[LLVMdev] Could metadata ever be mandatory for correctness in LLVM IR?

[Eric Christopher]

On Fri, Jun 19, 2015 at 3:32 PM Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Stephen Cross" <scross at scross.co.uk>
> > To: llvmdev at cs.uiuc.edu
> > Sent: Friday, June 19, 2015 5:19:33 PM
> > Subject: [LLVMdev] Could metadata ever be mandatory for correctness in
> LLVM   IR?
> >
> > Hi everyone,
> >
> > Currently it looks like metadata is used in LLVM IR essentially as an
> > 'optional extra', in that any pass is allowed to remove metadata
> > (though preserving metadata is useful) and the program should remain
> > valid. In other words the metadata only communicates information
> > about
> > code quality or associated information rather than issues relevant to
> > correctness.
> >
> > First question: Is this is a long term design intention OR something
> > that just currently happens to be true?
>
> That is intentional.
>
> >
> > The context for this is that I'm considering how to represent ABI
> > information in a language-agnostic way in LLVM IR. I've seen/heard
> > some suggestions about using metadata to represent ABI information
> > (e.g. to say this 'i32' is actually an 'int'), but it does seem like
> > metadata is not intended for this kind of use case since removing the
> > metadata would affect the correctness of the lowering in the backend.
>
> We might be able to make different guarantees for module-level metadata,
> but I'm not certain.
>
>
I'd definitely be against it. If we want to make it guaranteed then we
should just make it a part of the non-metadata IR.

-eric


> >
> > Second question: Given that context, would it be reasonable to say
> > that metadata would not be considered an appropriate mechanism for
> > communicating ABI information?
>
> That's right. Is there a reason you can't use function attributes for the
> ABI information?
>
>  -Hal
>
> >
> > Thanks,
> > Stephen
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>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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