fix(encode): copy the dictionary where the reference copies it, index it the way it does, and stop re-entering the parser per literal - #494
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The acceleration factor is the LEADING number of the argument: upstream takes a run of decimal digits plus an optional K/M multiplier and stops, never looking at what follows, clamps a factor past the minimum level instead of refusing it, and treats only a zero factor as an error (zstdcli.c:1133-1153 -> readU32FromCharChecked, 350-376). Four spellings behaved differently here, so a working command line failed against a drop-in build: - `--fast=3.5` / `--fast=3x` were refused; they are level -3. - `--fast=1K` / `--fast=2KiB` were refused; the multipliers are real. - `--fast=200000` was refused; it clamps to the minimum level. - `--fast=+3` was ACCEPTED, because Rust's integer parser takes a sign; a sign is not a digit, so the factor reads as zero and is an error. Every accepted and refused form now agrees with the reference across the whole surface, including the ones that already did.
…it at A dictionary made our ultra-fast frames BIGGER than our own no-dict frames on a 1 MiB corpus file (+1.9 to +2.2%), while it made the reference's smaller (-3.7 to -4.3%). Without a dictionary the two sides emit the identical 20,159 sequences on that file; with one the reference emits 27,546 to our 21,897, and only 0.1% of its extra sequences reach back into the dictionary at all. They are ordinary near matches, found everywhere because its table already holds the dictionary. The Fast attach cutoff was 2 GiB, so every source attached: a separate dictionary table, reached only where the scan's step happens to land, with the live table starting empty. The reference copies above 8 KiB (attachDictSizeCutoffs[ZSTD_fast], zstd_compress.c:2296) and takes the cutoff back to that. It had been raised on a speed argument with no byte column beside it, which is how a 6% ratio hole went unnoticed; the constant now carries both columns. Copy mode then had to index the dictionary the way the reference does. It builds the table once with ZSTD_fillHashTableForCDict (stride 3, the step position winning its slot and the two after it filling only an empty one) and installs it by stripping the tags. We were filling densely, which keeps a nearer occurrence per bucket and fragments one long dictionary match into several short ones. On log-shaped input that alone was 78 bytes against the reference's 71; with the fill it is 73. Alternating the two modes on one compressor then hit a stale table: the cached dictionary table survived a copy frame, and the borrowed-scan dispatch reads exactly that flag, so a copy frame's scan went to the dual-base kernel and read raw positions as virtual ones. A frame that is not an attach frame now drops the cached table with the live one. The existing alternation test covers it and had never run before, because at a 2 GiB cutoff its 64 KiB payload attached like everything else. Bytes, on z000033 (1,022,035 B) with its 16 KiB dictionary, ours against the reference: --fast=7 +5.95% -> -0.08%, --fast=5 +6.15% -> -0.04%, --fast=3 +6.43% -> -0.01%. Every level is now at or under it. No-dict output is byte-identical across 27 fixture-and-level rows, as are all dictionary frames at or under the cutoff and every non-Fast level.
…it away Invalidating it in `reset` also hit ATTACH frames, whose whole point is living off that cache: rebuilding the dictionary table every frame cost 12% on a reused 4 KiB dictionary frame (i9, wall clock, three interleaved rounds, 0.623 s -> 0.706 s). That size is below the attach cutoff, so it is a path this work does not otherwise touch — the regression showed up as a control arm that would not stay flat. The copy prime is the only place that must not keep the cache, so it drops it there.
The decision walked the literals TWICE, a byte and a dependent table load at a time, once for the previous table and once for the new one. It is the same sum read off the histogram the frame has already built: a symbol contributes its code length once per occurrence, so summing over at most 256 symbols gives the identical number. That is where upstream reads it from as well (HUF_estimateCompressedSize over count, huf_compress.c:1416-1417, after HUF_validateCTable checks representability off the same histogram). The two walks were the largest single item outside the matcher in a 4 KiB dictionary frame's profile. Output is byte-identical over 80 frames (five fixture shapes x eight levels x with and without a dictionary).
