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https://issues.apache.org/jira/browse/SYSTEMML-1444?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16104312#comment-16104312
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Matthias Boehm commented on SYSTEMML-1444:
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great - after thinking about the design a little more, I'd like to recommend to
go with approach (2) that would handle functions with a single output similar
to any other hop, while multi-output functions would use the same mechanism as
of today.
In detail, this would entail the following steps (which can be created as
subtasks and addressed via PRs individually):
a) Hop/Lop extensions: Extend the existing {{FunctionOp}} to be used in two
modes (single and multi output). Only in multi-output mode the list of outputs
are used (always DAG outputs), while in single-output mode the {{FunctionOp}}
can be used as input to any other HOP and hence be used in expressions. Besides
changing the construction of hops, this also requires some minor extensions to
the lop construction and instruction generation (e.g., using the
compiler-provides name of temporary outputs when generating single-output
instructions). At this point, all {{FunctionOps}} would still be created as
multi-output functions at language level.
b) Language changes / tests: According to the change of HOP and LOPs, we can
then construct differently configured HOPs for single-output functions at
language level (see DMLTranslator). In order to use single output functions, we
likely also need some changes of validation. This step should also introduces a
couple of tests for functions in expressions.
c) Size propagation and IPA: Having functions in expressions poses a challenge
to size propagation because there is no natural recompilation point after the
function call anymore. We should address this as follows: First, flag
dimension-preserving {{FunctionOps}} during {{InterProceduralAnalysis}} and
accordingly modify {{FunctionOp.refreshSizeInformation}} and
{{FunctionOp.inferOutputCharacteristics}} to allow size propagation over
{{FunctionOps}} during dynamic recompilation. Second, introduce a rewrite to
split DAGs after {{FunctionOps}} that return matrices/frames and are not
dimension-preserving (see {{RewriteSplitDagDataDependentOperators}} for an
example).
Finally, let's separate the discussion on "structs" (or "tuples") as it's not
really related. We would most likely implemented structs as syntactic sugar at
parser level. In contrast, the discussion above was referring to multi outputs
in HOP and LOP DAGs, which is much more involved.
> UDFs w/ single output in expressions
> ------------------------------------
>
> Key: SYSTEMML-1444
> URL: https://issues.apache.org/jira/browse/SYSTEMML-1444
> Project: SystemML
> Issue Type: Sub-task
> Components: APIs, Compiler, Runtime
> Reporter: Matthias Boehm
> Assignee: Janardhan
> Fix For: SystemML 1.0
>
>
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