[llvm-dev] Backward compatibility of LLVM IR - ll/bc files

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 27 18:22:23 PDT 2020


On Mon, Apr 27, 2020 at 6:42 AM Robinson, Paul via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Older releases are still available for download at releases.llvm.org; I
> believe the 3.0 release was supposed to be able to read 2.x bitcode, so you
> should be able to upgrade the bitcode with 3.0 tools and proceed from
> there.  I **think** everything since 3.0 is still readable by current
> tools.
>
>
>
> The project generally has not favored indefinite backward compatibility.
> There have not been any major bitcode format changes since then, so there
> has been little motivation to declare another compatibility break.  Being
> able to read 9-year-old bitcode does not seem like a valuable feature,
> given the above workaround.
>
>
>
> Let me say, though, that I fully support efforts to review and address old
> bugs.  The LLVM project as a whole fixes (or otherwise closes) only about
> 2/3 of reported bugs.  Improving that statistic would be very nice.
>

Agreed - though not sure what the chance of bugs that old still being
relevant, given how much LLVM changes over time. If most of those older
bugs are no longer relevant, it might be more useful to mass-close them and
spend the effort that might've gone into reproducing/updating them into
looking at more recent bugs. But certainly "dealer's choice" - if looking
at older bugs takes someone's fancy especially, that's cool :)

- Dave


> --paulr
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Ehud
> Katz via llvm-dev
> *Sent:* Monday, April 27, 2020 3:52 AM
> *To:* Craig Topper <craig.topper at gmail.com>
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] Backward compatibility of LLVM IR - ll/bc files
>
>
>
> I admit I didn't know about that, but that is because I am handling a lot
> of *old* bugs, older than LLVM version 3.0.
>
> Version 3.0 has been released in Dec 1st 2011, and there are still many
> bugs open from before that point.
>
>
>
> The current BC Reader can't parse files produced by LLVM version 2.9 and
> below (I've checked).
>
> So, I wonder if there is anyone in favor, against (, or just doesn't care)
> for supporting those versions?
>
>
>
> On Mon, Apr 27, 2020 at 8:18 AM Craig Topper <craig.topper at gmail.com>
> wrote:
>
> I thought the binary bitcode reader was backwards compatible already.
> That's what is documented here
> https://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility
>
>
>
> ~Craig
>
>
>
>
>
> On Sun, Apr 26, 2020 at 9:29 PM Ehud Katz via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Quite often I get to work on an old bug, where an old ll/bc file is
> attached as a testcase. These files, in most cases (if not all), need to be
> converted somehow to the latest format, for the trunk version to be able to
> parse it without an error.
>
>
>
>
>
> So a few questions arise:
>
>
>
> 1. Is there a standard way to convert an old ll/bc to the latest? If not,
> what is the common approach for these cases?
>
>
>
> 2. If there is no such tool, should we have one? (I think we should, of
> course.)
>
>
>
> 3. What about automatically let the ll and bc Readers do the conversion
> (internally)?
>
>
>
> 4. What about versions? Is there an RFC (probably old) for ll/bc file
> versioning? (I know there is an optional record for version in the bc file
> format, but it is not really utilized.)
>
>
>
>
>
> Cheers,
>
> Ehud.
>
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