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huff0: reshape the Decompress4X asm loops around a sentinel bit container - #1209
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…iner Rewrite the 4X main loops in the shape of zstd's HUF_decompress4X1 fast loop. Each of the four streams keeps its bit container and output pointer in a register across iterations; only the input pointers stay in memory, touched once per reload. A sentinel bit sits just below the unread bits, so the consumed count is a trailing-zero count and there is no bitsRead register, no per-symbol fill branch, and no AL/AH byte packing of the output (which lowered to BFI chains on arm64). Five symbols per stream are decoded between reloads for tablelog 9..11 and seven for tablelog <= 8; the reload is branch-free. Bounds are hoisted to an outer loop that bounds iterations by both the output limit and the lowest input pointer, so every read stays inside the block even for corrupt input. The inner decode is 5 instructions per symbol on both amd64 and arm64 against roughly 9-11 before, and there is now a BMI2 twin on amd64. The hand-off back to the Go tail loop is the delicate part. The asm returns the sentinel container and a signed distance from the stream start; a stream can legitimately end with its 8-byte window up to 7 bytes below its own start (those bytes belong to the previous stream), and the sentinel is ORed over the window's lowest data bit, so restoreFromAsm re-reads the window from memory rather than trusting the register, exactly as reference zstd does. prepareForAsm slides the window down a byte for streams whose final byte is 0x01, which would otherwise start with 8 consumed bits and let a full group shift the sentinel out. Reloads use TZCNT in both twins: avo models BSF's destination as read-write (undefined on zero input), which makes the temporary live across the whole loop and overflows the register budget. TZCNT is a pure write, and for the non-zero input the sentinel guarantees it runs as BSF on pre-BMI1 CPUs. Tests: unit tests for the two conversions, a no-BMI2 twin of the 4X roundtrip test, and a no-BMI2 leg in FuzzCompress. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
With one bit per symbol the fixed reload cadence of the 7-symbol loop reloads every 7 bits where the old loop filled every 32, which showed up as +4.7% on the two lowest-entropy benchmark cells on Sapphire Rapids (it was still -39% on Neoverse N1). 14 symbols of at most 4 bits fit the same 56-bit budget, so this is the same generator with a third symbol count. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review. 📝 WalkthroughWalkthroughDecompress4X now uses sentinel-form bit containers and parameterized assembly loops. The fast path selects 5, 7, or 14 symbols per reload by tablelog, supports generic and BMI2 amd64 paths, adds the 4-bit-table path, and restores reader state with corruption checks. ChangesDecompress4X rewrite
Estimated code review effort: 5 (Critical) | ~90+ minutes Sequence Diagram(s)sequenceDiagram
participant Decompress4X
participant bitReaderShifted
participant AssemblyLoop
Decompress4X->>bitReaderShifted: canUseAsm() and prepareForAsm()
Decompress4X->>AssemblyLoop: dispatch tablelog-specific loop
AssemblyLoop-->>Decompress4X: offsets and sentinel containers
Decompress4X->>bitReaderShifted: restoreFromAsm()
Suggested reviewers: Merge Risk: ⚪ Minimal · up to The Decompress4X fast path now uses sentinel-based architecture-specific loops with restored reader state. Coverage includes state handling, output bounds, corruption, CPU variants, and marker-ending streams, leaving no concrete merge-blocking risk in the supplied evidence. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Go fills ctx.ip[i] and ctx.ilowest from the bit reader slices, the asm advances ip, and off is written once in the epilogue as the signed distance from the stream start. The loop keeps the context pointer as its memory base instead of the reader array, which also removes the reload of the context pointer from the stack on every inner iteration. No field changes meaning while the call is in flight. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
The window may reach into the 6-byte jump table below stream 0, which is inside src and already handled by restoreFromAsm as consumed bytes. This keeps the asm running about six bytes longer per stream, which for streams with very short codes (pngdata at ~1.1 bits/symbol, tablelog 1 inputs) shrinks the Go tail from ~115 to ~70 symbols per stream and removes a +9% regression on 10 KB inputs on Sapphire Rapids. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
