optimize StringPiece memcmp for RISC-V with RVV

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",
@@ -189,6 +188,7 @@ BUTIL_SRCS = [
     "src/butil/strings/string_number_conversions.cc",
     "src/butil/strings/string_split.cc",
     "src/butil/strings/string_piece.cc",
+    "src/butil/string_compare_rvv.cc",
     "src/butil/strings/string_util.cc",
     "src/butil/strings/string_util_constants.cc",
     "src/butil/strings/stringprintf.cc",

CMakeLists.txt

@@ -478,6 +478,7 @@ set(BUTIL_SOURCES
     ${PROJECT_SOURCE_DIR}/src/butil/strings/string_number_conversions.cc
     ${PROJECT_SOURCE_DIR}/src/butil/strings/string_split.cc
     ${PROJECT_SOURCE_DIR}/src/butil/strings/string_piece.cc
+    ${PROJECT_SOURCE_DIR}/src/butil/string_compare_rvv.cc
     ${PROJECT_SOURCE_DIR}/src/butil/strings/string_util.cc
     ${PROJECT_SOURCE_DIR}/src/butil/strings/string_util_constants.cc
     ${PROJECT_SOURCE_DIR}/src/butil/strings/stringprintf.cc

src/butil/string_compare_rvv.cc

@@ -0,0 +1,55 @@
+// 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>
+
+namespace butil {
+
+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);
+    size_t remaining = n;
+
+    while (remaining > 0) {
+        size_t vl = __riscv_vsetvl_e8m8(remaining);
+        vuint8m8_t v1 = __riscv_vle8_v_u8m8(src1, vl);
+        vuint8m8_t v2 = __riscv_vle8_v_u8m8(src2, vl);
+        vbool1_t neq = __riscv_vmsne_vv_u8m8_b1(v1, v2, vl);
+        long first = __riscv_vfirst_m_b1(neq, vl);
+
+        if (first >= 0) {
+            return static_cast<int>(src1[first]) - static_cast<int>(src2[first]);
+        }
+
+        src1 += vl;
+        src2 += vl;
+        remaining -= vl;
+    }
+    return 0;
+}
+
+}  // namespace butil
+
+#endif  // __riscv && __riscv_vector

src/butil/strings/string_piece.h

@@ -43,6 +43,11 @@
 
 namespace butil {
 
+// RVV-accelerated byte comparison (implemented in string_compare_rvv.cc)
+#if defined(__riscv) && defined(__riscv_vector)
+BUTIL_EXPORT int rvv_memcmp(const void* p1, const void* p2, size_t n);
+#endif
+
 template <typename STRING_TYPE> class BasicStringPiece;
 typedef BasicStringPiece<std::string> StringPiece;
 typedef BasicStringPiece<string16> StringPiece16;
@@ -286,6 +291,11 @@ template <typename STRING_TYPE> class BasicStringPiece {
   static int wordmemcmp(const value_type* p,
                         const value_type* p2,
                         size_type N) {
+#if defined(__riscv) && defined(__riscv_vector)
+    if (sizeof(value_type) == 1 && N >= 16) {
+      return rvv_memcmp(p, p2, N);
+    }
+#endif
     return STRING_TYPE::traits_type::compare(p, p2, N);
   }