buffer: optimize byteLength for short strings

PR-URL: #54345

src/node_buffer.cc

@@ -35,6 +35,7 @@
 #include "v8-fast-api-calls.h"
 #include "v8.h"
 
+#include <bit>
 #include <climits>
 #include <cstring>
 #include "nbytes.h"
@@ -752,12 +753,25 @@ uint32_t FastByteLengthUtf8(Local<Value> receiver,
   if (source.length > 128) {
     return simdutf::utf8_length_from_latin1(source.data, source.length);
   }
+
   uint32_t length = source.length;
   uint32_t result = length;
-  const uint8_t* data = reinterpret_cast<const uint8_t*>(source.data);
-  for (uint32_t i = 0; i < length; ++i) {
+  const auto data = reinterpret_cast<const uint8_t*>(source.data);
+
+  uint32_t i = 0;
+  const auto length8 = length & ~0x7;
+  while (i < length8) {
+    uint64_t x;
+    memcpy(&x, data + i, sizeof(uint64_t));
+    result += std::popcount(x & 0x8080808080808080);
+    i += 8;
+  }
+
+  while (i < length) {
     result += (data[i] >> 7);
+    i++;
   }
+
   return result;
 }