[LLVMdev] Clarification on the backward compatibility promises

[Eric Christopher]

On Tue, Jun 17, 2014 at 2:42 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
>>> 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)
>
> I think we mean the same thing in the end. Since metadata can be
> dropped, one particular way in which upgrade can be done for an old
> format is to drop it. This would miss any optimization that the old
> format was intended to allow, but we don't promise to keep
> optimizations.
>
> What would not be OK is for us to change the format (for example, make
> range take a closed interval) and start miscompiling IR that had the
> old format in it.
>

*shrug* Before we lock down any metadata format like that I'd prefer
it was versioned.

> Debug metadata is special only in that we accept more than a missed
> optimization when it is dropped, and I agree it should remain like
> that for now.
>

:)

-eric