[LLVMdev] Union type efforts and ComputeLinearIndex

[Carter Cheng]

I just looked over your diff and it would seem to me that additional changes would be needed to be done to some of the LLVM-IR -> DAG stuff to make things fully functional (this could be a bit of supposition on my part since I do not fully understand all of the code).


--- On Sun, 8/2/09, Andrew Lenharth <andrewl at lenharth.org> wrote:

> From: Andrew Lenharth <andrewl at lenharth.org>
> Subject: Re: [LLVMdev] Union type efforts and ComputeLinearIndex
> To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Date: Sunday, August 2, 2009, 3:55 PM
> I posted an initial implementation
> that would generate code correctly,
> though I don't know that all optimization pass were safe
> still.
> 
> see:
> 
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20090511/077443.html
> 
> As far as I know, there isn't enough demand right now for
> unions.
> They are mainly useful for target-independent
> front-ends.  I don't
> have the time right now to push them, though they would be
> useful for
> my work, just not critical.
> 
> Andrew
> 
> On Sun, Aug 2, 2009 at 3:57 PM, Carter Cheng<carter_cheng at yahoo.com>
> wrote:
> > Hi,
> >
> > I really dont know I came rather late to the
> discussion and some kind folks on this list gave me a few
> links. I didnt see much mention of any controversy. Are
> there issues still that need to be resolved?
> >
> > So far I am still quite the novice with LLVM internals
> but looking over the code it looks like one could hack in
> most of the required functionality up to the DAG building
> stage. I am not sure if this would work so perhaps I could
> get some opinions.
> >
> > My thoughts were as follows- introduce a new
> CompositeType called union (possibly refactoring some stuff
> into a common parent class from StructType).
> >
> > Recycle GEP (using RTTI) to handle UnionType field
> lookups as well. Add type codes into bitreader/bitwriter
> which would cope with the new union type. Add u { ... } into
> the AsmParser.
> >
> > Revise some of the target classes to cope with the
> unusual data layout and report back correct size for the
> union type. Add support for DAG generation into the
> DAGBuilder visitor class.
> >
> > I am just seeing this UnionType as a type of struct
> where all the fields have the same offset and the UnionType
> itself is as large as the largest member. I would speculate
> that otherwise it behaves pretty much like a StructType- but
> then again I am new to LLVM.
> >
> >
> > --- On Sun, 8/2/09, Dan Gohman <gohman at apple.com>
> wrote:
> >
> >> From: Dan Gohman <gohman at apple.com>
> >> Subject: Re: [LLVMdev] Union type efforts and
> ComputeLinearIndex
> >> To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> >> Date: Sunday, August 2, 2009, 12:05 PM
> >> Hello,
> >>
> >> My understanding of the discussions about union
> types was
> >> that
> >> there was trouble working out how unions should
> work. Have
> >> the issues been resolved?  If so, could you post
> a
> >> summary?
> >>
> >> Thanks,
> >>
> >> Dan
> >>
> >> On Aug 2, 2009, at 7:45 AM, Carter Cheng wrote:
> >>
> >> > I am curious how the efforts were progressing
> in
> >> implementing a
> >> > union type. I am will to try to contribute
> somewhat to
> >> this even
> >> > though I am perhaps somewhat of a beginner
> with
> >> LLVM(so it might be
> >> > nice to collaborate with someone). I had a
> look at
> >> some of the
> >> > subsystems that would require changes though
> I may
> >> have missed some.
> >> >
> >> > I however have a few questions-
> >> >
> >> > 1) What is ComputeLinearIndex for
> specifically? I
> >> would expect that
> >> > perhaps the RTTI stuff in there might need to
> support
> >> a new case for
> >> > a union type.
> >> >
> >> > 2) The subsystems I have catalogued that
> probably need
> >> patching are
> >> > AsmParser, BitCode(Reader/Writer), Target,
> CodeGen and
> >> some stuff in
> >> > VMCore. Did I miss anything?
> >> >
> >> > Thanks in advance.
> >> >
> >> >
> >> >
> >> >
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> >
> >
> >
> >
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