`sufficient_match_len_for_pass` is a block constant (the profile's value against `target_len`), and the DP body derived it on entry. On input the search finds nothing in, that body is entered once per LITERAL, so the clamp was an out-of-line call per literal — it shows up as its own symbol at 2.4% of a level-13 dictionary frame on random input. Both passes now clamp before their segment loop, and the body asserts the caller did. Byte-identical over 36 frames across the optimal band.
The two paths had drifted to different answers for one question. A block the driver wrote off is not searched; the only reason to index it at all is that a LATER block may duplicate it, that duplicate is recognised on the seen-content grid and then searched, and the search sweeps positions — so an entry every stride bytes is met within a stride of scanning, immaterial against a block-sized match. The fast path reasons exactly that way and indexes every 512th position; the lazy/row path was still indexing every 8th, which is 131,000 stores per mebibyte of input that nothing will search, and it was 82% of the encode on a high-entropy megabyte. Wall clock, i9, three interleaved rounds: incompressible 1 MiB, level 5 0.223 s -> 0.052 s (1343 -> 5627 MB/s) 1 MiB repeated verbatim, L19 0.0446 s -> 0.0142 s Level 5 on that input is 287 us per frame against the reference's 400. Output is unchanged on 65 of 66 fixture-and-level rows — including the block-duplicate fixture at every level but 19, where the wider stride costs 506 bytes in 524,879 (0.1%). That fixture is a megabyte of random bytes repeated exactly; input that genuinely repeats compresses, so it never reaches this path at all.
Upstream's optimal loop takes a position the search finds nothing at as
one literal and moves on INSIDE its own loop (ZSTD_compressBlock_opt_generic,
if (!nbMatches) { ip++; continue; }). Ours returned it to the caller, so on
input the search finds nothing in — which is every position there — the
caller re-entered the DP body per literal and paid its 440-byte frame each
time. The hot instructions of that body on such input were its prologue and
epilogue, not any loop in it.
The run is now walked in place and handed back whole. The number of
searches is unchanged: the run stops at the first position with candidates
and the caller re-enters there. That position must NOT be searched twice —
the search inserts it into the binary tree, and inserting one position
twice corrupts the tree — so the run records what it searched and the
re-entry reads the answer out of the candidate buffer instead of asking
again.
Skipped when the LDM producer is active: its state machine is rebuilt per
call from the segment's block offset, so advancing inside one call is not
the same thing. HAS_LDM is a const generic, so that folds away.
Byte-identical over 36 fixture-and-level rows across the optimal band.
Two field reads and a clamp, taken once per searched position the way upstream takes ZSTD_getLowestMatchIndex — but it was standing in the profile as its own symbol, so it was paying a call and a return for four instructions of work. Its neighbours in the same file already carry the attribute; this one had been missed. Byte-identical over 24 fixture-and-level rows.
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📝 WalkthroughWalkthroughThe change updates Fast-level parsing, Huffman estimation, Fast dictionary priming, optimal-parser candidate reuse, match-table behavior, coordinate handling, decode documentation, and reference-parity tests. ChangesFast-level CLI parsing
Compression-path updates
Priority: ➖ Normal — Impact reflects medium issue severity. Estimated code review effort: 4 (Complex) | ~60 minutes Severity of issue fixed: Medium Merge Risk: 🔵 Low · up to The new alternating dictionary benchmark can misreport the amount of input compressed, which can mislead performance comparisons when alternate frame sizes are enabled. This is a bounded reporting issue and should be corrected before relying on those measurements. Sequence Diagram(s)sequenceDiagram
participant MatchGenerator
participant FastMatcher
participant HashTable
MatchGenerator->>FastMatcher: select copy-mode dictionary path
FastMatcher->>HashTable: prime dictionary with stride-three fill
HashTable-->>FastMatcher: preserve next fill position
FastMatcher-->>MatchGenerator: update loaded_dict_end
sequenceDiagram
participant OptimalCaller
participant OptimalParser
participant PlanBuffers
OptimalCaller->>OptimalParser: start block with clamped profile
OptimalParser->>PlanBuffers: check candidates_searched_at
PlanBuffers-->>OptimalParser: return cached candidates or empty state
