Fix null counts for output of row/column conversion:

src/main/cpp/src/row_conversion.cu

@@ -17,6 +17,7 @@
 #include <cooperative_groups.h>
 #include <cudf/column/column_factories.hpp>
 #include <cudf/detail/iterator.cuh>
+#include <cudf/detail/null_mask.hpp>
 #include <cudf/detail/sequence.hpp>
 #include <cudf/detail/utilities/cuda.cuh>
 #include <cudf/detail/utilities/integer_utils.hpp>
@@ -2031,6 +2032,19 @@ convert_to_rows_fixed_width_optimized(table_view const &tbl, rmm::cuda_stream_vi
   }
 }
 
+namespace {
+
+/// @brief Calculates and sets null counts for specified columns
+void fixup_null_counts(std::vector<std::unique_ptr<column>>& output_columns,
+                       rmm::cuda_stream_view stream) {
+  for (auto &col : output_columns) {
+    col->set_null_count(
+      cudf::detail::null_count(col->view().null_mask(), 0, col->size(), stream));
+  }
+}
+
+}
+
 /**
  * @brief convert from JCUDF row format to cudf columns
  *
@@ -2260,23 +2274,21 @@ std::unique_ptr<table> convert_from_rows(lists_column_view const &input,
       if (schema[i].id() == type_id::STRING) {
         // stuff real string column
         auto string_data = string_row_offset_columns[string_idx].release()->release();
-        auto const null_count = [&] {
-          // Null-count not set previously. Calculate, on the fly.
-          auto const &null_mask = *string_data.null_mask;
-          return null_mask.data() ?
-                     cudf::null_count(static_cast<bitmask_type const *>(null_mask.data()), 0,
-                                      num_rows) :
-                     0;
-        }();
         output_columns[i] =
             make_strings_column(num_rows, std::move(string_col_offsets[string_idx]),
                                 std::move(string_data_cols[string_idx]),
-                                std::move(*string_data.null_mask.release()), null_count);
+                                std::move(*string_data.null_mask.release()), 0);
+        // Null count set to 0, temporarily. Will be fixed up before return.
         string_idx++;
       }
     }
   }
 
+  // Set null counts, because output_columns are modified via mutable-view,
+  // in the kernel above.
+  // TODO(future): Consider setting null count in the kernel itself.
+  fixup_null_counts(output_columns, stream);
+
   return std::make_unique<table>(std::move(output_columns));
 }
 
@@ -2335,9 +2347,8 @@ std::unique_ptr<table> convert_from_rows_fixed_width_optimized(
     // Set null counts, because output_columns are modified via mutable-view,
     // in the kernel above.
     // TODO(future): Consider setting null count in the kernel itself.
-    for (auto &col : output_columns) {
-      col->set_null_count(cudf::null_count(col->view().null_mask(), 0, col->size()));
-    }
+    fixup_null_counts(output_columns, stream);
+
     return std::make_unique<table>(std::move(output_columns));
   } else {
     CUDF_FAIL("Only fixed width types are currently supported");

src/main/java/com/nvidia/spark/rapids/jni/RowConversion.java

@@ -163,7 +163,7 @@ public static Table convertFromRowsFixedWidthOptimized(ColumnView vec, DType ...
       scale[i] = schema[i].getScale();
 
     }
-    return new Table(convertFromRows(vec.getNativeView(), types, scale));
+    return new Table(convertFromRowsFixedWidthOptimized(vec.getNativeView(), types, scale));
   }
 
   private static native long[] convertToRows(long nativeHandle);