RISC-V：Optimize decompression throughput by mirroring AVX fast-path for RVV short memcpy +15%

[anthony-zy]

PR Title

Optimize RVV memcpy path to mirror AVX fast-path for short copies

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Summary

Refactors the RISC-V Vector (RVV) acceleration path in the short memcpy helper to mirror the existing AVX implementation. Instead of a generic vsetvl loop, this version performs a fixed 32-byte vector copy (with an optional second 32-byte segment), matching the profile-driven assumption that nearly all copies are short.

This eliminates loop overhead in the hot path and aligns RVV behavior with the well-tuned AVX code path.

Performance

Tested with lzbench on Spacemit(R) X60 (RISC-V, RVV 1.0):

• Before: 269 MB/s
• After: 310 MB/s
• Improvement: +15%

Implementation Details

• Uses __riscv_vsetvl_e8m2(32) to configure a 32-byte vector operation, matching the kShortMemCopy constant used in the AVX path.
• Performs an unconditional first 32-byte load/store.
• A second 32-byte segment is conditionally executed (with SNAPPY_PREDICT_FALSE) only when size > kShortMemCopy, since profiling shows long copies are rare.
• Note: with e8m1 and VLEN < 256 (e.g., VLEN=128, vl=16), two segments per 32 B would be required; using e8m2 ensures 32 B per op on common configurations.

Verification

• Environment: Spacemit(R) X60 (RISC-V, RVV 1.0)
• Benchmark tool: lzbench
• Compiled with -march=rv64gcv
• Passed all existing Snappy unit tests.