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perf(ldm): close the level-1 long-distance gap to libzstd - #632
Brings level-1 long-distance compression close to libzstd: the gear hash runs one cycle a byte on its dependency chain, and a Fast-level long-distance frame is scanned in place instead of being copied into the owned history. Compressed output is byte-identical.
On an x86_64 host with the CI bench flags (-C target-cpu=x86-64-v3), a fresh context per frame as the level_1_fast_ldm cell runs it, ours against libzstd in cycles per frame: low-entropy 1 MiB 1.52x -> 1.12x (gnu) and 1.41x -> 1.11x (musl), high-entropy 1 MiB 1.36x -> 1.12x (gnu). Page faults per frame on musl drop from about 300 to libzstd's 44.
Changes
Gear hash step as one lea on x86_64 (plain code compiled to add hash, hash and a load-folded add); the feed walks four-byte array chunks; a split is recorded by a cold out-of-line call, so the scan is the fall-through path.
Fast-level long-distance frames are scanned in place: the search reads the input slice, each gap between long matches is scanned where it lies, each match body's tail is indexed up to the match's end, and the borrowed block loop moves a block cut out of a long match as the owned loop does. Dictionary frames and the other backends keep the owned history.
The borrowed block path keeps every block on the search under long-distance matching and takes the long matches of a block it writes out unsearched, as the owned path already did; a long-distance frame with an attached dictionary keeps the owned loop.
encode_loop_oneshot_z000033 / ffi_encode_loop_z000033 take an ldm argument and decode_loop_z000033 an ldm encode option, to run the benchmark's LDM cells.
Testing
Frames byte-identical to main with and without long-distance matching across levels and fixture shapes, and borrowed against owned long-distance frames by regression tests (repeats past the window, unsearched one-byte blocks, an attached dictionary). Formatting, the CI clippy configurations, the full test suite and doctests pass on aarch64 (macOS); the x86_64 kernels are built and checked on Linux.
`encode_loop_oneshot_z000033` and `ffi_encode_loop_z000033` take an
optional `ldm` argument that times what the benchmark's `level_<N>_*_ldm`
cells time: a fresh context per frame, long-distance matching, content
checksum and content size on.
The long-distance matcher's rolling hash, `(hash << 1) + GEAR_TAB[b]`,
is one serial chain through every byte of the input. Written plainly,
x86_64 codegen folds the table load into an `add` behind `add hash,
hash`, two cycles a byte on that chain; upstream's compiled
`ZSTD_ldm_gear_feed` takes one `lea`. The step is now that `lea` on
x86_64; elsewhere the shift already folds into the add.
Each chunk's length is known, so its loads carry no bounds check, and
the loop's only counter is the chunk pointer, as upstream's is its
index. Split positions are derived from the pointer on the rare split
only.
`as_chunks::<4>` instead of `chunks_exact(4)`: the chunk is a `[u8; 4]`
by type. Measured with `chunks_exact` on runner4 against the lea step,
3 interleaved rounds, frames identical: L1 ldm low1m cycles -3.5%,
high1m -2%, no overlap; instructions equal.
A split comes about once per `2 ^ hash_rate_log` bytes. Without a hint
the record was laid out on the fall-through path, so every other byte
took a branch around it; under `target-cpu=x86-64-v3` that layout cost
the lea-step loop 17% of the frame on runner4. The record is now a cold
function the scan branches out to, upstream's compiled shape.
A one-shot frame under long-distance matching copied its whole input
into the owned history, because the long-distance block is matched in
pieces by re-committing it there. Upstream's one-shot frame reads its
input where it lies. The Fast backend now does the same: the
long-distance search reads the input slice, each gap between long
matches is scanned in place ending where the owned window would end,
and each match body's tail is indexed up to the match's end. A block
cut is moved out of a long match as the owned loop moves it.
The borrowed block path also keeps every block on the search under
long-distance matching, as the owned path does: a sample cannot see a
match from outside the block.
Frames are byte-identical to the owned loop's (regression test
a_long_distance_frame_scanned_in_place_is_the_owned_frame, including an
over-window input). Dictionary frames and the other backends keep the
owned history.
Reviewing files that changed from the base of the PR and between 31dfe46 and bc06519.
Files selected for processing: 3
zstd/examples/decode_loop_z000033.rs
zstd/src/encoding/match_generator/mod.rs
zstd/src/tests/roundtrip_integrity.rs
Walkthrough
The encoder optimizes LDM hashing and adds in-place LDM matching for eligible borrowed input. The examples add LDM benchmark modes, and a regression test compares borrowed and owned compression output.
