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perf(encode): keep the level-1 bit writers' flushes and FSE states off the critical path - #630
Takes the level-1 bit writers' flush guards and the sequence loop's FSE-state spills off the critical path, as upstream's loops have them, and trims the sequence-code pass, the literal gather and the Huffman table build. Compressed output is byte-identical.
runner4 against main (quiet, interleaved prebuilt bench-profile binaries, decode control flat): cycles 1 KiB frames level 1 -6.8% and level 3 -6.4%, 16 KiB frames level 1 -5.1%, decodecorpus z000033 level 1 -1.7% and level 3 -2.2%; instructions fall 1.2..8.3% at every measured level from -5 to 9.
Changes
Huffman literal stream: flush with one masked shift instead of a zero guard (HUF_flushBits shape).
Sequence bit burst: the accumulator keeps a bit free (nbBits + bitPos < 64, debug-asserted at every add), so a flush shifts without a full-word guard.
Sequence loop: the parts of the three streams' tables it reads are gathered into one view at fixed offsets on the stack, built only for a block that runs the loop, so two of the three FSE states stay in registers; an RLE stream is a one-state table that writes nothing.
Sequence codes: a short length's code and extra-bit width come from one fused table; from 64 (128 for a match) the width is the length's high bit.
Literal gather: a run's wildcopy slack is tested with one compare.
Huffman table: code lengths are set top-down in one pass over the tree, and codes are built from one count per weight, clearing only the packed codes a reused table wrote past this one.
Border pre-split: a byte fingerprint clears only the 256 slots it uses.
Testing
Frames byte-identical to main across levels -5..22 on four fixture shapes and on 2,090 dictionary frames. Formatting, the CI clippy configurations, the full test suite and doctests pass on aarch64 (macOS); the x86_64 BMI2/AVX2 tier builds and its frames are checked on Linux.
The flush guarded `container >> (64 - nb_bits)` against an empty
container with a test and a conditional move on the store's data path,
twice per unrolled group. Upstream's HUF_flushBits shifts unguarded; a
wrapping shift does the same without the guard: with no bits pending
the cursor does not move, so the bytes written are never committed.
Frames byte-identical across levels and dictionary frames. runner4,
one-shot loop, quiet host: instructions log8m L1 -0.4%, z000033 L1
-0.63%; cycles within noise.
The burst let its accumulator fill all 64 bits, so every flush guarded
the shift by the bytes it wrote with a compare and a conditional move on
the data path. Upstream asserts the accumulator keeps a bit free
(`nbBits + bitPos < 64` in BIT_addBitsFast) and flushes unguarded; the
sequence loop's budget never passes 63 bits, so the burst now holds the
same precondition, debug-asserted at every add, and flushes with a plain
shift.
Frames byte-identical across levels and dictionary frames. runner4:
instructions log8m L1 -0.15%, z000033 L1 -0.1%, L9 flat; the host was
loaded, so no cycle delta is claimed.
The sequence loop held three table pointers in registers and spilled
the three stream states, which are on its dependency chain, so every
transition paid a store and a reload. The parts the loop reads are now
gathered per call into one view at fixed offsets on the stack, addressed
from the stack pointer; two of the three states now stay in registers,
as upstream's loop keeps them. An RLE stream is a one-state table that
writes nothing, so the loop no longer asks per sequence whether each
stream has a table.
Frames byte-identical across levels and dictionary frames. runner4,
quiet, 3 interleaved rounds, decode control flat:
- log8m L1: instructions -1.2%, cycles median -1.2%
- z000033 L1: instructions -0.8%, cycles -1.0%, no overlap
- z000033 L9: instructions -0.2%, cycles about -1%
- small 1 KiB L1: instructions +0.27% (view setup), cycles -2.5%, no
overlap
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Walkthrough
The sequence encoder now uses shared FSE state tables. Bit-writer flushes, length encoding, literal-copy bounds, Huffman table construction, and byte histogram clearing also change.
Changes: Entropy encoding
Layer / File(s)
Summary
Sequence FSE state transitions zstd/src/fse/fse_encoder.rs, zstd/src/encoding/blocks/compressed.rs
SequenceStateTables stores transition data for three streams. The sequence encoder uses it for LL, ML, and OF transitions, including streams without an FSE table.
Bit-writer accumulator and flush behavior zstd/src/bit_io/bit_writer.rs, zstd/src/bit_io/bit_writer/tests.rs, zstd/src/huff0/huf_cstream.rs, zstd/src/encoding/blocks/compressed.rs
BitBurst::add requires fewer than 64 accumulated bits, and BitBurst::flush uses a direct shift. Tests cover a 63-bit flush and a debug-build panic for a 64-bit addition. Huffman flush paths use wrapping shifts without changing byte counts or cursor positions.
Length encoding and literal-copy bounds zstd/src/encoding/blocks/compressed.rs, zstd/src/encoding/blocks/compressed/literal_runs.rs, zstd/src/encoding/blocks/compressed/tests.rs
Literal and match lengths use fused lookup tables for short values and runtime upper-bound assertions. Literal-copy paths use revised bounds checks, with a regression test for an oversized trailing literal count.
Huffman code and depth construction zstd/src/huff0/huff0_encoder.rs
Huffman code construction counts symbols by weight and assigns canonical codes in one pass. Leaf depths are derived from parent depths in a descending traversal.
The oversized-tail copy path is guarded, and no remaining merge-blocking issue was established. This change is ready for normal checks.
Pre-merge checks: 5
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Explanation
Description Check
Check skipped - CodeRabbit’s high-level summary is enabled.
