[llvm-dev] Unionizing a struct

[Paul C. Anagnostopoulos via llvm-dev]

C++ seems willing to compile a PointerUnion of three unique_ptr's. Does 
that make sense, or does it have to be unique_ptr * ?

There is only one use of unique_ptr with PointerUnion in the system, and 
it uses unique_ptr *.

On 4/15/2021 1:47 PM, Craig Topper wrote:
> 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 <mailto: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?
>

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