verified identical. Complete automatic and forced-scalar full conformance passes at 213fc6491acc3d85516d6372fa09377285f4fa46, including the registered census and the automatic run's Linux/macOS kernel differential campaigns.

Exact-head similarity at 213fc6491acc3d85516d6372fa09377285f4fa46: 112 spans of 50+ tokens, 0 not allowlisted, 0 problems, 0 stale entries.

Links: Automatic full conformance; forced-scalar full conformance; Refs #22; design; unsafe inventory.

• Every commit is signed off (git commit -s, DCO).
• I copied nothing from other implementations.

---

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Summary by CodeRabbit

• Bug Fixes

  • Predictor samples outside the permitted range are now consistently rejected during combination, including when processed through the optimized validation path.
  • Malformed predictor rows continue to return a buffer error, and invalid samples return a sample error before combination proceeds.

• Documentation

  • Updated HEVC documentation with details on predictor validation and CPU-dependent processing.

[coderabbitai]

Important

Review skipped

Review was skipped as selected files did not have any reviewable changes.

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Commits

Reviewing files that changed from the base of the PR and between 28a5192 and 213fc64.

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Walkthrough

The HEVC combine path now uses AVX2 for predictor-domain validation when the width and CPU conditions are met. Other paths retain scalar validation. The fuzz target now injects varied out-of-range values, and the documentation describes the validation paths.

Changes: HEVC predictor validation

Layer / File(s)

Summary

Predictor validation and fallback crates/coreav-kernels-hevc/src/inter.rs, crates/coreav-kernels-hevc/src/simd.rs, crates/coreav-kernels-hevc/src/simd/combine.rs, crates/coreav-kernels-hevc/fuzz/fuzz_targets/combine.rs, docs/design/hevc-inter-combine.md, docs/unsafe-sites.md

combine uses AVX2 validation when the first predictor is at least 8 pixels wide and AVX2 is detected. The AVX2 path checks active samples in both predictors and returns Error::Buffer for short rows or Error::Sample for out-of-range values. Scalar validation remains the fallback. Tests cover bounds, widths, and offsets; fuzz inputs now inject varied out-of-range values. The design and unsafe-sites documentation describe the validation path.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Refactor

Merge Risk: Moderate · previous review 28a51

The PR’s commit-shape check fails on two merge-commit subjects. Shorten those subjects and rerun the check before merging.

Security architecture risk: Low · up to 28a51

The inspected change preserves existing input limits, validates both predictors before writing output, and retains scalar fallback. No introduced security weakness was demonstrated. Incomplete downstream coverage limits assurance beyond the examined decoding path.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The directly examined exposure remains caller-supplied predictor data entering existing in-process HEVC APIs. Checked rectangles limit each block to dimensions of 1 through 64, and the operation writes only the borrowed caller-owned output. Downstream decoder reachability was not exhaustively established.

Trust Boundaries and Controls

• observed — Caller-controlled samples still cross geometry and sample-domain checks before arithmetic and stores. The new unsafe implementation is reached through runtime AVX2 detection and returns the existing Buffer and Sample errors rather than granting additional authority.

Resilience and Maintainability Implications

• observed — Added source-level checks exercise invalid values in every lane across widths and offsets. The differential fuzz target can corrupt either predictor, compares errors and output with the reference, and asserts unchanged output on rejection. These checks support validation parity but were not executed during this assessment.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title "perf(hevc): validate predictor lanes with AVX2" follows Conventional Commits with the perf type and a scope. It names the primary change: AVX2 validation of predictor samples in the HEVC de…
Description check		The description follows the repository template closely. It has a TL;DR with one bullet, Background with a link to the source issue (Refs #22), What changes with covered and not-covered scope and a po…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title "perf(hevc): validate predictor lanes with AVX2" follows Conventional Commits with the perf type and a scope. It names the primary change: AVX2 validation of predictor samples in the HEVC decoder. The raw summary shows the AVX2 predictor validation in combine.rs, the dispatch in inter.rs, and the matching design and unsafe-site docs. The title is 46 characters, under the 50-character guideline, and it describes one outcome.

