llvm dev - Apr 2021 - Unionizing a struct

In TableGen, there is the following struct. In any given instance, only 
one of the pointers is filled in.

       struct RecordsEntry {
         std::unique_ptr<Record> Rec;
         std::unique_ptr<ForeachLoop> Loop;
         std::unique_ptr<Record::AssertionTuple> Assertion;

         void dump() const;

         RecordsEntry() {}
         RecordsEntry(std::unique_ptr<Record> Rec) : Rec(std::move(Rec))
{}
         RecordsEntry(std::unique_ptr<ForeachLoop> Loop)
           : Loop(std::move(Loop)) {}
         RecordsEntry(std::unique_ptr<Record::AssertionTuple> Assertion)
           : Assertion(std::move(Assertion)) {}
       };

It has been suggested that the three pointers be made a union. This 
requires a discrimination enum. I believe that would reduce the struct 
from 24 to 12 bytes, and I think it requires a custom destructor.

What do people think about this? If I should do it, could someone help 
me with the changes?


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Depending on the alignment requirement of the 3 types involved, you might
be able to use PointerUnion to store the discrimination enum in the lower 2
bits of the pointer.

~Craig


On Thu, Apr 15, 2021 at 10:33 AM Paul C. Anagnostopoulos via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> In TableGen, there is the following struct. In any given instance, only
> one of the pointers is filled in.
>
>   struct RecordsEntry {
>     std::unique_ptr<Record> Rec;
>     std::unique_ptr<ForeachLoop> Loop;
>     std::unique_ptr<Record::AssertionTuple> Assertion;
>
>     void dump() const;
>
>     RecordsEntry() {}
>     RecordsEntry(std::unique_ptr<Record> Rec) : Rec(std::move(Rec))
{}
>     RecordsEntry(std::unique_ptr<ForeachLoop> Loop)
>       : Loop(std::move(Loop)) {}
>     RecordsEntry(std::unique_ptr<Record::AssertionTuple> Assertion)
>       : Assertion(std::move(Assertion)) {}
>   };
>
> It has been suggested that the three pointers be made a union. This
> requires a discrimination enum. I believe that would reduce the struct from
> 24 to 12 bytes, and I think it requires a custom destructor.
>
> What do people think about this? If I should do it, could someone help me
> with the changes?
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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