mapleFU commented on code in PR #6155:
URL: https://github.com/apache/arrow-rs/pull/6155#discussion_r1711279460


##########
arrow-buffer/src/buffer/null.rs:
##########
@@ -131,9 +176,20 @@ impl NullBuffer {
     }
 
     /// Returns the null count for this [`NullBuffer`]
-    #[inline]
     pub fn null_count(&self) -> usize {
-        self.null_count
+        match &self.null_count {
+            NullCount::Eager(v) => *v,
+            NullCount::Lazy(v) => {
+                let cached_null_count = v.load(Ordering::Acquire);
+                if cached_null_count != UNINITIALIZED_NULL_COUNT {
+                    return cached_null_count as usize;
+                }
+
+                let computed_null_count = self.buffer.len() - 
self.buffer.count_set_bits();
+                v.store(computed_null_count as i64, Ordering::Release);

Review Comment:
   ( They would be similiar in x86, just affect reordering )?



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