Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/13680#discussion_r79561758
  
    --- Diff: 
sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeArrayData.java
 ---
    @@ -25,55 +25,58 @@
     import org.apache.spark.sql.types.*;
     import org.apache.spark.unsafe.Platform;
     import org.apache.spark.unsafe.array.ByteArrayMethods;
    +import org.apache.spark.unsafe.bitset.BitSetMethods;
     import org.apache.spark.unsafe.hash.Murmur3_x86_32;
     import org.apache.spark.unsafe.types.CalendarInterval;
     import org.apache.spark.unsafe.types.UTF8String;
     
     /**
      * An Unsafe implementation of Array which is backed by raw memory instead 
of Java objects.
      *
    - * Each tuple has three parts: [numElements] [offsets] [values]
    + * Each array has four parts:
    + *   [numElements][null bits][values or offset&length][variable length 
portion]
      *
    - * The `numElements` is 4 bytes storing the number of elements of this 
array.
    + * The `numElements` is 8 bytes storing the number of elements of this 
array.
      *
    - * In the `offsets` region, we store 4 bytes per element, represents the 
relative offset (w.r.t. the
    - * base address of the array) of this element in `values` region. We can 
get the length of this
    - * element by subtracting next offset.
    - * Note that offset can by negative which means this element is null.
    + * In the `null bits` region, we store 1 bit per element, represents 
whether a element has null
    + * Its total size is ceil(numElements / 8) bytes, and  it is aligned to 
8-byte boundaries.
      *
    - * In the `values` region, we store the content of elements. As we can get 
length info, so elements
    - * can be variable-length.
    + * In the `values or offset&length` region, we store the content of 
elements. For fields that hold
    + * fixed-length primitive types, such as long, double, or int, we store 
the value directly
    --- End diff --
    
    we should also mention that fixed-length values portion is also word-aligned


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