The Fast backend searches and matches eligible borrowed blocks in place with LDM. Block staging, cut handling, and raw-skip conditions account for LDM. A regression test compares borrowed and owned frame bytes and verifies decompression.
The encode examples add optional LDM benchmark loops. The decode example accepts comma-separated checksum and LDM options and sets their compression parameters independently.
sequenceDiagram
participant Encoder
participant MatchGeneratorDriver
participant FastKernelMatcher
Encoder->>MatchGeneratorDriver: Provide borrowed block
MatchGeneratorDriver->>MatchGeneratorDriver: Search borrowed input for LDM matches
MatchGeneratorDriver->>FastKernelMatcher: Scan gaps between LDM matches
FastKernelMatcher-->>MatchGeneratorDriver: Return sequences for scanned gaps
MatchGeneratorDriver->>FastKernelMatcher: Index the tail of each long-match range
MatchGeneratorDriver-->>Encoder: Emit sequences and stage the borrowed block
Long-distance matching with an attached Fast dictionary can enter the in-place path that reset did not select, and full-block RLE blocks can skip long-distance matches. Both should be fixed before merge. Other frames appear unaffected.
Pre-merge checks: 3 · 1 · 1
Failed checks: 1 warning · 1 inconclusive
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Explanation
Resolution
Docstring Coverage
Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 28 functions across 10 files.
Write docstrings for the functions missing them to satisfy the coverage threshold.
Linked Issues check
Issue #631 coding requirements are implemented in the diff. Gear hash: the summary of gear_hash.rs shows a x86_64 lea step, a four-byte chunk feed, and a cold push_split helper. Fast-level in-plac…
Provide (a) the high-entropy 1 MiB result on x86_64-musl for level_1_fast_ldm against libzstd, in cycles per frame, to test the 1.15x bound. (b) Evidence that non-LDM levels are unchanged. Either show matches_beyond_window_tables() retu…
Passed checks: 3 passed
Check name
Status
Explanation
Description Check
Check skipped - CodeRabbit’s high-level summary is enabled.
Title check
The title clearly and concisely describes the main change: improving level-1 long-distance matching performance to reduce the gap with libzstd.
Out of Scope Changes check
Each changed area ties to the #631 objectives or to the in-place LDM path. Gear hash changes (gear_hash.rs) serve the gear-hash objective. In-place scan changes (long_distance.rs, mod.rs, `fast_…
Full details: Out of Scope Changes check
Explanation
Each changed area ties to the #631 objectives or to the in-place LDM path. Gear hash changes (gear_hash.rs) serve the gear-hash objective. In-place scan changes (long_distance.rs, mod.rs, fast_matcher.rs, fastest.rs, frame_compressor.rs) serve the in-place Fast-level LDM objective. The fastest.rs and frame_compressor.rs changes keep borrowed blocks on the search or cut blocks at long-match boundaries under LDM, as the PR description states. The encode and decode example changes serve the benchmark harness objective. The regression test supports the byte-identity objective. No change was shown to be unrelated. The frame_compressor.rs diff could not be read, so this verdict relies on the summary and PR description for that file.
Full details: Linked Issues check
Explanation
Issue #631 coding requirements are implemented in the diff. Gear hash: the summary of gear_hash.rs shows a x86_64 lea step, a four-byte chunk feed, and a cold push_split helper. Fast-level in-place scan: mod.rs gates long_distance_in_place on BackendTag::Simple and no dictionary. It also calls start_matching_long_distance_borrowed first in the borrowed block path, and fast_matcher.rs adds skip_long_match_borrowed. Byte-identical frames: the new regression test a_long_distance_frame_scanned_in_place_is_the_owned_frame compares in-place and owned frames for levels 1, -5, Default and 5. Benchmark harness: the encode and decode examples take an ldm option. Two acceptance items lack evidence. (1) Performance: the PR reports low-entropy 1 MiB on gnu (1.12x) and musl (1.11x), high-entropy 1 MiB on gnu (1.12x), and musl page faults (44). It does not report high-entropy 1 MiB on musl, which the issue requires within 1.15x. (2) No change at levels without LDM: fastest.rs now sets dict_rejects_raw = dict_active || state.matcher.matches_beyond_window_tables(). The definition of matches_beyond_window_tables() was not read, so it is not shown to be false without LDM. The frame_compressor.rs diff could not be read, so its effect on non-LDM levels is not established.