OptimalParser->>PlanBuffers: store candidates and searched query
OptimalParser-->>OptimalCaller: return literal run or optimal sequence
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✅ Passed checks (4 passed)
Full details: Out of Scope Changes checkExplanation The pull request includes changes outside [ ✨ Finishing Touches📝 Generate docstrings
🧪 Generate unit tests (beta)
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After a search the parser jumps its lazy tree-insert cursor to where the match it found ENDS, on the reasoning that positions inside a match are covered. That end is the end of the match in the SOURCE (upstream `matchEndIdx = matchIndex + matchLength`, zstd_opt.c:747-748 for the live walk and :794-795 for the dictionary one), and the live walk here already used that form. The dictionary walk measured it from the position being SEARCHED instead. A dictionary candidate sits before that position, so the cursor ran ahead by the offset and the positions it passed never entered the tree. A later search then finds an empty bucket where the reference finds a long match. It shows only with a dictionary attached, because only a dictionary candidate reaches back far enough, and hardest where the search is shallowest: upstream resolves level 11 at 4 KiB to btopt with `searchLog` 3, so a search gets eight candidates and cannot afford an empty bucket. Traced on the benchmark's `small-4k-log-lines` scenario: a dictionary match at position 319 (offset 161, length 42) moved the cursor to 353 where upstream moves it to 192, so positions 200-353 were never indexed, and the search at 701 walked one node and stopped. The reference codes the rest of that block as a single 3695-byte match; we spent four sequences on the same span. `compare_ffi` REPORT_DICT on that scenario, ours against the reference: level_11_lazy 52 -> 45 bytes against 46, level_12_lazy 51 -> 45 against 46. Both now come in under it; level_10 and level_13 are unchanged. No-dict output is byte-identical over 24 fixture-and-level rows, and no dictionary row anywhere in the sweep grew. Also widened the cparams parity grid to every level: the sampled list skipped 11 and 13, and a level row is exactly the kind of thing that can be wrong on its own while its neighbours are right. It passes, which is what ruled the resolved parameters out as the cause here. Closes #495
Taking the attribute off is worth 2.7% on a dictionary decode, where it is the common path (23 matches a frame on the benchmark's small-10k-random scenario), and costs 3.5% on an ordinary decode, where it is not taken at all and only its placement matters. Measured on the i9, wall clock, three interleaved rounds of prebuilt binaries, with instruction counts unchanged either way — so the difference is code placement, and the ordinary decode is the path that runs far more often.
…' into fix/#323-fast-dict-attach-cutoff
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`dict_idx` indexes the live history, while `match_end_abs` and the tree's insert cursor are absolute. A reused dictionary context advances `history_abs_start` between frames, so from the second frame on the comparison put a small relative end against an absolute one, the cursor never advanced, and the parser went back to inserting every covered position: the work this was meant to skip. The first frame hid it, because the base is zero there. The invariant is now asserted in debug beside the computation, and it fires on the reused-context test with the relative form restored, so the whole debug suite carries the check rather than one fixture. The optimal-band ratio test also compresses three frames on ONE compressor now, which is the shape that moves the base.
…gainst us The evidence for the wider stride was ours-before against ours-after: no same-run reference figure, and no instruction counts, on either fixture — including the repeated-block one whose output the change grows. Taken now, three arms in one session (before, after, and the C reference through `ffi_encode_loop_z000033`), `perf stat -r 3`, three rounds each. Incompressible 1 MiB at level 5: cycles 1.66 G -> 0.34 G against the reference's 1.03 G, instructions 1.93 G -> 0.30 G against 0.83 G — from 1.58x of the reference to 0.33x. The 1 MiB block repeated verbatim at level 19: 0.82 G -> 0.083 G cycles against the reference's 12.7 G, and it is the row that costs bytes: 524,365 -> 524,871 against the reference's 524,361, so 510 bytes in 524,871 (0.1%) while running 150 times faster than it. The constant now carries those tables instead of the internal before/after pair.
