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

[Stephen Cross]

> That's right. Is there a reason you can't use function attributes for the ABI information?

I'm basically gathering information in advance of making a proposal
for modifications to LLVM IR for encoding ABI information. However, no
aspects of the current design I'm thinking about involve function
attributes; the focus is instead to encode ABI information for types
so we can map from each IR parameter to the relevant source language
type (but clearly in a language-agnostic way). It is of course a
complex area and I'll be writing up an extensive proposal in the next
week or so and submitting that to the mailing list.

> 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.

I probably should've mentioned that I am also strongly against using
metadata for this purpose; the intention is to safely rule out this
option and to clarify the use cases for metadata.

On Fri, Jun 19, 2015 at 11:39 PM, Eric Christopher <echristo at gmail.com> wrote:
>
>
> 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