[llvm-dev] Changes to the Developer Policy / IR Backwards Compatibility

[Moritz Angermann via llvm-dev]

Hi,

thank you for clearing that up! Maybe it’s worth to extend the wording then?

I’m essentially generating bitcode without using the llvm bitcode writer, and
hence have some interest in knowing what compatibility the bitcode I generate
today will provide, and when the bitcode generator will need to be adapted.

The basic reasoning behind this is, that the textual ir is rather unstable and
the bitcode ir is more stable and compatible across a larger set of llvm
versions.

If there’s going to be 4.0 followed by 5.0, will 5.0 be able to read all bc
from 3.0 as well? From the old old documentation I felt assured that generating
bc as of 3.7 or 3.8, would still be readable by 4.0. However with the current
documentation, it’s not clear to me when I can expect the bitcode generated 
will stop working.

Also, will there be a 5.X series? Or will there be 5.0 be followed by 6.0?

Thanks again!

Cheers,
 Moritz

> On Oct 9, 2016, at 3:07 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:
> 
> 
>> On Oct 8, 2016, at 10:41 AM, Eric Christopher via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> 
>> 
>> On Fri, Oct 7, 2016 at 11:57 PM Moritz Angermann via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> Hi,
>> 
>> I’ve noted some change in wording on the current (4.0) developer policy[1],
>> compared to the 3.9 developer policy[2]. Specifically the part about IR
>> Backwards Compatibility.
>> 
>> The change from 3.9 to 4.0 is the following:
>> 
>>   - The textual format is not backwards compatible. We don’t change it too often, but there are no specific promises.
>>   - Additions and changes to the IR should be reflected in test/Bitcode/compatibility.ll.
>> - - The bitcode format produced by a X.Y release will be readable by all following X.Z releases and the (X+1).0 release.
>> + - The current LLVM version supports loading any bitcode since version 3.0.
>>   - After each X.Y release, compatibility.ll must be copied to compatibility-X.Y.ll. The corresponding bitcode file should be assembled using the X.Y build and committed as compatibility-X.Y.ll.bc.
>>   - Newer releases can ignore features from older releases, but they cannot miscompile them. For example, if nsw is ever replaced with something else, dropping it would be a valid way to upgrade the IR.
>>   - Debug metadata is special in that it is currently dropped during upgrades.
>>   - Non-debug metadata is defined to be safe to drop, so a valid way to upgrade it is to drop it. That is not very user friendly and a bit more effort is expected, but no promises are made.
>> 
>> This raises the following question for me: does this imply that bitcode generated by 4.X won’t necessarily be as
>> stable as it was for the 3.X series, specifically that the backwards compatability guarantee will be scraped in
>> LLVM 4.0+?
>> 
>> 
>> The intent here was that while typically a major revision would enable a bitcode break - we actually didn't do that this time and so we're just keeping compatibility even with the major version number change.
> 
> 
> To pile-up in case it is not clear: the intent is to *extend* the compatibility.
> 
> Also worth mentioning that there won’t be any 4.X series: 4.0 is scheduled for January, and the summer release will be 5.0.
> 
> — 
> Mehdi
> 
>