The outer loop divided the remaining output budget by 8 to get the number of inner iterations it could run before re-checking bounds. That was only valid while an iteration wrote at most 8 bytes per stream, which the 14-symbol tablelog <= 4 loop broke: a batch could advance the output pointers 1.75x past the limit, and with corrupt input (streams whose read pointers stay high, so the input bound never fires) stream 3 wrote past the end of the output buffer before the Go tail reported the corruption. Through zstd's literal buffer that is a heap overrun from untrusted input. Found by review; valid input was not observed to trigger it but was not proven safe either. Divide the output budget by the next power of two at or above the symbol count instead (5 and 7 -> 8, 14 -> 16), and make the generator panic if a future symbol count does not fit its shift. Add a guard-byte test that decodes non-draining random streams under tables of every size into buffers with 4 KiB of canary after the capacity, on both amd64 twins; it fails on the previous assembly at +0 and passes now. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
lizthegrey
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September 10, 2026 05:47
…nt fixes From review of the branch: a canary-checked Decompress4X helper, valid roundtrips under every table size with the output capacity exact (the allocator's size classes otherwise hide small overruns from every other test), an end-to-end case for streams whose final byte is 0x01 so the window slide in prepareForAsm is exercised through the public API, a note on the generic twins that avo's "Requires: BMI" stamp comes from TZCNT which runs as BSF without BMI1, and a pointer from the Go-side limit comment to where the batch bound lives. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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In `@huff0/decompress_test.go`:
- Around line 313-314: Update the test’s encoded stream-length setup around the
src assignments so each length remains within the 16-bit encoding limit, capping
it at 65535 instead of allowing 65536 to truncate to zero. Preserve the test’s
65536 and 262143 output-capacity cases independently from the encoded stream
lengths.
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huff0/_generate/gen.gohuff0/bitreader.gohuff0/decompress_amd64.gohuff0/decompress_amd64.shuff0/decompress_arm64.gohuff0/decompress_arm64.shuff0/decompress_asm.gohuff0/decompress_test.gohuff0/fuzz_test.go
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Review nit: with the four prepareForAsm calls chained by &&, a late false left the earlier readers already slid (a valid but surprising state). canUseAsm is now a pure check and prepareForAsm only runs once all four readers qualify. Also cap the corrupt-input test's stream lengths at 65535: the jump table stores 16-bit lengths, so the 65536 and 262143 cases were encoding a zero length and never reached the assembly. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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Fuzzing completed on amd64 and arm64. Nit addressed. |
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…tinel reload A single stream is one serial dependency chain (peek, table load, shift) with no other stream to hide a reload behind, and the sentinel reload (trailing zero count, pointer arithmetic, load, OR, shift) sat on that chain once per group. On Sapphire Rapids that regressed tablelog 9..11 inputs by 8-14% while still winning on N1. Keep the consumed count in its own register instead, as the low byte of a running sum of whole table entries (the symbol halves never carry into it), and refill without a branch: OR the consumed whole-byte bits back in from the 8 bytes below the window with every shift count computed from the count alone, so the container chain sees a single OR. The right shift by 64-r is two shifts (1, then 63-r, which is 63^r) so that r == 0 contributes nothing. The container invariant is then the Go bit reader's own, so the asm stores off, value and bitsRead directly and restoreFromAsm is not needed for 1X. The 4X code goes back to exactly its klauspost#1209 form apart from the shared constant names. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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…tinel reload A single stream is one serial dependency chain (peek, table load, shift) with no other stream to hide a reload behind, and the sentinel reload (trailing zero count, pointer arithmetic, load, OR, shift) sat on that chain once per group. On Sapphire Rapids that regressed tablelog 9..11 inputs by 8-14% while still winning on N1. Keep the consumed count in its own register instead, as the low byte of a running sum of whole table entries (the symbol halves never carry into it), and refill without a branch: OR the consumed whole-byte bits back in from the 8 bytes below the window with every shift count computed from the count alone, so the container chain sees a single OR. The right shift by 64-r is two shifts (1, then 63-r, which is 63^r) so that r == 0 contributes nothing. The container invariant is then the Go bit reader's own, so the asm stores off, value and bitsRead directly and restoreFromAsm is not needed for 1X. The 4X code goes back to exactly its klauspost#1209 form apart from the shared constant names. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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…#1210) * huff0: reshape the Decompress1X asm loop around the sentinel bit container Generate the single-stream loop from the same sentinel-container decode and reload that the Decompress4X loops use: the consumed count is the container's trailing zero count, so per symbol the loop is a shift, a table load, a shift and a store (5 instructions on both arches, down from 7 plus two fill branches and a byte swap per four symbols), and the reload is branch-free. Three variants decode 5, 7 or 14 symbols per reload for tablelog 9..11, 5..8 and <= 4, sharing the 4X constants (renamed fastSymbols, fast8bSymbols, fast4bSymbols). One stream, so the container, input pointer, output pointer and both loop limits stay in registers; the outer loop bounds a batch by the output capacity and by the distance to the stream start exactly as the 4X loop does, and the Go tail drains the rest. The asm no longer reports ErrMaxDecodedSizeExceeded itself: it stops at the capacity and the Go tail reports the error when bits remain, so the error is the same as before. The hand-off reuses canUseAsm, prepareForAsm and restoreFromAsm. Tests: the guard-byte corrupt-input test and the stream-ends-in-0x01 test now run over both one and four streams, with the small output sizes the 1X entry threshold and batch rounding produce, and the 1X roundtrips gain a BMI2-disabled twin. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> * huff0: refill the 1X container from below the window instead of a sentinel reload A single stream is one serial dependency chain (peek, table load, shift) with no other stream to hide a reload behind, and the sentinel reload (trailing zero count, pointer arithmetic, load, OR, shift) sat on that chain once per group. On Sapphire Rapids that regressed tablelog 9..11 inputs by 8-14% while still winning on N1. Keep the consumed count in its own register instead, as the low byte of a running sum of whole table entries (the symbol halves never carry into it), and refill without a branch: OR the consumed whole-byte bits back in from the 8 bytes below the window with every shift count computed from the count alone, so the container chain sees a single OR. The right shift by 64-r is two shifts (1, then 63-r, which is 63^r) so that r == 0 contributes nothing. The container invariant is then the Go bit reader's own, so the asm stores off, value and bitsRead directly and restoreFromAsm is not needed for 1X. The 4X code goes back to exactly its #1209 form apart from the shared constant names. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> * huff0: share the output-bound shift between the 1X and 4X generators Generated assembly unchanged. Addresses the CodeRabbit nit on #1210. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> --------- Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
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Rewrites the
Decompress4Xassembly main loops in the shape of zstd'sHUF_decompress4X1fast loop, generated from the same avo program for amd64 and arm64.What changed
bitsReadregister, no per-symbol fill branch, and no AL/AH byte packing of the output (which lowered toBFIchains on arm64).consumed>>3bytes, load 8 bytes, OR the sentinel, shift byconsumed&7.SHRX/SHLX), selected viacpuinfo.HasBMI2(); the generic twin keeps the count inCL.The inner decode step is 5 instructions per symbol on both architectures (was roughly 9–11), and the reload is 10 (amd64) / 11 (arm64) per 5–7 symbols.
