Remove UNKNOWN_NULL_COUNT where it can be easily computed

cpp/include/cudf/structs/structs_column_view.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, NVIDIA CORPORATION.
+ * Copyright (c) 2022-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,9 @@
 
 #include <cudf/column/column.hpp>
 #include <cudf/column/column_view.hpp>
+#include <cudf/utilities/default_stream.hpp>
+
+#include <rmm/cuda_stream_view.hpp>
 
 /**
  * @file
@@ -87,9 +90,12 @@ class structs_column_view : public column_view {
    * @throw cudf::logic error if this is an empty column
    *
    * @param index The index of the child column to return
+   * @param stream The stream on which to perform the operation. Uses the default CUDF
+   * stream if none is specified.
    * @return The child column sliced relative to the parent's offset and size
    */
-  [[nodiscard]] column_view get_sliced_child(int index) const;
+  [[nodiscard]] column_view get_sliced_child(
+    int index, rmm::cuda_stream_view stream = cudf::get_default_stream()) const;
 };         // class structs_column_view;
 /** @} */  // end of group
 }  // namespace cudf

cpp/src/lists/copying/copying.cu

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020-2022, NVIDIA CORPORATION.
+ * Copyright (c) 2020-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@
 #include <cudf/detail/get_value.cuh>
 #include <cudf/detail/null_mask.hpp>
 #include <cudf/lists/lists_column_view.hpp>
+#include <cudf/types.hpp>
 
 #include <rmm/cuda_stream_view.hpp>
 #include <rmm/exec_policy.hpp>
@@ -81,10 +82,13 @@ std::unique_ptr<cudf::column> copy_slice(lists_column_view const& lists,
   // Compute the null mask of the result:
   auto null_mask = cudf::detail::copy_bitmask(lists.null_mask(), start, end, stream, mr);
 
+  auto null_count = cudf::detail::null_count(
+    static_cast<bitmask_type const*>(null_mask.data()), 0, end - start, stream);
+
   return make_lists_column(lists_count,
                            std::move(offsets),
                            std::move(child),
-                           cudf::UNKNOWN_NULL_COUNT,
+                           null_count,
                            std::move(null_mask),
                            stream,
                            mr);

cpp/src/strings/copying/copying.cu

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2022, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -73,10 +73,13 @@ std::unique_ptr<cudf::column> copy_slice(strings_column_view const& strings,
   auto null_mask = cudf::detail::copy_bitmask(
     strings.null_mask(), offsets_offset, offsets_offset + strings_count, stream, mr);
 
+  auto null_count = cudf::detail::null_count(
+    static_cast<bitmask_type const*>(null_mask.data()), 0, strings_count, stream);
+
   return make_strings_column(strings_count,
                              std::move(offsets_column),
                              std::move(chars_column),
-                             UNKNOWN_NULL_COUNT,
+                             null_count,
                              std::move(null_mask));
 }
 

cpp/src/structs/structs_column_view.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020-2022, NVIDIA CORPORATION.
+ * Copyright (c) 2020-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,7 +15,9 @@
  */
 
 #include <cudf/column/column.hpp>
+#include <cudf/detail/null_mask.hpp>
 #include <cudf/structs/structs_column_view.hpp>
+#include <cudf/types.hpp>
 #include <cudf/utilities/error.hpp>
 
 namespace cudf {
@@ -27,22 +29,24 @@ structs_column_view::structs_column_view(column_view const& rhs) : column_view{r
 
 column_view structs_column_view::parent() const { return *this; }
 
-column_view structs_column_view::get_sliced_child(int index) const
+column_view structs_column_view::get_sliced_child(int index, rmm::cuda_stream_view stream) const
 {
   std::vector<column_view> children;
   children.reserve(child(index).num_children());
   for (size_type i = 0; i < child(index).num_children(); i++) {
     children.push_back(child(index).child(i));
   }
-  return column_view{child(index).type(),
-                     size(),
-                     child(index).head<uint8_t>(),
-                     child(index).null_mask(),
-                     // TODO: could potentially compute the actual count here, but at
-                     // the moment this interface doesn't take a stream.
-                     UNKNOWN_NULL_COUNT,
-                     offset(),
-                     children};
+
+  return column_view{
+    child(index).type(),
+    size(),
+    child(index).head<uint8_t>(),
+    child(index).null_mask(),
+    child(index).null_count()
+      ? cudf::detail::null_count(child(index).null_mask(), offset(), offset() + size(), stream)
+      : 0,
+    offset(),
+    children};
 }
 
 }  // namespace cudf

cpp/src/structs/utilities.cpp

@@ -270,11 +270,12 @@ std::unique_ptr<column> superimpose_nulls_no_sanitize(bitmask_type const* null_m
   auto content              = input->release();
 
   // Build new children columns.
-  std::for_each(
-    content.children.begin(), content.children.end(), [current_mask, stream, mr](auto& child) {
-      child = superimpose_nulls_no_sanitize(
-        current_mask, cudf::UNKNOWN_NULL_COUNT, std::move(child), stream, mr);
-    });
+  std::for_each(content.children.begin(),
+                content.children.end(),
+                [current_mask, new_null_count, stream, mr](auto& child) {
+                  child = superimpose_nulls_no_sanitize(
+                    current_mask, new_null_count, std::move(child), stream, mr);
+                });
 
   // Replace the children columns.
   return cudf::make_structs_column(num_rows,