optimize StringPiece::find(char) for RISC-V with RVV memchr#3396
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Pull request overview
This PR adds a RISC-V RVV-accelerated byte-search implementation (rvv_memchr) and wires it into butil::StringPiece::find(char) to speed up single-byte searches on RVV-capable targets.
Changes:
- Add
rvv_memchr()implementation alongsidervv_memcmp()insrc/butil/string_compare_rvv.cc. - Expose RVV helpers in
string_piece.hand dispatchStringPiece::find(char)torvv_memchr()when RVV is available. - Update build files to compile the RVV implementation (CMake + Bazel).
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| src/butil/strings/string_piece.h | Declares RVV helpers and uses rvv_memcmp in wordmemcmp for byte strings. |
| src/butil/strings/string_piece.cc | Dispatches StringPiece::find(char) to RVV memchr on RVV-enabled RISC-V. |
| src/butil/string_compare_rvv.cc | Implements RVV memcmp and new RVV memchr. |
| CMakeLists.txt | Adds string_compare_rvv.cc to BUTIL sources for CMake builds. |
| BUILD.bazel | Adds string_compare_rvv.cc to BUTIL sources for Bazel builds (but currently drops modp_b64_rvv.cc). |
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| "src/butil/third_party/icu/icu_utf.cc", | ||
| "src/butil/third_party/superfasthash/superfasthash.c", | ||
| "src/butil/third_party/modp_b64/modp_b64.cc", | ||
| "src/butil/third_party/modp_b64/modp_b64_rvv.cc", | ||
| "src/butil/third_party/symbolize/demangle.cc", | ||
| "src/butil/third_party/symbolize/symbolize.cc", |
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| // RVV-accelerated memcmp for StringPiece operations. | ||
| // Algorithm follows glibc's RVV memcmp pattern: | ||
| // - e8m8 LMUL with hardware-adaptive VL via vsetvl | ||
| // - vfirst.m for early-out on first mismatch | ||
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| const void* result = butil::rvv_memchr(self.data() + pos, c, self.size() - pos); | ||
| if (result != nullptr) { | ||
| return static_cast<const char*>(result) - self.data(); | ||
| } | ||
| return BasicStringPiece<std::string>::npos; |
Contributor
|
LGTM |
Felix-Gong
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chenBright
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Jul 22, 2026
chenBright
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The BUTIL_SOURCES of Makefile also need to be updated.
This patch adds RVV-accelerated memcmp and memchr for StringPiece operations
on RISC-V 64-bit platforms.
The implementation follows glibc's official RVV memcmp/memchr patterns:
- e8m8 LMUL for maximum throughput
- Hardware-adaptive vector length via vsetvl
- vfirst.m for early-out on first difference/match
Performance on SOPHGO SG2044 (RVV 1.0, VLEN >= 128, GCC 15.1):
glibc 2.38 scalar memcmp (no RVV acceleration) as baseline.
memcmp (worst-case full scan, 50000 iterations):
64 B: 20 ns -> 6 ns (3.4x)
256 B: 54 ns -> 15 ns (3.6x)
1 KB: 120 ns -> 55 ns (2.2x)
4 KB: 435 ns -> 225 ns (1.9x)
16 KB: 1968 ns -> 1258 ns (1.6x)
64 KB: 10962 ns -> 9044 ns (1.2x)
256 KB: 46893 ns -> 43108 ns (1.1x)
1 MB: 446161 ns -> 454717 ns (1.01x)
memchr (worst-case full scan, 50000 iterations):
64 B: 5 ns -> 0.9 ns (5.8x)
256 B: 12 ns -> 3 ns (4.0x)
1 KB: 40 ns -> 12 ns (3.3x)
4 KB: 148 ns -> 44 ns (3.4x)
16 KB: 558 ns -> 158 ns (3.5x)
64 KB: 2150 ns -> 635 ns (3.4x)
256 KB: 9200 ns -> 2900 ns (3.2x)
1 MB: 39000 ns -> 13000 ns (3.0x)
Correctness:
- 59/59 expanded correctness tests passed (64B - 1MB)
- string_piece_unittest: 24/24 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 declarations
- src/butil/strings/string_piece.cc: RVV dispatch in find(char) for memchr
- BUILD.bazel: added string_compare_rvv.cc to BUTIL_SRCS
- CMakeLists.txt: added string_compare_rvv.cc
- Makefile: added string_compare_rvv.cc
Signed-off-by: Xiaofei Gong <gongxiaofei24@iscas.ac.cn>
Signed-off-by: YuanSheng <yuansheng@isrc.iscas.ac.cn>
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@chenBright Thanks for pointing this out! Rebased onto latest master (PR #3390 is merged), so the Makefile fix is already in master. No separate Makefile change needed in this PR. Force-pushed the update. |
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What this PR does
Adds RVV-accelerated
rvv_memchr()for byte search inStringPiece::find(char)on RISC-V.Implementation
rvv_memchr()instring_compare_rvv.cc: uses RVV vector compare (vmseq) +vfirstto scan for target byte across bufferstring_piece.hwithBUTIL_EXPORTfor shared library buildsstring_piece.cc::find(const StringPiece, char, size_t)— routes torvv_memchr()when RVV available, falls back to genericfindT()otherwisePerformance (RISC-V SG2044, 50000 iterations)
Dependency
Depends on #3390 (RVV memcmp for StringPiece) being merged first, as this branch is based on top of it.
Tests