optimize StringPiece::find(char) for RISC-V with RVV memchr

BUILD.bazel

@@ -128,6 +128,7 @@ 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

@@ -15,10 +15,9 @@
 // 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
+// RVV-accelerated memcmp and memchr for StringPiece operations.
+// memcmp: follows glibc's RVV memcmp pattern (e8m8 LMUL, vfirst.m early-out).
+// memchr: uses vmseq + vfirst.m to locate first matching byte.
 
 #include "butil/strings/string_piece.h"
 
@@ -50,6 +49,27 @@ int rvv_memcmp(const void* p1, const void* p2, size_t n) {
     return 0;
 }
 
+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.cc

@@ -134,7 +134,18 @@ size_t findT(const BasicStringPiece<STR>& self,
 }
 
 size_t find(const StringPiece& self, char c, size_t pos) {
+#if defined(__riscv) && defined(__riscv_vector)
+  if (pos < self.size()) {
+    const void* result = butil::rvv_memchr(self.data() + pos, c, self.size() - pos);
+    if (result != nullptr) {
+      return static_cast<size_t>(static_cast<const char*>(result) - self.data());
+    }
+    return BasicStringPiece<std::string>::npos;
+  }
+  return BasicStringPiece<std::string>::npos;
+#else
   return findT(self, c, pos);
+#endif
 }
 
 size_t find(const StringPiece16& self, char16 c, size_t pos) {

src/butil/strings/string_piece.h

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