fix(dictionary): load raw content where upstream does, and scan dictionary blocks the way it scans them by polaz · Pull Request #492 · structured-world/structured-zstd

polaz · 2026-09-07T15:23:53Z

Dictionaries: what they are allowed to be, and how a block that has one gets scanned.

What a dictionary may be (#470)

ZSTD_CCtx_loadDictionary takes its buffer in ZSTD_dct_auto mode — bytes that do not start with ZSTD_MAGIC_DICTIONARY are raw content, which is why any file can be handed to zstd -D. Our two setters for that entry point rejected such a buffer as BadMagicNum, so a caller that works against libzstd failed here.

FrameCompressor::set_dictionary_from_bytes and the streaming encoder's namesake now load either kind. EncoderDictionary::from_bytes stays strict, so ZSTD_dct_fullDict remains reachable.
DictionaryHandle::from_serialized_or_raw_content gives the decode side the same pair (ZSTD_createDDict is auto too).
An empty buffer is a dictionary with nothing in it rather than a malformed one, as ZSTD_createDDict(NULL, 0) is — and on the encoder's setters it clears whatever was attached and succeeds, as ZSTD_CCtx_loadDictionary does. The constructor that names raw content still refuses it.
C ABI: a fullDict selector over bytes that are not a dictionary now answers dictionary_wrong on the compression side, as upstream does; the decompression side keeps dictionary_corrupted.
CLI: zstd -22 without --ultra warns and compresses at 19 rather than failing, which is what upstream does — we refused the run, so a script that worked against upstream broke against us.

How a dictionary block is scanned (#469)

The dictionary path ran on the no-dictionary loop with the dictionary probes bolted on. Upstream keeps ZSTD_compressBlock_doubleFast_dictMatchState_generic and ..._noDict_generic apart, and the difference is not decoration: the no-dictionary loop carries a second cursor, precomputing the long hash of ip+1 and carrying its index and slot across iterations. That buys a hash per two positions at the price of keeping that cursor's state live — and the dictionary loop cannot afford it, because it must also keep two table pointers, the dictionary's two hash shifts and its region bound live. The profile showed exactly that: 93.4% of the dictionary path inside this loop, reloading its own invariants from the stack at each probe.

The dictionary now gets its own kernel: one cursor, one hash per position, and upstream's step (ip += ((ip - anchor) >> 8) + 1). The probe order is upstream's too, and one part of it does real work — the dictionary's short table is consulted only when the live short slot is empty or out of window, an else if on the slot rather than on the compare, so with the tables a small dictionary sizes that arm all but stops firing within a few hundred positions.

The two-cursor loop loses its dictionary code and its USE_DICT axis with it.

The Fast dictionary probe (part of #323)

The borrowed dictionary kernel called the segmented dict/input match counter for every occupied dictionary slot and only then asked whether what came back reached four. On input the dictionary does not describe, almost none of those are matches, so the counter ran from byte zero for each: 21.2% of a level-1 profile sat in it.

Four bytes decide it first now, as upstream does, on both dictionary-candidate arms; and the counter resumes past those four rather than re-reading them. (Doing the gate without that resume made the fixtures a dictionary actually describes 2.3% slower — measured, then fixed.)

Measured

i9, compress-dict groups, criterion, µs per frame. c_ffi is the same libzstd arm in the same runs; readings interleaved between prebuilt binaries.

fixture	level	before	after	c_ffi	vs c_ffi
small-10k-random	-1 fast	64.52	42.97	22.3	2.90× → 1.93×
small-10k-random	1 fast	68.25	46.53	25.3	2.70× → 1.84×
small-10k-random	3 dfast	95.91	80.14	60.4	1.59× → 1.33×
small-4k-log-lines	-1 fast	1.348	1.274	0.955	1.41× → 1.33×
small-4k-log-lines	1 fast	1.357	1.284	0.947	1.43× → 1.36×
small-4k-log-lines	3 dfast	2.034	1.926	1.169	1.74× → 1.65×
synthetic-1m	-1 fast	51.37	50.44	123.0	0.42×
synthetic-1m	1 fast	49.78	49.58	122.8	0.40×
synthetic-1m	3 dfast	103.50	103.57	129.5	0.80×

The synthetic-1m rows are flat: interleaved re-runs put both builds inside 49.5–51.4 µs.

