[llvm-dev] Why does new llvm-as reject old IR format?

James Y Knight via llvm-dev llvm-dev at lists.llvm.org
Fri Aug 12 07:12:43 PDT 2016

On Fri, Aug 12, 2016 at 10:05 AM, Madhur Amilkanthwar via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Surprised to know that backward compatibility is not honored across the
> tools.
> (i.e. you can read old .bc but NOT old .ll files)
> Supporting latter is more useful, IMO, because then I wouldn't have to
> modify all my sources.

It's been that way for LLVM's entire existence. One could imagine ways to
allow for source-level IR compatibility as well, but, at the moment there
isn't any such promise.


And who are "we" here?

"We" is the LLVM community, since that is the official policy of the LLVM

>  On Fri, Aug 12, 2016 at 7:21 PM, Tim Northover <t.p.northover at gmail.com>
> wrote:
On 12 August 2016 at 06:42, Madhur Amilkanthwar via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > Ideally, I would expect backward compatibility from LLVM tools; and not
>> > requiring to modify the code again.
>> >
>> > What is LLVM's philosophy here?
>> We support backwards compatibility (i.e. newer tools can read older
>> files) on .bc files but not .ll files. This goes back to version 3.0
>> so nothing new can read files from the 2.9 or earlier era and the
>> other direction (older tools reading newer files) is never going to
>> work.
>> We're in the middle of changing our version numbering and official
>> policy, but the general sense is that we have no intent to break this.
>> So for your particular problem you should be able to use a 3.2 llvm-as
>> and then do what you like with the .bc file.
>> Cheers.
>> Tim.
> --
> *Disclaimer: Views, concerns, thoughts, questions, ideas expressed in this
> mail are of my own and my employer has no take in it. *
> Thank You.
> Madhur D. Amilkanthwar
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