[llvm-dev] Unionizing a struct

[Paul C. Anagnostopoulos via llvm-dev]

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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