Resolution
Provide (a) the high-entropy 1 MiB result on x86_64-musl for level_1_fast_ldm against libzstd, in cycles per frame, to test the 1.15x bound. (b) Evidence that non-LDM levels are unchanged. Either show matches_beyond_window_tables() returns false when LDM is off, and that the frame_compressor.rs borrowed-block cut change runs only under LDM. Or show byte-identical frames and comparable cycles against main for levels without LDM.
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Actionable comments posted: 4
🪄 Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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Inline comments:
Review comments at @zstd/examples/decode_loop_z000033.rs:
- Around line 27-28: Update the XXH64 verification comment to tell users to pass
checksum,ldm when reproducing the benchmark’s LDM decode cell; ldm alone does
not enable checksum verification.
Review comments at @zstd/src/encoding/match_generator/mod.rs:
- Around line 1728-1733: Update borrowed_dict_supported so it returns false when
matches_beyond_window_tables() is true, preventing the borrowed dictionary path
when LDM is enabled; preserve the existing producer and storage checks.
- Around line 1057-1065: In the borrowed skip branch that consumes
borrowed_pending and calls skip_matching_borrowed, take the block’s
long-distance matches before returning whenever LDM is enabled, so they are not
discarded at the next alignment. Add a regression case comparing borrowed and
owned frames with a full 128 KiB repeated-byte block between far_repeats data.
Review comments at @zstd/src/tests/roundtrip_integrity.rs:
- Around line 1039-1112: Extend
a_long_distance_frame_scanned_in_place_is_the_owned_frame with a full-block RLE
run between the repeated runs and compare borrowed and owned output for that
input. Add a separate case using an attached dictionary with level 1 and
long-distance matching enabled; keep the existing round-trip and byte-for-byte
checks for both paths.
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- A block written out unsearched (one byte repeated over the block) was
skipped by the in-place loop without taking its long-distance matches,
so the store was reset at the next block and the search ahead of the
block was lost; the owned loop takes them past. The borrowed skip now
does too. Regression test:
a_long_distance_frame_with_an_unsearched_block_is_the_owned_frame.
- An attached Fast dictionary with long-distance matching: the reset
keeps such a frame owned, but the dictionary's in-place check still
admitted it, tripping the reset's prediction (and in release running
the long-distance search without the dictionary ahead of the input).
The check now refuses long-distance frames. Regression test:
a_long_distance_dictionary_frame_is_the_owned_frame.
- The borrowed-vs-owned comparison is one helper shared by the tests.
- The decode loop's options say the benchmark's LDM cells need
`checksum,ldm`.
## 🤖 New release
* `structured-zstd`: 0.2.5 -> 0.2.6
<details><summary><i><b>Changelog</b></i></summary><p>
<blockquote>
##
[0.2.6](v0.2.5...v0.2.6)
- 2026-10-26
### Added
- *(encode)* faster entropy stage and a lazy match-finder switch
([#626](#626))
### Performance
- *(ldm)* close the level-1 long-distance gap to libzstd
([#632](#632))
- *(encode)* keep the level-1 bit writers' flushes and FSE states off
the critical path
([#630](#630))
- *(encode)* bring the level-1 entropy stage to libzstd's pace
([#628](#628))
</blockquote>
</p></details>
---
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[release-plz](https://github.com/release-plz/release-plz/).
Co-authored-by: sw-release-bot[bot] <255865126+sw-release-bot[bot]@users.noreply.github.com>
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Summary
-C target-cpu=x86-64-v3), a fresh context per frame as thelevel_1_fast_ldmcell runs it, ours against libzstd in cycles per frame: low-entropy 1 MiB 1.52x -> 1.12x (gnu) and 1.41x -> 1.11x (musl), high-entropy 1 MiB 1.36x -> 1.12x (gnu). Page faults per frame on musl drop from about 300 to libzstd's 44.Changes
leaon x86_64 (plain code compiled toadd hash, hashand a load-foldedadd); the feed walks four-byte array chunks; a split is recorded by a cold out-of-line call, so the scan is the fall-through path.encode_loop_oneshot_z000033/ffi_encode_loop_z000033take anldmargument anddecode_loop_z000033anldmencode option, to run the benchmark's LDM cells.Testing
Frames byte-identical to
mainwith and without long-distance matching across levels and fixture shapes, and borrowed against owned long-distance frames by regression tests (repeats past the window, unsearched one-byte blocks, an attached dictionary). Formatting, the CI clippy configurations, the full test suite and doctests pass on aarch64 (macOS); the x86_64 kernels are built and checked on Linux.Closes #631