Wall clock and cycles, 20 000 dictionary frames of the 10 KiB fixture, three interleaved rounds, with the other level as the control arm each time:

Against libzstd itself, on the same fixture with the same counters in the same runs — a CDict parsed once, then 20 000 frames through a reused context on each side, three interleaved rounds:

level	cycles before	cycles after	libzstd	insn before	insn after	libzstd
1 (Fast)	5675 / 5663 / 5689 M	3883 / 3908 / 3901 M	2105 / 2103 / 2108 M	15,936 M	9,936 M	4,996 M
3 (dfast)	8161 / 8125 / 8412 M	6594 / 6574 / 6626 M	5035 / 5044 / 5033 M	11,980 M	9,292 M	9,750 M

The gap to upstream goes 2.70× → 1.85× at level 1 and 1.62× → 1.31× at level 3; at level 3 we now execute slightly FEWER instructions than it does, so what remains there is execution density rather than work.

Wall clock over the same 20 000 frames: level 1 1.36 / 1.35 / 1.35 → 0.93 / 0.92 / 0.93 s, level 3 1.95 / 2.03 / 1.94 → 1.61 / 1.59 / 1.60 s.

Each level is also the other's control arm — the Fast kernel cannot execute the dfast loop and vice versa — and the control moved 1.4% while the measured arm moved 27%. On the other two dictionary fixtures at level 1 the gate moves less and in the shape it should: structured input 339 / 345 / 351 → 333 / 333 / 327 M cycles, and input the dictionary describes is flat, since there the count would have succeeded anyway.

Bytes, all 93 scenario/level rows of the dictionary matrix: one row moved and downward — level_3_dfast/small-4k-log-lines 56 → 44, which is exactly what libzstd emits there and the one dictionary row where we were losing to it. Total 15,896,896 → 15,896,884; the ratio against libzstd is unchanged at 1.017.

What each change emits, judged on frame md5 rather than on length — two different parses can weigh the same:

The Fast gate is byte-identical: 30 rows (three dictionary fixture shapes × five Fast levels × with dictionary / without) all unchanged.
The dfast ip+1 insertion changes output by design, and both rows it moves get smaller: the matrix row above, and small-4k-log-lines at CLI level -3 with a dictionary, 63 → 52 bytes against libzstd's 54 there.

Allocations: identical on both revisions — 56 blocks / 236,297 bytes at level 3 and 53 blocks / 192,107 bytes at level 1 for a 50-frame run, i.e. setup only, nothing per frame.

Against libzstd's own CLI on a real 80 MB access log

Same machine, single-threaded on both sides, alternating per level, best of two. Every frame we emit was decoded back with zstd -d and compared: ok on all 21 levels.

level	ours (bytes)	libzstd	bytes	ours (s)	libzstd	time
1	10,267,998	10,283,940	0.998	0.25	0.11	2.27×
3	11,424,458	11,414,689	1.001	0.36	0.20	1.80×
5	11,323,945	11,304,365	1.002	0.73	0.43	1.70×
8	10,395,722	10,382,929	1.001	1.09	0.75	1.45×
11	10,455,688	10,442,475	1.001	1.71	1.37	1.25×
13	10,455,262	10,445,898	1.001	2.09	2.20	0.95×
15	10,413,467	10,404,855	1.001	3.01	2.97	1.01×
17	9,492,125	9,594,298	0.989	14.21	13.23	1.07×
18	9,763,883	9,770,574	0.999	17.76	17.64	1.01×
19	9,188,505	9,184,245	1.000	28.45	35.48	0.80×
22	9,216,267	9,211,905	1.000	37.71	46.38	0.81×

Size lands within 0.35% of libzstd at every level and beats it at 1, 17 and 18; speed trails in the fast band, reaches parity around 13–15 and wins from 19 up.

On the dictionary path over the same log — trained on one half, compressing the held-out half, with each side's own trainer — sizes agree within 0.2% and both trainers' dictionaries are interchangeable; at level 19 ours is both smaller (938,571 against 939,529 bytes) and faster (1.98 s against 2.56 s).