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…dths The coverage report named four lines in the copy-mode fill that nothing reached: its two early returns and the hash-width arms past 4. - A dictionary shorter than one hash read leaves instead of computing `history.len() - HASH_READ_SIZE`, and still records the boundary. - A slice carrying nothing hashable past the stride cursor indexes nothing, rather than walking the same positions again. That is also the shape a history shrunk by eviction leaves behind. - The fill runs at every width the table accepts (4 through 8): the widths are a `match`, so only the ones a test builds are exercised, and 4 alone left the rest cold. What remains uncovered there is the `unreachable!()` arm, which the table's constructor makes unreachable by rejecting any other width.
A copy-mode frame must not reach the attached table, and said so by discarding it. A compressor whose source sizes cross the attach cutoff then rebuilt that table on every attach frame it came back to, which the neighbouring note already prices at 12% of a reused 4 KiB dictionary frame. It is set aside instead: while it is away every reader sees exactly the state a discard leaves, so no frame's output can depend on it still existing, and the next attach prime takes it back and runs the same shape check it would have run on a table that never left. A dictionary change still goes through `invalidate`, which drops the stash with it. Byte-identical over 24 fixture-and-level rows against the branch tip. The dictionary encode loop grows an `alt<N>` argument that alternates each frame between the input and its first N bytes, so the mode-switching shape this is about can be measured rather than argued about.
Setting the attached table aside on a copy frame, so the next attach frame would not hash the dictionary again, was built and then measured on the shape it exists for: frames alternating either side of the attach cutoff on one compressor (`encode_loop_dict … alt<N>`, 20 000 frames of 64 KiB and 4 KiB with a 16 KiB dictionary, i9, `perf stat -r 3`, three rounds). Retired instructions came out identical — 30,163,334,509 against 30,163,334,255, a difference of 254 in 30 billion. So the attach frame that follows a copy frame is not rebuilding anything to begin with, and the 0.9% of cycles that moved is the size of a code-layout change. The stash was state and two methods for no work removed, so it goes. The measurement is recorded where the discard happens, so the next reader does not build it again to find out.
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In `@zstd/examples/encode_loop_dict.rs`:
- Around line 118-121: Update the encode loop’s final input-byte reporting to
accumulate each selected frame’s frame.len() value, rather than calculating
src.len() multiplied by iters. Ensure alternating src and alt frames contribute
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…tionary `repeat_inner` is `inline(always)`, so calling it from the dictionary path pulled the whole copy machinery — overlapping copies, the wildcopy variants, their error paths — into that path's body, and every dictionary match paid the prologue and epilogue of a frame sized for code most of them never run. On a dictionary-heavy frame that is the common path: 23 matches a frame at 112 instructions a call on the benchmark's small-10k-random scenario. Behind a call the frame belongs to the tail. 13.2 million retired instructions on 200 000 frames of that scenario (3.1032 -> 3.0900 G, i9, `perf stat -r 3`). Cycles and wall clock did not move with it (1245-1266 -> 1259-1266 ns a frame, ranges overlapping), so this is fewer operations and not a speed claim. It was dropped a commit ago for exactly that reason, which was wrong: a timer too coarse to resolve a fraction of a percent is not evidence against work that is provably gone. Only a measured increase in cycles would be.
The Fast attach-vs-copy cutoff, the parser's literal-run walk and the folded window-floor helper each landed with an argument and an incomplete number. All three are now measured the same way: two prebuilt binaries and libzstd alternated in one ssh session, perf stat -r 3 for cycles AND retired instructions, three rounds, plus a control arm the change cannot execute. The cutoff table gains its two missing cycle cells, an instruction column and absolute byte counts. Copy takes 18-28% fewer cycles and 22-27% fewer instructions at the positive levels and matches or beats the reference's bytes there; at the ultra-fast levels it spends 9-21% more cycles to save 5.5-6.0% of the bytes, which is the trade the cutoff exists to make. The parser's literal-run walk is a speed win and is now stated as one: -29.5% cycles and -22.8% instructions per frame on near-random input at level 19, 1.94x -> 1.37x of libzstd, byte-identical. The folded window floor is NOT a speed win and no longer reads like one. It removes 164,648 retired instructions a frame on that fixture, but the control arm moves 3.6% of cycles on its own, so the clock cannot resolve it. Kept for the operations that are provably gone. Also fixes the encode loop reporting src.len() x iters as its input total under alt<N>, where the frames are two different sizes.
