[LLVMdev] Clarification on the backward compatibility promises

[Owen Anderson]

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.

—Owen



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140709/d739eb23/attachment.html>