Considered and rejected

Folding the dictionary probe's index and tag into one shift (upstream packs them that way for its short cache) measured deterministically WORSE: +1.63% instructions on the dictionary arm with the cycle readings overlapping. When the CDict tables carry the same width as the live ones, the old form's dictionary index shift is the same shift as the live long index and the compiler shared one shrx between them; offsetting by the tag bits breaks that sharing. The transform is sound — byte-identical over 72 rows of frame md5 — it is simply not faster.

Verification

1039 tests on aarch64, 1061 on x86_64 (the per-kernel monomorphs differ, so the new kernel's AVX2/SSE2 forms are only exercised there), 58 C-ABI tests, 23 doc tests; clippy clean on both arches over the CI feature set.
Every CPU tier of the new dictionary loop is run over the same block and must emit identical sequences, so the scalar fallback and the SIMD kernels are pinned to each other — on aarch64 too, where the dispatch resolves NEON at compile time and the scalar loop is reachable only through a cfg(test) branch kept for exactly this.
Cross-arch agreement: frame md5 with a dictionary matches between the x86_64 and aarch64 builds at levels 1, 3 and 5.
-D round trips both ways against zstd 1.5.7 for a raw-content dictionary and a trained one.

Closes #470
Closes #469

Summary by CodeRabbit

New Features
- Dictionary inputs can now be provided as serialized dictionaries or raw content.
- Empty dictionary input clears the currently attached dictionary.
- Compression level requests above 19 are automatically limited to level 19 unless ultra mode is enabled.
Bug Fixes
- Dictionary validation now reports more accurate errors for compression and decompression paths.
- Dictionary handling now supports empty inputs consistently across compression workflows.

`ZSTD_CCtx_loadDictionary` takes its buffer in `ZSTD_dct_auto` mode (`ZSTD_compress_insertDictionary`, zstd_compress.c:5216-5222): bytes that do not start with `ZSTD_MAGIC_DICTIONARY` are raw content, which is why any file can be handed to `zstd -D`. Our two setters for that same entry point rejected such a buffer as `BadMagicNum`, so a caller that works against libzstd failed here. - `FrameCompressor::set_dictionary_from_bytes` and the streaming encoder's namesake now load either kind. `EncoderDictionary::from_bytes` stays strict, so `ZSTD_dct_fullDict` remains reachable by parsing first and attaching the result. - `DictionaryHandle::from_serialized_or_raw_content` mirrors the `Dictionary` constructor, giving the decode side the same pair (`ZSTD_createDDict` is auto too, zstd_ddict.c:102-107). - C ABI: a `fullDict` selector over bytes that are not a dictionary now answers `dictionary_wrong` on the compression side, as upstream does (zstd_compress.c:5207 and 5223); the decompression side keeps `dictionary_corrupted` (zstd_ddict.c:99 and 105). A caller that branches on the code saw the wrong one. Each fix carries the regression test that failed without it. The CLI and the rest of the C ABI already classified on the magic, so `-D` over a plain file works in both directions and is left alone: `structured-zstd -6 -D rawdict4k` produces 3289 bytes where `zstd -6 -D` produces 3368, and each decodes the other's frame. Upstream additionally skips the dictionary content entirely below 8 bytes while still applying its dictionary-sized cParams, which costs it: on a 64 KiB record fixture with a 6-byte `-D`, `zstd -6` emits 5256 bytes against 3965 with no dictionary at all, while ours emits 3865 and decodes there. Not copied — it would trade ratio for a byte-identity that is not a goal. Closes #470

The raw-content fallback existed because `set_dictionary_from_bytes` rejected a blob without the magic; it now loads either kind, so the fallback was reachable only for a corrupt serialized dictionary, where re-reading the bytes as raw content with a made-up id is the wrong answer anyway. Part of #470

