[LLVMdev] Suggestion: Support union types in IR

[Jon Harrop]

On Friday 02 January 2009 20:48:25 Chris Lattner wrote:
> On Jan 1, 2009, at 6:25 AM, Jon Harrop wrote:
> >> Exactly. I'm not especially interested in C-style unions, I'm
> >> interested in discriminated unions. But the actual discriminator field is
> >> easily represented in LLVM IR already, so there's no need to extend the
> >> IR to support them. That's why I am only asking for C-style union
> >> support - it's a lower-level primitive that you can use to build
> >> discriminated unions on top of.
> >
> > I don't think you would want to build discriminated unions on top of
> > C-style unions though.
>
> Why?

Uniformity when nesting and space efficiency. Users of a language front-end 
will want to nest discriminated unions, e.g. to manipulate trees. C-style 
unions are designed to be unboxed so they are useless for that unless you 
introduce pointer indirections but if you store all unions by reference then 
the uniformity of the 1-word pointer size obviates the need to use same 
(largest) size union cases so you would drop C-style unions in favor of a 
more space-efficient alternative anyway. Otherwise, you might opt for a 
non-uniform representation where local unions are unboxed but unions stored 
within them are boxed, which is needless complexity but it might buy a little 
performance.

So I think most LLVM users will represent their discriminated unions as 
something like:

  {i32, {..} *}

or:

  {i32, {..}} *

but not as C-style unions.

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e