Title check
The title names the main change: performance work on level-1 bit writers, moving flushes and FSE state handling off the critical path. The raw summary and objectives show this is the central aim, with…
Linked Issues check
The PR satisfies the coding requirements in [#629]. HUF_flushBits uses an unguarded masked shift. BitBurst enforces the strictly-below-64-bit invariant and flushes without the full-word guard. `Se…
Out of Scope Changes check
The changes remain within [#629]. The fused sequence-code lookup, literal gather check, Huffman-table construction, and fingerprint clearing optimize the same encoding path. The added tests support bi…
Docstring Coverage
Docstring coverage is 85.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 9 files.
Full details: Title check
Explanation
The title names the main change: performance work on level-1 bit writers, moving flushes and FSE state handling off the critical path. The raw summary and objectives show this is the central aim, with the bit-writer flush changes, FSE state tables, and sequence encoding work all serving it. The title also uses the conventional perf(...) prefix. It does not mention the secondary Huffman, literal-gathering, and frame-compressor changes, which is acceptable because a title need not cover every detail.
Full details: Linked Issues check
Explanation
The PR satisfies the coding requirements in [#629]. HUF_flushBits uses an unguarded masked shift. BitBurst enforces the strictly-below-64-bit invariant and flushes without the full-word guard. SequenceStateTables gathers FSE data into a stack view and supports one-state RLE streams. Tests cover the 63-bit flush, the debug assertion, and the literal tail bound. The PR reports byte-identical output across levels and dictionary frames, lower level-1 cycles, and no test regressions.
Full details: Out of Scope Changes check
Explanation
The changes remain within [#629]. The fused sequence-code lookup, literal gather check, Huffman-table construction, and fingerprint clearing optimize the same encoding path. The added tests support bit-writer and encoder safety. No unrelated change is demonstrated.
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A block of one sequence is coded through the start states alone and
never runs the transition loop, but it still cleared and filled the
view. The view is now built inside the loop's branch, and the final
states are written from the tables themselves. It stays a local: off
the stack pointer it needs no base register, and kept on the compressor
behind a pointer the loop spilled one more state (+0.4..0.6%
instructions at level 1).
The gather tested the room left in the block and the run against it
separately, which compiled to a subtract, two setcc and a test per run.
The run, its start and the slack fit 64 bits with no overflow, so one
compare decides whether the chunked copy stays inside the block.
Each leaf's depth was found by walking its parents to the root, a chain
of dependent loads per leaf. Every node's parent is built after it and
so sits at a higher index; one descending pass that gives each node its
parent's depth plus one sets every depth, upstream's HUF_buildTree
shape. The depths are the same.
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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Inline comments:
Review comments at @zstd/src/encoding/blocks/compressed/literal_runs.rs:
- Line 110: Replace the overflow-prone sum in the chunked-copy condition with a
remaining-room check: ensure at least WILDCOPY_SLACK bytes remain after pos,
then verify len fits within the remaining room after reserving that slack. This
must reject oversized custom-matcher literal lengths before the raw copy.
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The code pass looked each length's code up, bounds-checked it, then
looked the code's extra-bit width up in a second table. A short length
now reads both from one fused table, and from 64 up (128 for a match)
the width is the length's high bit, so that branch reads no table. The
range check that kept the codes inside the histograms moves into that
rare branch as an assert, with no value in its message: formatting one
kept the length on the stack for the whole loop.
runner4, frames byte-identical: instructions log8m L1 -0.79%,
z000033 L1 -0.52%, L3 -0.24%; cycles within noise.
Building codes from weights walked the weights twice before assigning
any: once for the sum that gives the table log, once for the symbols
per code length. Both follow from the thirteen symbols-per-weight
counts, which one walk collects. The codes are then appended in the
assigning pass, and a reused table clears only the packed codes the
previous table wrote past this one, not all 256.
runner4, frames byte-identical: instructions small 1 KiB L1 -2.06%,
small 16 KiB L1 -0.72%, z000033 L1 -0.23%; cycles small 1 KiB about
-1.5%, the rest within noise.
The border pre-split records byte histograms, which fill, compare and
read 256 slots, but each recording cleared the whole 1024-slot table,
on top of the zeroed table it was given: two 3 KiB clears a block that
nothing reads.
The gather's single-compare room test sums the read position, the run
length and the slack in 64 bits, which a run's u32 length cannot wrap.
The tail is not a run: a custom matcher's literal reports are added to
it unchecked, so a count near usize::MAX wrapped the sum and let a copy
of that length through before the coverage check. The tail is now held
to the block once, before the loop, so the per-run test stays one
compare.
Regression test: a_tail_past_the_block_is_refused_before_the_copy.
## 🤖 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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Co-authored-by: sw-release-bot[bot] <255865126+sw-release-bot[bot]@users.noreply.github.com>
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Summary
main(quiet, interleaved prebuilt bench-profile binaries, decode control flat): cycles 1 KiB frames level 1 -6.8% and level 3 -6.4%, 16 KiB frames level 1 -5.1%, decodecorpus z000033 level 1 -1.7% and level 3 -2.2%; instructions fall 1.2..8.3% at every measured level from -5 to 9.Changes
HUF_flushBitsshape).nbBits + bitPos < 64, debug-asserted at every add), so a flush shifts without a full-word guard.Testing
Frames byte-identical to
mainacross levels -5..22 on four fixture shapes and on 2,090 dictionary frames. Formatting, the CI clippy configurations, the full test suite and doctests pass on aarch64 (macOS); the x86_64 BMI2/AVX2 tier builds and its frames are checked on Linux.Closes #629
Summary by CodeRabbit