The dictionary path ran on the no-dictionary loop with the dictionary probes bolted on. Upstream keeps the two apart (`ZSTD_compressBlock_doubleFast_dictMatchState_generic` against `..._noDict_generic`) and the difference is not decoration: the no-dictionary loop carries a second cursor, precomputing the long hash of ip+1 and carrying its index and slot across iterations, which buys a hash per two positions at the price of keeping that cursor's state live. The dictionary loop cannot afford it — it must also keep two table pointers, the dictionary's two hash shifts and its region bound live — and the profile showed exactly that: 93.4% of the dictionary path inside this loop, reloading its own invariants from the stack at each probe. So the dictionary gets its own kernel, scanning one cursor, hashing each position once, and stepping the way upstream's dictionary loop steps (`ip += ((ip - anchor) >> 8) + 1`, accelerating with the distance from the last match). The probe order is upstream's too, and one part of it does real work: the dictionary's short table is consulted only when the live short slot is empty or out of window, an `else if` on the slot rather than on the compare, so with the tables a small dictionary sizes that arm all but stops firing within a few hundred positions — where the old arrangement paid a hash, a load and a tag compare at every position. The two-cursor loop loses its dictionary code and its USE_DICT axis with it; a borrowed window never carries a dictionary, so the new kernel needs no BORROWED axis either. Compressed size across the whole dictionary matrix (93 scenario/level rows): one row moved, and downward — level_3_dfast/small-4k-log-lines 56 -> 55 bytes. Total 15,896,896 -> 15,896,895; the ratio against libzstd is unchanged at 1.01704. Closes #469

The borrowed dictionary kernel called the segmented dict/input match counter for every occupied dictionary slot and only then asked whether what came back reached four. On input the dictionary does not describe, almost none of those candidates are matches, so the counter ran from byte zero for each of them: a fifth of the whole path's time sat in it (`count_forward_dict_2segment`, 21.2% of a level-1 profile on 10 KiB of random input with a trained dictionary). Compare four bytes first, as upstream does (`MEM_read32(dictMatch) == MEM_read32(ip0)` before `ZSTD_count_2segments`, zstd_fast.c:578-584), on both dictionary-candidate arms: the main dict probe and the repcode probe's dict-side candidate. The gate is exactly the condition the caller already tested — a count reaches four only when the first four bytes agree — so nothing that was accepted is rejected now. The counter then RESUMES past those four rather than re-reading them, which is why it takes what the caller established as a parameter. Doing the gate without that resume made the fixtures a dictionary actually describes 2.3% slower, since there the count would have succeeded anyway and the four bytes were simply compared twice. A candidate within three bytes of the dictionary's end has four bytes only by crossing into the input, which upstream reads across for free (one contiguous window) and we cannot (two buffers), so those positions keep the plain counting form. Byte-identical: all 93 scenario/level rows of the dictionary matrix unchanged, total 15,896,895 bytes. Part of #323

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zstd/src/encoding/dfast/mod.rs
zstd/src/encoding/frame_compressor.rs
zstd/src/encoding/frame_compressor/tests.rs
zstd/src/encoding/match_generator/tests.rs
zstd/src/encoding/simple/fast_kernel/count.rs
zstd/src/encoding/simple/fast_kernel/count/tests.rs
zstd/src/encoding/simple/fast_kernel/kernel.rs
zstd/src/encoding/simple/fast_kernel/kernel/tests.rs
zstd/src/encoding/streaming_encoder.rs
zstd/src/encoding/streaming_encoder/tests.rs

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📝 Walkthrough

Walkthrough

The change adds automatic raw-content dictionary loading, preserves distinct C API full-dictionary errors, clamps non-ultra CLI levels, and introduces a dedicated dfast dictionary matching loop with verified-prefix counting.

Changes

Dictionary loading behavior

Layer / File(s)	Summary
Raw and serialized dictionary parsing `zstd/src/decoding/dictionary.rs`, `zstd/src/decoding/dictionary/tests.rs`, `zstd/src/encoding/frame_compressor.rs`, `zstd/src/encoding/frame_compressor/tests.rs`, `zstd/src/encoding/streaming_encoder.rs`, `zstd/src/encoding/streaming_encoder/tests.rs`, `zstd/examples/encode_loop_dict.rs`	Dictionary APIs now detect serialized dictionaries, raw content, and empty buffers. Raw-content frames omit the dictionary ID and require the same dictionary during decoding.
C API full-dictionary classification `c-api/src/attach.rs`, `c-api/src/tests.rs`	Compression reports `ZSTD_error_dictionary_wrong` for unmagicked bytes with `ZSTD_dct_fullDict`. Decompression reports `ZSTD_error_dictionary_corrupted`.

