[LLVMdev] Union support in LLVM

[Kenneth Uildriks]

On Sun, Dec 12, 2010 at 6:15 AM, James Lyon <jameslyon0 at gmail.com> wrote:
> Hi,
>
> I'm working on developing a programming language using LLVM as a backend,
> and it'd be very handy for me if LLVM had union support. I've been looking
> into getting the previous union implementation working properly for the last
> week or so, but I'm entirely new to the LLVM codebase so I thought I'd ask
> whether I'm barking up the wrong tree before doing a full-blown
> implementation. At the moment it seems like the best approach to get unions
> working is to treat them as a byte array and then have the
> insertvalue/extractvalue instructions automatically perform conversions to
> and from other types by bitcasting to/from an equivalent size i8 vector
> where the bytes can be got at individually.
>
> This approach seems to have a few problems. It gets vector instructions
> involved without any really good reason (I'm looking at the assembler
> output). It also seems to violate the ABI - my test function is trying to
> return the result in memory where I think it should be using registers (I'm
> JITing a function and calling it using GCC compiled code). The x86-86 ABI
> isn't very clear to me though.
>
> Anyway, please let me know if anyone sees major problems with this approach,
> or has any thoughts on the ABI issues.

An alternate approach would be to not define a union type as such, but
to introduce metadata (using the LLVM metadata support) that marks a
memory object as a discriminated union with a particular discriminator
value.  Then optimizers could be taught to make assumptions such as
"the discriminator value has changed if and only if the type of the
object's data has changed".  If your language lends itself to having
all your discriminated unions live in memory until mem2reg time, that
should make it work well without having to rework every layer of LLVM.