[jira] [Commented] (ARROW-13121) [C++][Compute] Extract preallocation logic from KernelExecutor

["Antoine Pitrou (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-13121?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17365633#comment-17365633 ] 

Antoine Pitrou commented on ARROW-13121:
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I'd also mention that when I worked on ARROW-13042, I spent a lot of time trying to figure out which code paths exactly got executed in {{exec.cc}} and I never fully figured it out (one particularly case was implicit casting and broadcasting with a NullScalar LHS and a ChunkedArray RHS on a scalar kernel). I ended up trying to find clues in other places instead.

> [C++][Compute] Extract preallocation logic from KernelExecutor
> --------------------------------------------------------------
>
>                 Key: ARROW-13121
>                 URL: https://issues.apache.org/jira/browse/ARROW-13121
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Ben Kietzman
>            Priority: Major
>
> Currently KernelExecutor handles preallocation of null bitmaps and other buffers based on simple flags on each Kernel. This is not very flexible and we end up leaving a lot of performance on the table in cases where we can preallocate but the behavior can't be captured in the available flags. For example, in the case of {{binary_string_join_element_wise}}, it would be possible to preallocate all buffers (even the character buffer) and write output into slices.
> Having this as a public function would enable us to unit test it directly (currently Executors are only tested indirectly through calling of compute::Functions) and reuse it, for example to correctly preallocate a small temporary for pipelined execution
> One way this could be added is as a new method on each Kernel:
> {code}
> // Output preallocated Datums sufficient for execution of the kernel on each ExecBatch.
> // The output Datums may not be identically chunked to the input batches, for example
> // kernels which support contiguous output preallocation will preallocate a single Datum
> // (and can then output into slices of that Datum).
> Result<std::vector<Datum>> Kernel::prepare_output(
>   const Kernel*,
>   KernelContext*,
>   const std::vector<ExecBatch>& inputs)
> {code}



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