CLI compression-level limits

Layer / File(s)	Summary
Non-ultra level clamping `zstd/src/bin/structured-zstd/main.rs`, `zstd/src/bin/structured-zstd/tests.rs`	Levels above 19 are reduced to level 19 without `--ultra`, including benchmark ranges. Ultra mode preserves higher requested levels.

Dictionary match-loop optimization

Layer / File(s)	Summary
Verified-prefix match counting `zstd/src/encoding/simple/fast_kernel/count.rs`, `zstd/src/encoding/simple/fast_kernel/count/tests.rs`, `zstd/src/encoding/simple/fast_kernel/kernel.rs`, `zstd/src/encoding/simple/fast_kernel/kernel/tests.rs`	Dictionary counting resumes after verified bytes and handles transitions into the input segment. Four-byte checks reject mismatched candidates before segmented counting.
Dedicated dfast dictionary loop `zstd/src/encoding/dfast/mod.rs`, `zstd/src/encoding/match_generator/tests.rs`	Dictionary probing moves from the two-cursor fast loop into a dedicated single-cursor loop. Architecture-specific dispatch selects the dictionary loop when dictionary tables are present. Tests compare emitted sequences across available kernel tiers.

Estimated code review effort: 4 (Complex) | ~60 minutes

Merge Risk: ⚪ Minimal · up to 6dedb

This change adds upstream-compatible dictionary loading and optimized dictionary matching while retaining strict serialized-dictionary handling. The supplied regression coverage supports the new behaviors, with no current merge-blocking risk identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	Most changes support issues `#470` and `#469`, but the compression-level ceiling added in zstd/src/bin/structured-zstd/main.rs and its tests is unrelated to either linked issue.	Remove the compression-level clamping changes, or link an issue that requires this CLI behavior and explicitly include it in the pull request scope.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies both primary changes: raw dictionary loading and dictionary-block scanning aligned with upstream behavior.
Linked Issues check	✅ Passed	The changes satisfy issue `#470` by supporting serialized, raw-content, and empty dictionaries across relevant loaders, preserving strict fullDict behavior, and adding coverage for IDs and error classif…
Docstring Coverage	✅ Passed	Docstring coverage is 96.49% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 57 functions across 17 files.

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codecov · 2026-09-07T15:26:48Z

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❌ Patch coverage is 97.14286% with 3 lines in your changes missing coverage. Please review.

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In `@zstd/src/encoding/dfast/mod.rs`:
- Around line 3348-3353: In the lookahead block of the two-cursor commit path,
update the long-hash table at the computed hl1_idx by storing packed_next to
long_hash_ptr.add(hl1_idx) under the existing ip + 1 + HASH_READ_SIZE <=
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c-api/src/attach.rs
c-api/src/tests.rs
zstd/examples/encode_loop_dict.rs
zstd/src/decoding/dictionary.rs
zstd/src/decoding/dictionary/tests.rs
zstd/src/encoding/dfast/mod.rs
zstd/src/encoding/frame_compressor.rs
zstd/src/encoding/frame_compressor/tests.rs
zstd/src/encoding/simple/fast_kernel/count.rs
zstd/src/encoding/simple/fast_kernel/count/tests.rs
zstd/src/encoding/simple/fast_kernel/kernel.rs
zstd/src/encoding/streaming_encoder.rs
zstd/src/encoding/streaming_encoder/tests.rs

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`zstd -22` warns and compresses at 19 ("Warning : compression level higher than max, reduced to 19", exit 0). We failed the run instead, so a script that works against upstream broke against us. Verified against zstd 1.5.7 on the same host: with --ultra it compresses at 22; without it warns, reduces and exits 0. The benchmark range reduces the same way, since `-b20` reaches an ultra level as surely as `-20` does. Found while evaluating both encoders over a real 32 MiB access log: the sweep's level-22 row came back empty against ours. Part of #128

