Felix-Gong opened a new pull request, #3390:
URL: https://github.com/apache/brpc/pull/3390
## Summary
This patch adds RVV-accelerated `memcmp` and `memchr` implementations for
`StringPiece` operations on RISC-V 64-bit platforms.
The implementation follows glibc's official RVV pattern:
- **e8m8 LMUL** for maximum throughput
- **Hardware-adaptive vector length** via `vsetvl`
- **`vfirst.m`** for early-out on first difference/match
## Performance
Test environment: SOPHGO SG2044 (RVV 1.0, VLEN >= 128, GCC 15.1)
Baseline: glibc 2.38 scalar memcmp/memchr (no RVV acceleration)
### memcmp (worst-case full scan, 50000 iterations)
| Size | glibc (ns) | RVV (ns) | Speedup |
|------|-----------|---------|---------|
| 64 B | 25 | 5 | **5.00x** |
| 256 B | 52 | 15 | **3.47x** |
| 1 KB | 120 | 55 | **2.18x** |
| 4 KB | 419 | 224 | **1.87x** |
| 16 KB | 1,640 | 864 | **1.90x** |
| 64 KB | 10,937 | 9,508 | **1.15x** |
| 256 KB | 54,005 | 45,537 | **1.19x** |
| 1 MB | 324,601 | 413,379 | 0.79x |
### memchr (worst-case full scan, 50000 iterations)
| Size | glibc (ns) | RVV (ns) | Speedup |
|------|-----------|---------|---------|
| 64 B | 28 | 3 | **9.33x** |
| 256 B | 60 | 9 | **6.67x** |
| 1 KB | 184 | 36 | **5.11x** |
| 4 KB | 637 | 154 | **4.14x** |
| 16 KB | 2,568 | 594 | **4.32x** |
| 64 KB | 10,664 | 3,614 | **2.95x** |
| 256 KB | 41,660 | 17,262 | **2.41x** |
| 1 MB | 163,354 | 55,354 | **2.95x** |
## Correctness
| Test | Result |
|------|--------|
| Expanded correctness (24,950 cases, 64B–1MB) | **24,950/24,950 PASSED** |
| `string_piece_unittest` | **22/22 PASSED** |
| `test_butil` | **724/725 PASSED** (1 pre-existing StackTrace failure) |
| `test_bvar` | **64/64 PASSED** |
## Files Changed
- `src/butil/string_compare_rvv.cc` (new): RVV memcmp/memchr implementations
- `src/butil/strings/string_piece.h`: RVV dispatch in `wordmemcmp` (N >= 16)
- `BUILD.bazel`: added `string_compare_rvv.cc` to `BUTIL_SRCS`
- `CMakeLists.txt`: added `string_compare_rvv.cc`
## Build
```bash
cmake ..
-DCMAKE_CXX_COMPILER=/opt/gcc-15.1/bin/riscv64-unknown-linux-gnu-g++ \
-DCMAKE_CXX_FLAGS='-O2 -march=rv64gcv'
make -j$(nproc)
```
RISC-V optimization for butil StringPiece operations.
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