[LLVMdev] [PATCH] - Union types, attempt 2

[Talin]

On Fri, Jan 15, 2010 at 12:41 AM, Joachim Durchholz <jo at durchholz.org>wrote:

> Talin schrieb:
>
>  Well, the fact that union members have to be indexed by number means that
>> the ordering has to be part of the type - so even though type-theoretically
>> union { i32, float } is the same as union { float, i32 }, in my
>> implementation they are distinct types. However, from the standpoint of a
>> frontend, this is not a great concern, because the frontend will most likely
>> sort the list of types before constructing the IR type.
>>
>
> Hm... it's placing a burden on the frontend developer.
>
> More importantly, it's something that the fronend developer must not forget
> to do, so you better make sure this is documented in capital letters in a
> place where the frontend developer is likely to look when preparing code
> generation.
>
> Most importantly, however, this will create a lot of hassles when making
> code interoperable between compilers: Compiler writers need to agree on a
> language-independent canonical ordering.
> That said, if the ordering is canonical, it could be established at the IR
> level. E.g. by ordering alphabetically.
>
> When coding, please consider that many languages establish assignment
> compatibility between union types. E.g. a union {i32, float} value could be
> assigned to a name that's typed as a union {i32, i64, float}.
> This probably means the need for conversion operators, and it definitely
> means that indexes aren't meaningful by themselves, only in conjunction with
> their union type.
>
> I really feel that these issues should be addressed on a layer above IR.
LLVM IR always requires that all types match exactly, and any conversions or
promotions must be inserted explicitly by the frontend. Making unions do
automatic conversions would make them dramatically different from every
other IR type.

>
> > By always putting the types in a canonical order, regardless of
>
>> the order that they appear in the source code, you can ensure that unions
>> of equal types are always compatible. In other words, you can treat the
>> members like an ordered set rather than like a list.
>>
>
> Yes, that's closer to the frontend semantics: the variants of a union type
> don't have any natural ordering, so list semantics could cause problems.
>
> Regards,
> Jo
>



-- 
-- Talin
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