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Summary
The dictionary made our ultra-fast frames bigger than our own no-dict frames
on anything past 8 KiB, while it made the reference's smaller. Root-causing that
led through the parser and the block-skip path as well, so this also closes three
rows the dashboard had flagged as outside band.
Every number below is from the i9, wall clock, three interleaved rounds of
prebuilt binaries, with the reference (
c_ffi) measured in the same session.The dictionary defect (#323)
The issue's own fixture (
small-4k-log-lines, 437 B dictionary) is nowbyte-identical to the reference at every Fast level: -7/-6 at 51 B, -5/-2/-1/1/2
at 44 B, -4 at 51 B, -3 at 50 B, against the issue's reported 61 B versus 48-55 B.
The larger defect was above the attach cutoff.
ZSTD_shouldAttachDict(zstd_compress.c:2309) copies the dictionary for a Fast source over 8 KiB, and the
copy is what makes the dictionary pay: the scan then runs over a table already
holding the dictionary's positions, so ordinary near matches improve everywhere.
Our cutoff was 2 GiB, set on a speed argument with no byte column beside it, so
every source attached instead. On
decodecorpus-z000033(1,022,035 B) with its16 KiB dictionary, ours against the reference:
--fast=7--fast=5--fast=3The comparator says why: without a dictionary the two sides emit the identical
20,159 sequences; with one the reference emits 27,546 to our 21,897, and only
0.1% of its extra sequences reach back into the dictionary at all.
Copy mode then had to index the dictionary the way the reference does. It builds
the table once with
ZSTD_fillHashTableForCDict(stride 3, the step positionwinning its slot, the two after it filling only an empty one) and installs it by
stripping the tags (
ZSTD_copyCDictTableIntoCCtx, zstd_compress.c:2386-2400). Wewere filling densely, which keeps a nearer occurrence per bucket and fragments one
long dictionary match into several short ones: on log-shaped input that alone was
78 B against the reference's 71, and 73 B with the fill.
Alternating the two modes on one compressor then hit a stale table: the cached
dictionary table survived a copy frame, and the borrowed-scan dispatch reads
exactly that flag, so a copy frame's scan went to the dual-base kernel and read
raw positions as virtual ones. The existing alternation test covers it and had
never run, because at a 2 GiB cutoff its 64 KiB payload attached like everything
else.
Speed, on the rows the dashboard flagged
All three turned out to be slower than the reference, not faster, and all
three are input the search finds nothing in.
compress-dict/level_13_lazy/small-10k-randomcompress-dict/level_16_btopt/small-10k-randomcompress/level_5_greedy/high-entropy-1mLevel 5 on incompressible input now beats the reference at 0.54x. What the
profile said, and what each change did:
lazy/row path while the fast path indexes every 512th with the identical
documented reasoning. That was 82% of a high-entropy megabyte, for entries
nothing will search. Aligned: 0.223 s -> 0.052 s on that input.
loop takes a position with no match as one literal and moves on inside its own
loop; ours handed it back, so on input without matches the caller re-entered the
DP body per literal and paid its 440-byte frame each time. Walking the run in
place put the parse at 0.85x of
ZSTD_compressBlock_opt2, from 6.1x.table load at a time, where upstream reads the same sum off the histogram
(
HUF_estimateCompressedSize, huf_compress.c:1416-1417). On dictionary framesthat was -17% at 4 KiB and -28% at 10 KiB of wall clock.
and
window_low_abs_for_targetfolded into its caller (it was standing in theprofile as its own symbol, paying a call and a return for four instructions).
The lazy-band ratio defect (#495)
Two levels emitted more than the reference on
small-4k-log-lineswith itsdictionary (52 B and 51 B against 46) while the levels either side were fine.
