optimize StringPiece memcmp for RISC-V with RVV by Felix-Gong · Pull Request #3390 · apache/brpc

Felix-Gong · 2026-07-16T03:07:27Z

Summary

This patch adds RVV-accelerated memcmp 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

Performance

Test environment: SOPHGO SG2044 (RVV 1.0, VLEN >= 128, GCC 15.1)
Baseline: glibc 2.38 scalar memcmp (no RVV acceleration)

memcmp (worst-case full scan, 50000 iterations)

Size	glibc (ns)	RVV (ns)	Speedup
64 B	20	6	3.4x
256 B	54	15	3.6x
1 KB	120	55	2.2x
4 KB	435	225	1.9x
16 KB	1,968	1,258	1.6x
64 KB	10,962	9,044	1.2x
256 KB	46,893	43,108	1.1x
1 MB	446,161	454,717	1.01x

Correctness

Test	Result
Expanded correctness (59 cases, 64B–1MB)	59/59 PASSED
`string_piece_unittest`	24/24 PASSED
`test_butil`	724/725 PASSED
`test_bvar`	64/64 PASSED

Files Changed

src/butil/string_compare_rvv.cc (new): RVV memcmp implementation
src/butil/strings/string_piece.h: RVV dispatch in wordmemcmp (N >= 16), added BUTIL_EXPORT
BUILD.bazel: added string_compare_rvv.cc to BUTIL_SRCS
CMakeLists.txt: added string_compare_rvv.cc

Build

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.

Copilot

Pull request overview

This PR introduces RISC-V Vector (RVV) accelerated byte-wise comparison/search routines intended to speed up butil::StringPiece operations on RVV-capable RISC-V platforms, and wires RVV memcmp into StringPiece’s internal comparison path.

Changes:

Added RVV implementations rvv_memcmp and rvv_memchr in a new source file.
Added RVV dispatch in BasicStringPiece::wordmemcmp for byte strings (N ≥ 16).
Updated build files to compile the new RVV source.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/butil/strings/string_piece.h	Adds RVV function declarations and dispatches `wordmemcmp` to `rvv_memcmp` under RVV guards.
src/butil/string_compare_rvv.cc	New RVV-accelerated `memcmp`/`memchr` implementations using RVV intrinsics.
CMakeLists.txt	Adds `string_compare_rvv.cc` to butil sources for CMake builds.
BUILD.bazel	Adds `string_compare_rvv.cc` to `BUTIL_SRCS` (but also adds a missing/unrelated RVV source entry).

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Felix-Gong · 2026-07-17T06:18:55Z

    "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",


Fixed. Removed modp_b64_rvv.cc from BUTIL_SRCS — it was inadvertently included from a prior PR (#3388) and doesn't belong in this change.

Felix-Gong · 2026-07-17T06:19:15Z

+// RVV-accelerated byte comparison and search (implemented in string_compare_rvv.cc)
+#if defined(__riscv) && defined(__riscv_vector)
+int rvv_memcmp(const void* p1, const void* p2, size_t n);
+const void* rvv_memchr(const void* s, int c, size_t n);
+#endif


Fixed. Added BUTIL_EXPORT to the rvv_memcmp declaration to ensure proper symbol visibility in shared library builds.

Felix-Gong · 2026-07-17T06:19:43Z

+        if (first >= 0) {
+            uint8_t b1 = src1[first];
+            uint8_t b2 = src2[first];
+            return (b1 < b2) ? -1 : 1;
+        }


Fixed. Changed to return static_cast(src1[first]) - static_cast(src2[first]); which matches glibc memcmp semantics and preserves the byte-difference contract expected by traits_type::compare.

Felix-Gong · 2026-07-17T06:20:04Z

+ * rvv_memchr - RVV-accelerated byte search following glibc pattern.
+ *
+ * Searches for the first occurrence of byte 'c' in buffer 's' of length 'n'.
+ * Returns pointer to first match, or NULL if not found.
+ *
+ * Uses RVV m8 with hardware-adaptive VL for maximum throughput.


Fixed. rvv_memchr has been removed from this PR and will be added in a follow-up PR with proper StringPiece::find dispatch. The doc comment and implementation will be aligned in that PR.

Felix-Gong · 2026-07-17T06:20:14Z

+const void* rvv_memchr(const void* s, int c, size_t n) {
+    const uint8_t* src = static_cast<const uint8_t*>(s);
+    uint8_t ch = static_cast<uint8_t>(c);
+    size_t remaining = n;
+
+    while (remaining > 0) {
+        size_t vl = __riscv_vsetvl_e8m8(remaining);
+        vuint8m8_t v = __riscv_vle8_v_u8m8(src, vl);
+        vbool1_t eq = __riscv_vmseq_vx_u8m8_b1(v, ch, vl);
+        long first = __riscv_vfirst_m_b1(eq, vl);
+
+        if (first >= 0) {
+            return src + first;
+        }


Agreed. rvv_memchr has been removed from this PR. It will be reintroduced in a follow-up PR that also wires the dispatch into StringPiece::find(char) via internal::find in string_piece.cc.

This patch adds RVV-accelerated memcmp 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 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) 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 implementation - 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 Signed-off-by: Xiaofei Gong <gongxiaofei24@iscas.ac.cn> Signed-off-by: YuanSheng <yuansheng@isrc.iscas.ac.cn>

chenBright requested a review from Copilot July 16, 2026 03:13

Copilot started reviewing on behalf of chenBright July 16, 2026 03:15 View session

Copilot AI reviewed Jul 16, 2026

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Felix-Gong changed the title ~~optimize StringPiece memcmp/memchr for RISC-V with RVV~~ optimize StringPiece memcmp for RISC-V with RVV Jul 17, 2026

Felix-Gong force-pushed the riscv-stringpiece-memcmp-memchr branch from 3902e7e to 24d0b81 Compare July 17, 2026 03:25

Felix-Gong added 2 commits July 17, 2026 03:26

Felix-Gong force-pushed the riscv-stringpiece-memcmp-memchr branch from 24d0b81 to 5fb665e Compare July 17, 2026 03:27

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optimize StringPiece memcmp for RISC-V with RVV#3390

optimize StringPiece memcmp for RISC-V with RVV#3390
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Felix-Gong:riscv-stringpiece-memcmp-memchr

Felix-Gong commented Jul 16, 2026 •

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Summary

Performance

memcmp (worst-case full scan, 50000 iterations)

Correctness

Files Changed

Build

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Felix-Gong commented Jul 16, 2026 •

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