The dictionary scan loop has one monomorph per CPU tier and the runtime dispatch runs exactly one of them, so on any given machine the others never execute. Forcing the cached tier runs each in turn over the same dictionary-primed block and pins what the dispatch assumes: every tier emits the same sequences, so the scalar fallback and the SIMD kernels agree bit for bit. Mirrors the binary-tree tiers' existing test. Also covered, all of them boundaries the changed code introduced: - a Fast dictionary candidate in the last three bytes of the dictionary, which cannot be judged on four bytes of its own and keeps the counting form. The match crosses into the input and must still be found, which is what the four-byte gate must not cost. - the counter resuming past an established prefix that reaches exactly the end of the input, with and without dictionary left of its own. - its two contract assertions: it does raw pointer math from a safe signature, so a caller that broke either bound would read outside the buffers. - a magic-prefixed blob that does not parse, which is a corrupt dictionary and must be refused rather than re-read as raw content.

A repeat offset can point into the dictionary rather than the input — what a dictionary's own repeat offsets are for on the first block — and the probe then reads its candidate from the other buffer, through the four-byte gate the previous commit added there. Empty dictionary table in the fixture, so the emitted sequence can only have come through that arm.

`ZSTD_createDDict(NULL, 0)` builds a usable DDict that references no content (zstd_ddict.c:123-140), and an empty buffer is how `ZSTD_CCtx_loadDictionary` is told there is no dictionary. Loading either kind answered `DictionaryTooSmall` for that input, so a caller handed an empty file got an error where upstream gives them nothing. The constructor that names raw content still refuses it: there the emptiness is the caller asking for a dictionary that cannot exist. Also bounds the counter's established-prefix argument in release, for the same reason the two arguments beside it are bounded there: it feeds a subtraction and a raw-pointer add from a safe signature.

The dictionary loop probes the long table at ip+1 and, unlike upstream, never wrote that position back (`hashLong[hl3] = curr + 1`, zstd_double_fast.c:459). Nothing else covers it: the complementary insertion after a match writes `curr + 2` and the two positions before the match end, so the probed position was searched and then forgotten. Compressed size across the dictionary matrix: 15,896,895 -> 15,896,884, all of it on the one row where we were losing to libzstd — level_3_dfast/small-4k-log-lines 55 -> 44 bytes, which is exactly what libzstd emits there.

The commit that introduced it reported microseconds, wall clock and instructions; the cycle figures behind it now sit at the code they justify, with the control arm that makes them attributable.

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The tier-parity test was x86-only, and aarch64 is where it was needed most: the dispatch there resolves NEON at compile time, so the SIMD kernel always wins and nothing ever ran the scalar loop to compare it against — on that target the scalar dictionary wrapper was not even built. It is now built in test builds there, and the dispatcher keeps a `cfg(test)` branch that asks for it, which is the only way the target can run both. The test covers every tier the running CPU may legally execute, on whichever architecture it runs. Proven to bite: with the scalar loop stubbed out to emit nothing, the test fails on aarch64 with an empty sequence list rather than passing.

The commit that introduced it reported wall clock and instructions for the Fast rows; the cycle figures now sit at the code they justify, with the control arm that makes them attributable — 5387/5463/5373 -> 3935/3939/3943 M on the fixture the gate is for, against 1.4% of drift on a level that cannot run it.

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Inline comments:
In `@zstd/src/encoding/simple/fast_kernel/count.rs`:
- Around line 231-234: Update the precondition in count_forward_dict_2segment to
avoid computing cur + known; after the existing cur <= inp_len validation,
compare known against the remaining input length using subtraction so overflow
cannot bypass the assertion. Add a release-mode regression test using a one-byte
input, cur = 1, and known = usize::MAX, asserting the function panics before any
pointer arithmetic.

