[LLVMdev] Clarification on the backward compatibility promises

[Evan Cheng]

> On Jul 10, 2014, at 11:40 AM, Eric Christopher <echristo at gmail.com> wrote:
> 
> On Thu, Jul 10, 2014 at 11:29 AM, Evan Cheng <evan.cheng at apple.com> wrote:
>> 
>>> On Jul 9, 2014, at 9:52 PM, Eric Christopher <echristo at gmail.com> wrote:
>>> 
>>> On Wed, Jul 9, 2014 at 9:33 PM, Owen Anderson <resistor at mac.com> wrote:
>>>> 
>>>> On Jul 9, 2014, at 3:51 PM, Eric Christopher <echristo at gmail.com> wrote:
>>>> 
>>>> On Wed, Jul 9, 2014 at 3:12 PM, Evan Cheng <evan.cheng at apple.com> wrote:
>>>> 
>>>> 
>>>> On Jun 17, 2014, at 2:10 PM, Eric Christopher <echristo at gmail.com> wrote:
>>>> 
>>>> 2. Metadata compatibility. We already had precedence of introducing
>>>> incompatible changes into metadata format in the past within release.
>>>> Should we use relaxes rules for metadata compatibility?
>>>> 
>>>> 
>>>> I think we have a special case for debug metadata (and should document
>>>> that), but otherwise I think we should hold metadata to the same
>>>> standard as the rest of the IR.
>>>> 
>>>> 
>>>> The idea with metadata is that it can be removed and everything still
>>>> works. I'm definitely not ready to lock down the debug metadata format
>>>> and I really don't think we should for any of the other uses since
>>>> stripping already works. (Note, I don't consider function attributes
>>>> etc as metadata)
>>>> 
>>>> 
>>>> We may need to rethink this. If metadata is used only as optimization /
>>>> codegen hints, then yes I agree they can be dropped. But I suspect there is
>>>> a need for metadata that’s *required* for correctness. As LLVM continues to
>>>> gain clients beyond “just” compilers, we will need to be sensitive to their
>>>> needs. I anticipate use of LLVM bitcode files as persistent object format.
>>>> 
>>>> 
>>>> I think that metadata that's required for correctness should be baked
>>>> into the IR and not be metadata - so if there's something we need for
>>>> correctness we need to come up with an IR extension. See the recent
>>>> comdat work as an example.
>>>> 
>>>> 
>>>> That’s not really a practical suggestion for clients that aren’t essentially
>>>> clang.  The bar to changing the IR is (correctly) very high, essentially
>>>> unreachable if the client is out-of-tree.
>>>> 
>>> 
>>> Sure, but they likely have their own metadata format with their own
>>> needs and can keep their own local patches for their out of tree
>>> extensions right? As far as I know we don't have any metadata
>>> extensions in tree that are required for any correctness. If so,
>>> they've explicitly gone against the rules we set for metadata a long
>>> time back:
>>> 
>>> http://blog.llvm.org/2010/04/extensible-metadata-in-llvm-ir.html
>>> 
>>> Unless I'm missing your point completely of course :)
>> 
>> I don’t disagree with this. I am only cautioning against taking an absolutely hardline attitude towards metadata compatibility. As long as we take a pragmatic approach by providing ways for clients to maintain backward compatibility, I don’t think anyone will have problems with the stated policy. We will need to be very careful if / when we propose fundamental changes.
>> 
> 
> I'm not inclined to change any metadata consumer just for kicks, and
> if we want to change how metadata itself works then that should be a
> more reasoned and careful change.
> 
> I.e. I see a user of metadata as something that we don't guarantee
> compatibility for, but the metadata system itself I can see as a more
> formal IR construct.
> 
> Make sense?

Yes, this sounds generally reasonable.

evan

> 
> -eric