flate: Simplify matchlen (remove asm)

[klauspost]

With unsafe, there is no benefit from matchlen assembly. Remove it.

Summary by CodeRabbit

• Performance Improvements

  • Enhanced match length calculation efficiency in compression algorithm
  • Streamlined match length computation process
  • Removed architecture-specific assembly implementations

• Code Optimization

  • Simplified matching logic in encoding methods
  • Removed conditional complexity in match length determination

• Architecture Changes

  • Removed specialized AMD64 implementation of match length function
  • Updated generic match length handling

[coderabbitai]

📝 Walkthrough

Walkthrough

The pull request introduces modifications to the compression and matching logic in the flate package. The changes primarily focus on optimizing match length calculations by removing architecture-specific assembly implementations and updating the matching algorithms in Go code. The modifications include removing the matchlen_amd64.go and matchlen_amd64.s files, updating the fast_encoder.go and level1.go files to use a more streamlined approach for calculating match lengths, and removing build constraints in the matchlen_generic.go file.

Changes

File	Change Summary
flate/fast_encoder.go	- Added math/bits import - Modified matchlen and matchlenLong methods to use 8-byte chunk comparisons - Improved match length calculation efficiency
flate/level1.go	- Simplified match length calculation in Encode method - Removed conditional block for match length determination - Directly used e.matchlenLong method
flate/matchlen_amd64.go	- Completely removed architecture-specific matchLen function
flate/matchlen_amd64.s	- Deleted assembly implementation of matchLen function
flate/matchlen_generic.go	- Removed build constraints limiting file compilation

Sequence Diagram

sequenceDiagram
    participant Encoder as Fast Encoder
    participant MatchLen as Match Length Calculator
    
    Encoder->>MatchLen: Calculate match length
    MatchLen-->>MatchLen: Compare 8-byte chunks
    MatchLen-->>MatchLen: Compare remaining bytes
    MatchLen-->>Encoder: Return match length

Loading

Possibly related PRs

• flate: Simplify L4-6 loading #1043: Simplifies offset calculations in Encode methods across multiple compression levels, indicating a broader effort to improve encoding efficiency

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[coderabbitai]

Actionable comments posted: 1

🧹 Nitpick comments (1)

flate/fast_encoder.go (1)

154-179: Optimize matchlen with 64-bit comparisons

The matchlen function has been efficiently rewritten to compare 8 bytes at a time using le.Load64 and detect differences with bits.TrailingZeros64. This improves performance by reducing the number of iterations required for matching sequences.

Consider adding comments to explain the use of bits.TrailingZeros64(diff) >> 3 for clarity, as this bit manipulation may not be immediately evident to all readers.

📜 Review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between dbaa9c1 and 8b2e695.

📒 Files selected for processing (5)

• flate/fast_encoder.go (3 hunks)
• flate/level1.go (1 hunks)
• flate/matchlen_amd64.go (0 hunks)
• flate/matchlen_amd64.s (0 hunks)
• flate/matchlen_generic.go (0 hunks)

💤 Files with no reviewable changes (3)

• flate/matchlen_generic.go
• flate/matchlen_amd64.s
• flate/matchlen_amd64.go

⏰ Context from checks skipped due to timeout of 90000ms (16)

• GitHub Check: fuzz-other ("noasm,nounsafe")
• GitHub Check: fuzz-other (nounsafe)
• GitHub Check: build (1.23.x, windows-latest)
• GitHub Check: build (1.23.x, macos-latest)
• GitHub Check: fuzz-zstd ("noasm,nounsafe")
• GitHub Check: build (1.23.x, ubuntu-latest)
• GitHub Check: fuzz-zstd (nounsafe)
• GitHub Check: build (1.22.x, windows-latest)
• GitHub Check: build (1.22.x, macos-latest)
• GitHub Check: fuzz-s2 ("noasm,nounsafe")
• GitHub Check: build (1.22.x, ubuntu-latest)
• GitHub Check: fuzz-s2 (nounsafe)
• GitHub Check: build (1.21.x, windows-latest)
• GitHub Check: build (1.21.x, macos-latest)
• GitHub Check: build (1.21.x, ubuntu-latest)
• GitHub Check: build-special

🔇 Additional comments (4)

flate/fast_encoder.go (3)

10-10: Import math/bits for optimized bit operations

The addition of "math/bits" is appropriate for utilizing bit manipulation functions like bits.TrailingZeros64, which enhances performance in match length calculations.

---

162-164: Ensure safe unaligned memory access

When using le.Load64 for loading 8-byte chunks, ensure that unaligned memory access is safe on all target architectures. While x86 architectures handle unaligned access gracefully, others like ARM may raise exceptions.

Please confirm that le.Load64 safely handles unaligned memory access across all supported architectures.

---

201-222: Optimize matchlenLong with bulk comparisons

The matchlenLong function now efficiently handles long matches by processing 8 bytes at a time, similar to matchlen. This change enhances performance for larger data sets.

flate/level1.go (1)

122-122: Simplify match length calculation using matchlenLong

Replacing the custom inlined match extension logic with a call to e.matchlenLong streamlines the code and reduces duplication. This enhances maintainability and leverages the optimized function from fast_encoder.go.