[PATCH] D135267: [DWARF] Share across CUs only when order free

[Adrian Prantl via Phabricator via llvm-commits]

aprantl added a comment.

In D135267#3849760 <https://reviews.llvm.org/D135267#3849760>, @dblaikie wrote:

> In D135267#3842862 <https://reviews.llvm.org/D135267#3842862>, @DianQK wrote:
>
>> There seems to be a lot of complexity to consider here, would adding an option to control `DINode` can be shareable be a good suggestion?
>> Pseudo-code:
>>
>>   bool DwarfUnit::isShareableAcrossCUs(const DINode *D) const {
>>     ...
>>     return ((isa<DIType>(D) && !option("disable-die-share-across-cus"))||
>>             (isa<DISubprogram>(D) && !cast<DISubprogram>(D)->isDefinition())) &&
>>            !DD->generateTypeUnits();
>>   }
>
> I'm not ready to understand what solutions might be appropriate as I'm still trying to understand the problem and why it arises for Swift but not C++ - sorry I haven't had enough time to try to get the context clearly in my head.
>
> I don't have a Swift compiler, and don't really know Swift (so I'm not sure what `Foo: Hashable` means - in C++ that would be derivation, and `Hashable` wouldn't have any reference to `Foo` (only the other way around, `Foo` would reference `Hashable`) - could you include the LLVM IR for this small example so I could take a look?

`struct Foo: Equatable, Hashable {}`

Is a definition of a struct with the name `Foo` that conforms to the protocols `Equatable` and `Hashable`. Protocols in Swift are //somewhat// similar in spirit to Concepts in C++. Defining `Foo` to conform to `Equatable` to //somewhat// similar to declaring `bool operator==(const Foo &) = default;` in C++. Because of the protocol conformance the compiler must emit a witness table (~ vtable) defining how to call the methods required by `Equatable` on a `Foo` object.


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