Full details: Description check

Explanation

The description follows the repository template closely. It has a TL;DR with one bullet, Background with a link to the source issue (Refs #22), What changes with covered and not-covered scope and a post-merge statement, Decisions with three bullets, and Evidence with verification details and both required checklist items checked. The How it works section is omitted, which is acceptable because the change stays inside one layer and the user-visible behavior is unchanged. Two gaps remain. The Background describes the scalar pass but not a symptom or measured cost, so the reason for the overhead reduction is thin. The Evidence section relies on many internal commit and artifact references that a reviewer cannot easily verify from the PR itself. The description is mostly complete, so it passes.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 4 functions across 4 files. (2 skipped: 2 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

Finishing Touches

Generate docstrings

• Commit to this branch
• Create a new PR

Generate unit tests (beta)

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Failed pre-merge checks:
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Review info

Run configuration

• Source mode: Auto
• Repository: coreav-dev/coreav
• Branch: main
• File: .coderabbit.yaml
• Commit: 383ede6
• Layers: Repository YAML
• Review profile: CHILL
• Plan: Advanced
• Run ID: 6be3b10a-8e87-4955-9248-8086ccd9e8bd

Commits

Reviewing files that changed from the base of the PR and between 383ede6 and 28a5192.

Files selected for processing: 6

• crates/coreav-kernels-hevc/fuzz/fuzz_targets/combine.rs
• crates/coreav-kernels-hevc/src/inter.rs
• crates/coreav-kernels-hevc/src/simd.rs
• crates/coreav-kernels-hevc/src/simd/combine.rs
• docs/design/hevc-inter-combine.md
• docs/unsafe-sites.md

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 7 remain after this review.

[capy-ai]

Review: no blocking issue

Reviewed head 213fc6491acc3d85516d6372fa09377285f4fa46 (diff against main 796acf71).

Unsafe soundness. simd/combine.rs is sound. The only load is _mm256_loadu_si256 on a chunks_exact(8) group, so each read is exactly 32 bytes inside the checked active slice row.get(..width); alignment is not assumed, padding is never read, and there are no stores or pointer arithmetic. The target-feature functions are reachable only through the Avx2 token, which Avx2::detect builds after cached runtime detection. The dispatch in inter.rs mirrors the existing not(feature = "scalar") + x86_64 gating, and narrow rows (width < 8), CPUs without AVX2 and scalar builds keep the original scalar loops. The new docs/unsafe-sites.md row covers every new site in the file: the capability-gated block entry, the inlined row predicate, the load, and the scalar tail. The lib.rs and simd.rs rows stay accurate, since mod combine; adds no unsafe code.

Rejection and error order. Signed cmpgt against the inclusive bounds gives exactly !(min..=max).contains(v) with no arithmetic, so i32::MIN and i32::MAX are safe. The block driver keeps the scalar order: rows in order, Buffer before Sample within a row, first predictor before second, and every predictor validated before any write. Rectangle and denominator checks still come first. Because the result is Sample whichever lane fails, finishing the row before returning changes nothing observable.

Differential (Docker linux/amd64, AVX2 present, development only). I added a throwaway integration test that compares weight_uni/weight_bi against combine_reference (both the error and the full output, with output left untouched on error). It covers both depths, widths 1–64, heights 1–3, random strides, and padding filled with i32::MIN. At every lane position and tail position of either predictor it injects min-1, max+1, min-2, max+2, i32::MIN, i32::MIN+1, i32::MAX-1, i32::MAX, min and max, then adds random multi-fault cases: 764,928 cases, 609,507 rejected, all identical. The same test passes with --features scalar, so automatic output equals forced-scalar output. As a mutation check, widening the SIMD upper bound by one fails the test at once (width 8, uni), which shows the vector path is exercised. Crate lib tests pass with 49 automatic and 45 scalar, including the new lane/offset test.

Evidence. I recomputed the paired median speedups from the retained per-call timings: c00 1.0771, c01 1.0630, c02 1.0046, c03 1.0196, with intervals that match. These match the claims. c02 is marginal (one of seven pairs is below 1.0, lower bound 1.0003). Between the measured SHA 3bb68a61 and this head, only one line of docs/design/hevc-inter-combine.md changes. Every commit is signed off with no co-author trailers, the PR text has no CoreAV-vs-FFmpeg figures, and CodeRabbit approved with 0 open threads.

Non-blocking

• fuzz_targets/combine.rs: the injected fault always lands on the first predictor for uni and the second for bi, so the fuzzer never puts an out-of-range value in the first predictor of a bi call. data[8] also drives both second_offset and the injection trigger. Choosing the side from an independent byte would widen coverage cheaply. The unit test and the differential above already cover this path.
• The final-head automatic and forced-scalar full conformance runs were still in progress when I reviewed, so this verdict assumes they pass.

[capy-ai]

Replying to #530 (comment)

Keeping the held head unchanged while full conformance finishes. The existing fault-side correlation is recorded in Not covered, with a follow-up in #22 to decouple it from uni/bi selection and offsets. Both-predictor rejection is covered by the unit and independent differential checks.