[jira] [Commented] (IMPALA-11643) Implement ColumnType::ToIR() for non-scalar types

["Daniel Becker (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/IMPALA-11643?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17614117#comment-17614117 ] 

Daniel Becker commented on IMPALA-11643:
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The reason why structs are not supported is that information about the children is stored in std::vectors, which are difficult to deal with in LLVM code. As the layout of std::vector depends on the compiler and can change (and is also quite complicated), we shouldn't touch it from hand-crafted LLVM code directly, only through IR functions (functions compiled from C++ to LLVM).

A possible solution is to add an IR function that takes a ColumnType* and inserts an element into its vectors. We could construct the non-vector parts of ColumnType in LLVM as we do now, then call this function from hand-crafted LLVM code repeatedly to insert the necessary elements into the vectors.

However, the resulting value is no longer an llvm::ConstantStruct*, which is the return type of ColumnType::ToIR(). Some callers depend on the result being a constant, so we can't change the return type in these cases.

A solution is to have two functions: one that returns a constant and is only valid for non-struct types, and a general function that supports all types and returns llvm::Value*. Creating a function that supports all types and returns a constant is not possible in my opinion without meddling with the internals of std::vector from LLVM code.

This is in a way just moving the problem a bit further as we still end up with a function that only supports scalar types. On the other hand, we would have full support for converting ColumnType objects to LLVM in the cases where constantness is not needed.

> Implement ColumnType::ToIR() for non-scalar types
> -------------------------------------------------
>
>                 Key: IMPALA-11643
>                 URL: https://issues.apache.org/jira/browse/IMPALA-11643
>             Project: IMPALA
>          Issue Type: Improvement
>          Components: Backend
>            Reporter: Daniel Becker
>            Assignee: Daniel Becker
>            Priority: Minor
>              Labels: codegen
>
> Currently ColumnType::ToIR() is only implemented for scalar types. It should be extended to support all types.



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