The hand-off back to Go
The delicate part is returning control to the Go tail loop, and it is where two edge cases live:
restoreFromAsmtreats those bytes as consumed and rejects anything further below, or more bits consumed than the stream holds. This mirrors reference zstd'sHUF_initRemainingDStream.restoreFromAsmre-reads the window from memory rather than trusting the register, again as reference zstd does.0x01starts with 8 consumed bits, which would let a 7-symbol group shift the sentinel out of the container.prepareForAsmslides that window down one byte on entry.Reloads use
TZCNTin both twins. avo modelsBSF's destination as read-write (it is undefined on zero input), which made the temporary live across the whole loop and overflowed the 15-register budget;TZCNTis a pure write and runs asBSFon pre-BMI1 CPUs for the non-zero input the sentinel guarantees.Benchmarks
PCALIGN-pinned before/after trees,
-count 10 -cpu 1, base/head alternated across 5 rounds. The N1 huff0 table is from the final revision (after the batch-bound fix); the Sapphire Rapids and A72 huff0 tables are from the revision before it, which differs only in the batch size of the tablelog ≤ 4 loop (it now re-checks bounds every 16 rather than 8 bytes of budget, so those cells hand a few more symbols to the Go tail); 100-byte cells are omitted (they barely enter the loop and are all within ±1%).Decompress1Xis the control and is unchanged everywhere.Neoverse N1 (oracle, arm64)
1X control (all cells):
geomean 20.07µ 20.05µ -0.06%zstd DecodeAll, Neoverse N1 (first-revision run; the inner loop is unchanged since):
Sapphire Rapids (AWS c7i.2xlarge, amd64, BMI2 twin)
1X control (all cells):
geomean 14.16µ 14.14µ -0.13%Residual on Sapphire Rapids:
pngdata.001on small inputs (+2.9% at 10 KB, +4.7% on the 43µs table cell). That file compresses to ~1.1 bits/symbol at tablelog 10, so the fixed 5-symbol reload cadence reloads every ~6 bits where the old loop filled every 32; on the wide x86 core the old loop was already near its limit there. The same cells are -34%/-28% on the N1. A conditional reload (skip when fewer than 64-S×tablelog bits were consumed) would remove it at the cost of a data-dependent branch per group; not done here. The generic (non-BMI2) amd64 twin is validated for correctness (tests, fuzz seeds) but not timed.zstd DecodeAll, Sapphire Rapids (first-revision run):
Cortex-A72 (daisy, arm64)
1X control (all cells):
geomean 30.60µ 30.62µ +0.07%zstd DecodeAll, Cortex-A72 (first-revision run):
Validation
go test ./huff0/ ./zstd/on: Zen 3 (amd64, both twins viaTestDecompress4XNoBMI2and the new no-BMI2 leg inFuzzCompress), Sapphire Rapids (amd64), Neoverse N1 (arm64), Cortex-A72 (arm64); plus-tags noasm,-tags nounsafe, and a 386 vet.New unit tests for
restoreFromAsmandprepareForAsmcovering the in-stream, below-start, exactly-drained, over-consumed and past-end cases.TestDecompress4XCorruptStaysInBounds(and its no-BMI2 twin): non-draining random streams under tables of every size, decoded into buffers with 4 KiB of guard bytes past the capacity. This exists because an independent review of the branch found that the 14-symbol loop's batch bound still used the 8-byte divisor of the other two loops and could write ~0.75×dstEvery past the output on corrupt input (reachable through zstd's literal buffer). The fuzzers had not seen it: Go's allocator size classes leave slack after small buffers. The test fails at +0 on the previous assembly and passes now (commit f63b5da).Fuzzing: complete, clean on the fixed revision (f63b5da), 60 minutes per fuzzer plus 15-minute
-asanlegs, all exit 0 with no crashers:huff0.FuzzCompresshuff0.FuzzDecompress1xzstd.FuzzDecodeAllzstd.FuzzDecAllNoBMI2zstd.FuzzDecoderhuff0.FuzzCompress -asanzstd.FuzzDecodeAll -asanA first campaign of the same shape on the revision before the batch-bound fix was also clean through its huff0 fuzzers, which is the point of the guard-byte test above: an overrun into allocator slack is invisible to fuzzing.
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