Use unpack<uint16_t> in BitReader

Author: AntoinePrv

cpp/src/arrow/util/bit_stream_utils_internal.h

@@ -323,19 +323,10 @@ inline int BitReader::GetBatch(int num_bits, T* v, int batch_size) {
     }
   }
 
-  if (sizeof(T) == 4) {
-    int num_unpacked =
-        internal::unpack(buffer + byte_offset, reinterpret_cast<uint32_t*>(v + i),
-                         batch_size - i, num_bits);
-    i += num_unpacked;
-    byte_offset += num_unpacked * num_bits / 8;
-  } else if (sizeof(T) == 8 && num_bits > 32) {
-    // Use unpack64 only if num_bits is larger than 32
-    // TODO (ARROW-13677): improve the performance of internal::unpack64
-    // and remove the restriction of num_bits
-    int num_unpacked =
-        internal::unpack(buffer + byte_offset, reinterpret_cast<uint64_t*>(v + i),
-                         batch_size - i, num_bits);
+  if constexpr (sizeof(T) >= 2) {
+    int num_unpacked = internal::unpack(buffer + byte_offset,
+                                        reinterpret_cast<std::make_unsigned_t<T>*>(v + i),
+                                        batch_size - i, num_bits);
     i += num_unpacked;
     byte_offset += num_unpacked * num_bits / 8;
   } else {