[jira] [Created] (ARROW-1011) [Format] Clarify in Layout.md what are the expectations for buffer padding vs. in-memory data

["Wes McKinney (JIRA)" <ji...@apache.org>]

Wes McKinney created ARROW-1011:
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             Summary: [Format] Clarify in Layout.md what are the expectations for buffer padding vs. in-memory data
                 Key: ARROW-1011
                 URL: https://issues.apache.org/jira/browse/ARROW-1011
             Project: Apache Arrow
          Issue Type: New Feature
          Components: Format
            Reporter: Wes McKinney


This has come up in https://github.com/apache/arrow/pull/673 and in prior discussions. 

The basic summary is that we should not write non-zero padding bytes in IPC messages. However: one cannot in general rely on the padding being non-zero when the data is in memory (for example: zero-copy slices of Arrow arrays/vectors).

I think it would be good to clarify this point in Layout.md -- namely that what gets written to the wire should be deterministic. However, in-memory algorithms should not in general expect the padding region to have a particular value. As an example, a popcount on a validity bitmap would want to exclude padding bytes from the computation. Other elementwise SIMD operations are free to use the padding bytes as they wish, with a known caveat. 

cc [~cloud_fan]



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