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zstd/src/bin/structured-zstd/main.rs
zstd/src/bin/structured-zstd/tests.rs
zstd/src/decoding/dictionary.rs
zstd/src/decoding/dictionary/tests.rs
zstd/src/encoding/dfast/mod.rs
zstd/src/encoding/frame_compressor/tests.rs
zstd/src/encoding/match_generator/tests.rs
zstd/src/encoding/simple/fast_kernel/count.rs
zstd/src/encoding/simple/fast_kernel/count/tests.rs
zstd/src/encoding/simple/fast_kernel/kernel.rs
zstd/src/encoding/simple/fast_kernel/kernel/tests.rs

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`cur + known <= inp_len` computes the sum before comparing it, and that sum wraps in a release build: `known = usize::MAX` with any cursor passes a bound it should have failed, after which the pointer arithmetic below leaves the input. `cur <= inp_len` is established directly above, so the same bound expressed as `known <= inp_len - cur` cannot wrap and cannot underflow. Carries the regression test: a one-byte input with `cur = 1` and `known = usize::MAX` must fail the contract, and before the fix it failed the ADD instead — a different panic in a debug build, and none at all in a release one. Also, an empty buffer now clears the encoder's dictionary instead of being reported as one too small to use, on both the one-shot and the streaming setter. That is what the upstream entry point they mirror does (`ZSTD_clearAllDicts` then `return 0`, zstd_compress.c:1293-1295), so a caller could previously neither say "no dictionary" nor undo an earlier one through the setter. The decode side was fixed a commit earlier; the encoder kept its own copy of the branch. And the tier-parity test reaches wasm: simd128 is resolved at compile time there, so without the same `cfg(test)` door the NEON arm has, that target ran its SIMD kernel under every tier name and compared it with itself. (The test does not build for wasm32 today — a dev-dependency fails to compile for that target — but the door and the tier are what make it meaningful when it does.)

The measurements so far compared our own before and after; the numbers now sit beside libzstd's, taken in the same runs on the same fixture with the same counters (a CDict parsed once, then 20 000 frames through a reused context on each side). Level 1 (Fast): 5675/5663/5689 -> 3883/3908/3901 M cycles against its 2105/2103/2108, and 15,936 -> 9,936 M instructions against its 4,996 — the gap goes 2.70x -> 1.85x in cycles. Level 3 (dfast): 8161/8125/8412 -> 6594/6574/6626 M cycles against its 5035/5044/5033, and 11,980 -> 9,292 M instructions against its 9,750 — 1.62x -> 1.31x in cycles, and slightly FEWER instructions than it runs. Also records what the Fast gate emits rather than only how much: frame md5 over three dictionary fixture shapes x five Fast levels x (with dictionary, without) is unchanged, 30 rows. Equal compressed lengths would not have shown that.

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Clearing the stream dictionary dropped the dictionary and kept the tables built from it: the Huffman and FSE allocations stayed alive, and in `heap_size`, for as long as the encoder lived, though nothing could reach them again. Measured by the regression test that carries this: a serialized dictionary left 37,912 bytes behind a clear that should have returned to zero. The match-finder snapshot needs no matching call, and the code says why: a dictionary is primed at the first write, which is also the point after which this setter refuses to run, so there is never one to drop. Also exercises the paths this round's code introduced: - clearing after the frame is open, and on a stream whose write failed — the two refusals a clear inherits from the attach it is. - a repcode candidate in the dictionary's last three bytes whose match falls short of four bytes: no four bytes of its own to be gated on, so it is counted, and the count rejects it. Both arms of that decision are now exercised. The three lines left in the dfast loop are the `DFTRACE` diagnostic, gated on an environment variable read into a process-wide latch: a test that enabled it would make every other test in the binary write to stderr, so it stays off deliberately.

polaz added 4 commits September 7, 2026 17:12

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polaz added 6 commits September 7, 2026 18:35

docs(dfast): record what the dictionary loop's shape measured

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The commit that introduced it reported microseconds, wall clock and instructions; the cycle figures behind it now sit at the code they justify, with the control arm that makes them attributable.

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polaz merged commit e1a802c into main Sep 7, 2026
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polaz deleted the fix/#470-raw-content-dict branch September 7, 2026 18:38

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chore: release v0.0.51 #491

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What a dictionary may be (#470)

How a dictionary block is scanned (#469)

The Fast dictionary probe (part of #323)

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Against libzstd's own CLI on a real 80 MB access log

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