optimize StringPiece memcmp for RISC-V with RVV

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>

Author: Felix-Gong

BUILD.bazel

@@ -128,7 +128,6 @@ BUTIL_SRCS = [
     "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",
     "src/butil/third_party/snappy/snappy-sinksource.cc",

src/butil/string_compare_rvv.cc

@@ -1,51 +1,32 @@
-/* -*- mode: c; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 4 -*- */
-/* vi: set expandtab shiftwidth=4 tabstop=4: */
-/**
- * \file
- * RISC-V Vector (RVV) accelerated memcmp/memchr for StringPiece operations.
- *
- * Algorithm follows glibc's RVV memcmp/memchr patterns:
- * - Use vsetvli with e8/m8 for maximum throughput (hardware-adaptive VL)
- * - Use vfirst.m to find first differing/matching byte (no vcpop needed)
- * - Simple loop: load → compare → find first → branch
- *
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * License); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *   http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing,
- * software distributed under the License is distributed on an
- * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
- * KIND, either express or implied.  See the License for the
- * specific language governing permissions and limitations
- * under the License.
- */
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+// 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
 
 #include "butil/strings/string_piece.h"
 
 #if defined(__riscv) && defined(__riscv_vector)
 #include <riscv_vector.h>
-#include <stdint.h>
-#include <string.h>
 
 namespace butil {
 
-/**
- * rvv_memcmp - RVV-accelerated byte comparison following glibc pattern.
- *
- * Compares two byte arrays and returns:
- *   < 0 if p1 < p2
- *     0 if p1 == p2
- *   > 0 if p1 > p2
- *
- * Uses RVV m8 with hardware-adaptive VL for maximum throughput.
- */
 int rvv_memcmp(const void* p1, const void* p2, size_t n) {
     const uint8_t* src1 = static_cast<const uint8_t*>(p1);
     const uint8_t* src2 = static_cast<const uint8_t*>(p2);
@@ -59,9 +40,7 @@ int rvv_memcmp(const void* p1, const void* p2, size_t n) {
         long first = __riscv_vfirst_m_b1(neq, vl);
 
         if (first >= 0) {
-            uint8_t b1 = src1[first];
-            uint8_t b2 = src2[first];
-            return (b1 < b2) ? -1 : 1;
+            return static_cast<int>(src1[first]) - static_cast<int>(src2[first]);
         }
 
         src1 += vl;
@@ -71,35 +50,6 @@ int rvv_memcmp(const void* p1, const void* p2, size_t n) {
     return 0;
 }
 
-/**
- * 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.
- */
-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;
-        }
-
-        src += vl;
-        remaining -= vl;
-    }
-    return nullptr;
-}
-
 }  // namespace butil
 
 #endif  // __riscv && __riscv_vector

src/butil/strings/string_piece.h

@@ -43,10 +43,9 @@
 
 namespace butil {
 
-// RVV-accelerated byte comparison and search (implemented in string_compare_rvv.cc)
+// RVV-accelerated byte comparison (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);
+BUTIL_EXPORT int rvv_memcmp(const void* p1, const void* p2, size_t n);
 #endif
 
 template <typename STRING_TYPE> class BasicStringPiece;