llvm dev - Jun 2015 - [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?

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.

Second question: Given that context, would it be reasonable to say
that metadata would not be considered an appropriate mechanism for
communicating ABI information?

Thanks,
Stephen

----- 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.
> 
> 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
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

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
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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