perf(encoding): varint encoding - manual loop-unroll in decodeSingleByteRun for single-byte varints (#578)#578
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perf(encoding): varint encoding - manual loop-unroll in decodeSingleByteRun for single-byte varints (#578)#578srsuryadev wants to merge 3 commits intomainfrom
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…Width, MainlyConstant for faster iteration for SST workload Summary: Add v2 encoding scaffoldings for the Varint, RLE, FixedBitWidth, and MainlyConstant for faster iteration or perf tuning Differential Revision: D96684714
Summary: Add `decodeSingleByteRun` fast path to `bulkVarintDecode32` and `bulkVarintDecode64` that processes leading runs of single-byte varints (values 0-127) using 8-byte word reads before falling through to the BMI2 switch-based decoder. For each 8-byte word where no continuation bits are set (`word & 0x8080808080808080 == 0`), all 8 varints are decoded with simple shifts, avoiding the `_pext_u64` and 64-case switch overhead. This is placed in the caller functions rather than inside `bulkVarintDecodeBmi2` to preserve the BMI2 function's code layout and icache behavior for mixed-width data. Benchmark results (1M elements, mode/opt): | Scenario | Before | After | Speedup | |-----------------------|-----------|-----------|-----------| | 1-byte (32-bit) | 465us | 260us | 1.79x | | 5-byte (32-bit) | slower | 1.22ms | fixed | | 3-byte (32-bit) | 1.04ms | 864us | 1.20x | | 4-byte (32-bit) | 1.50ms | 1.04ms | 1.44x | | 64-bit 1-byte | 294us | 232us | 1.27x | | batch1024 | 1.96us | 1.20us | 1.63x | | Uniform/2-byte/8-byte | unchanged | unchanged | no regress| Also enhances the varint benchmark with fixed byte-width benchmarks (1-5 byte for 32-bit, 1/4/8 byte for 64-bit), skip benchmarks, and batch size benchmarks. Differential Revision: D96617939
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…yteRun for single-byte varints (#578) Summary: Pull Request resolved: #578 Manually loop-unroll `decodeSingleByteRun` with a 3-tier approach: 1. 32-element (4-word) unrolled loop with combined high-bit check `(w0 | w1 | w2 | w3) & kHighBits` to minimize branch overhead 2. 8-element (1-word) loop for smaller runs 3. Single-element trailing loop to pick up individual single-byte varints before multi-byte values Also extracts the byte-expansion logic into a reusable `expandWord()` helper for clarity. Reviewed By: xiaoxmeng Differential Revision: D96619597
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…yteRun for single-byte varints (#578) Summary: Pull Request resolved: #578 Manually loop-unroll `decodeSingleByteRun` with a 3-tier approach: 1. 32-element (4-word) unrolled loop with combined high-bit check `(w0 | w1 | w2 | w3) & kHighBits` to minimize branch overhead 2. 8-element (1-word) loop for smaller runs 3. Single-element trailing loop to pick up individual single-byte varints before multi-byte values Also extracts the byte-expansion logic into a reusable `expandWord()` helper for clarity. Reviewed By: xiaoxmeng Differential Revision: D96619597
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…yteRun for single-byte varints (#578) Summary: Pull Request resolved: #578 Manually loop-unroll `decodeSingleByteRun` with a 3-tier approach: 1. 32-element (4-word) unrolled loop with combined high-bit check `(w0 | w1 | w2 | w3) & kHighBits` to minimize branch overhead 2. 8-element (1-word) loop for smaller runs 3. Single-element trailing loop to pick up individual single-byte varints before multi-byte values Also extracts the byte-expansion logic into a reusable `expandWord()` helper for clarity. Reviewed By: xiaoxmeng Differential Revision: D96619597
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…yteRun for single-byte varints (#578) Summary: Pull Request resolved: #578 Manually loop-unroll `decodeSingleByteRun` with a 3-tier approach: 1. 32-element (4-word) unrolled loop with combined high-bit check `(w0 | w1 | w2 | w3) & kHighBits` to minimize branch overhead 2. 8-element (1-word) loop for smaller runs 3. Single-element trailing loop to pick up individual single-byte varints before multi-byte values Also extracts the byte-expansion logic into a reusable `expandWord()` helper for clarity. Reviewed By: xiaoxmeng Differential Revision: D96619597
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…yteRun for single-byte varints (#578) Summary: Pull Request resolved: #578 Manually loop-unroll `decodeSingleByteRun` with a 3-tier approach: 1. 32-element (4-word) unrolled loop with combined high-bit check `(w0 | w1 | w2 | w3) & kHighBits` to minimize branch overhead 2. 8-element (1-word) loop for smaller runs 3. Single-element trailing loop to pick up individual single-byte varints before multi-byte values Also extracts the byte-expansion logic into a reusable `expandWord()` helper for clarity. Reviewed By: xiaoxmeng Differential Revision: D96619597
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…yteRun for single-byte varints (#578) Summary: Pull Request resolved: #578 Manually loop-unroll `decodeSingleByteRun` with a 3-tier approach: 1. 32-element (4-word) unrolled loop with combined high-bit check `(w0 | w1 | w2 | w3) & kHighBits` to minimize branch overhead 2. 8-element (1-word) loop for smaller runs 3. Single-element trailing loop to pick up individual single-byte varints before multi-byte values Also extracts the byte-expansion logic into a reusable `expandWord()` helper for clarity. Reviewed By: xiaoxmeng Differential Revision: D96619597
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…yteRun for single-byte varints (#578) Summary: Pull Request resolved: #578 Manually loop-unroll `decodeSingleByteRun` with a 3-tier approach: 1. 32-element (4-word) unrolled loop with combined high-bit check `(w0 | w1 | w2 | w3) & kHighBits` to minimize branch overhead 2. 8-element (1-word) loop for smaller runs 3. Single-element trailing loop to pick up individual single-byte varints before multi-byte values Also extracts the byte-expansion logic into a reusable `expandWord()` helper for clarity. Reviewed By: xiaoxmeng Differential Revision: D96619597
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Summary:
Manually loop-unroll
decodeSingleByteRunwith a 3-tier approach:(w0 | w1 | w2 | w3) & kHighBitsto minimize branch overheadvarints before multi-byte values
Also extracts the byte-expansion logic into a reusable
expandWord()helper for clarity.
Reviewed By: xiaoxmeng
Differential Revision: D96619597