After a search the parser jumps its lazy tree-insert cursor to where the match
ENDS, on the reasoning that positions inside a match are covered. That end is
the end of the match in the SOURCE (upstream
matchEndIdx = matchIndex + matchLength, zstd_opt.c:747-748 for the live walk and :794-795 for thedictionary one), and the live walk here already used that form. The dictionary
walk measured it from the position being SEARCHED instead, so the cursor ran
ahead by the offset and the positions it passed never entered the tree.
Traced on that fixture: a dictionary match at position 319 (offset 161, length
42) moved the cursor to 353 where upstream moves it to 192, so positions 200-353
were never indexed, and the search at 701 walked one node and stopped. The
reference codes the rest of that block as a single 3695-byte match; we spent four
sequences on the same span. It only shows with a dictionary attached, because
only a dictionary candidate reaches back that far, and hardest where the search
is shallowest: upstream resolves level 11 at 4 KiB to btopt with
searchLog3.compare_ffiREPORT_DICT, ours against the reference: level_11_lazy 52 -> 45against 46, level_12_lazy 51 -> 45 against 46. Both now come in under it; 10 and
13 are unchanged, no-dict output is byte-identical over 24 rows, and no
dictionary row anywhere in the sweep grew.
The cparams parity grid also now covers every level instead of a sample of them
(it skipped 11 and 13). It passes, which is what ruled the resolved parameters
out as the cause.
CLI
--fastnow reads its argument the way the reference reads it(zstdcli.c:1133-1153 ->
readU32FromCharChecked): the leading digit run plus anoptional K/M multiplier, ignoring what follows, clamping a factor past the minimum
level instead of refusing it, and treating only a zero factor as an error. Four
spellings behaved differently before:
--fast=3.5and--fast=3xwere refused(they are level -3),
--fast=1Kand--fast=2KiBwere refused,--fast=200000was refused, and
--fast=+3was accepted although a sign is not a digit.Verification
Output is unchanged wherever it should be, checked by frame md5 rather than
length: 27 no-dict rows against
main, all dictionary frames at or under theattach cutoff, every non-Fast level, 80 frames across the histogram-estimator
change, and 36 rows across the optimal band for the parser change. The one
deliberate exception is a synthetic megabyte of random bytes repeated verbatim at
level 19, where the wider skip stride costs 506 B in 524,879 (0.1%); input that
genuinely repeats compresses and never reaches that path.
Local gates: 1050 library tests, 63 FFI interop tests, doc tests,
clippy --workspace --all-targets,fmt, and all four no-std / embedded clippyconfigurations the CI job runs.
Measured, not fixed: dictionary DECODE
small-10k-randomdecompress-dict is 1173 ns against the reference's 604(1.94x). The same scenario without a dictionary is 134 vs 123 ns, but that frame
is stored raw; the honest no-dict comparison is a compressed frame, where we are
1.43x. So the dictionary adds roughly half an x on top of a general decode gap.
Per-frame, from a dual-arm profile (normalised for the frame-count difference),
ours against the reference: sequence stage 43.5 + 12.7 (
repeat_from_dict, whichthe reference inlines) against 30.2; FSE table build 15.1 against 8.6; per-frame
plumbing 6.4 against 1.1. Callgrind puts
repeat_from_dictat 23 calls a frameand 112 instructions a call, with its hot instructions in its own prologue,
epilogue and a GOT-indirect call rather than the copy.
One change was tried and rejected on its control arm: dropping
#[cold]fromthat function is worth 2.7% on a dictionary decode and costs 3.5% on an ordinary
one, with instruction counts unchanged either way. The attribute now carries that
measurement as a comment so the next reader does not repeat it. A real fix wants
a lean inline fast path for "the whole match lies inside the dictionary content"
that does not disturb the no-dict layout; that is not attempted here.
Closes #323
Closes #495
Summary by CodeRabbit
New Features
--fastvalue parsing with numeric multipliers such asK,M,KiB, andMB.Bug Fixes
--fastarguments.Performance