crc32: optimize CRC32C computation on arm64 using 6-way parallel processing

[zwtao40]

This PR optimizes CRC32C calculation on arm64 architecture by implementing 6-way parallel processing, achieving significant performance improvements on ARM64 processors.

Background

The current CRC32C implementation on arm64 uses single-lane computation, which cannot fully utilize the pipeline parallelism capabilities of modern ARM64 processors. The CRC32C instructions have a latency of several cycles, creating a bottleneck when processing data sequentially.

Implementation

The optimization extends the original single-lane CRC32 computation to six parallel lanes:

1. 6-way parallel lanes: Each lane operates independently without data dependencies, allowing the processor to schedule multiple CRC32C instructions concurrently.

2. Carry-less multiplication for merging: After all six lanes complete their computations, the intermediate results are merged using VPMULL (carry-less multiplication) instructions for subsequent iterations until the termination condition is reached.

3. Threshold-based dispatch: The parallel path is activated for data sizes >= 1024 bytes, ensuring optimal performance for both small and large buffers.

4. Loop unrolling: The inner loop processes 4 iterations (64 bytes per lane per iteration), totaling 384 bytes per loop cycle across all 6 lanes.

Technical Details

• Uses registers R9, R1-R5 for 6 parallel CRC32C accumulators
• Pre-computed constants (R1-R5) for carry-less multiplication merging
• Leverages ARM64 NEON VPMULL instruction for efficient result combination
• Processes 1024 bytes per large_loop iteration before merging

Performance Benchmark

Tested on Huawei Kunpeng 920 (ARMv8.2-A):

4K: 15 GB/s -> 34 GB/s (+126%)
8K: 14 GB/s -> 35 GB/s (+150%)

The optimization provides approximately 2.3x throughput improvement for typical buffer sizes.

Compatibility

• Requires ARM64 architecture with CRC32 and NEON extensions
• Falls back to the original sequential path for buffers < 1024 bytes
• No changes to API or behavior, fully backward compatible

Testing

• All existing crc32 tests pass
• Benchmark results are reproducible across multiple runs

Updates #79052

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