[cfe-dev] [LLVMdev] Proposal to add Bitcode version field to bitcode file wrapper

Mon Oct 6 14:39:14 PDT 2014

On Mon, Oct 6, 2014 at 2:24 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

>  One advantage to including the minor version number is that it allows
> the IR to evolve in more flexible ways (semantic difference without a
> syntactic difference) but auto-upgrade stays feasible. That is, it can help
> *avoid* breakages.
>
> Another advantage would be for cases like debug-info metadata, where IIRC
> there's no backward compatibility at all; we just throw away old stuff. If
> we had a minor version in there, it becomes more reasonable to support
> one-minor-version backward compatibility.  (This is a use case that we
> would be interested in.)
>

FWIW we already have a debug info metadata version flag which could be used
to facilitate this functionality. Kind of a painful thing to do, depending
on the particular breakage. Current strategy is that old version debug info
is dropped, no reason it couldn't be upgraded instead.

- David

>  --paulr
>
>
>
> *From:* cfe-dev-bounces at cs.uiuc.edu [mailto:cfe-dev-bounces at cs.uiuc.edu] *On
> Behalf Of *Sean Silva
> *Sent:* Monday, October 06, 2014 12:27 PM
> *To:* Yung, Douglas
> *Cc:* cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
>
> *Subject:* Re: [cfe-dev] [LLVMdev] Proposal to add Bitcode version field
> to bitcode file wrapper
>
>
>
>
>
>
>
> On Sun, Oct 5, 2014 at 8:10 PM, Yung, Douglas <
> douglas_yung at playstation.sony.com> wrote:
>
> Hi –
>
>
>
> I realize the thread has drifted a little, but I wanted to get back to my
> original proposal. I would like to make a change to the bitcode file
> wrapper to include the version of llvm that produced the bitcode. I would
> like to write this version into the unused version field that currently
> exists. Would there be any objections to this change?
>
>
>
> If the version field is currently unused, I don't see the harm in filling
> it in. It probably would be fine to just use it to store the LLVM major
> version, so that we can detect incompatibilities. I still would be opposed
> to using a granularity finer than the "intended" breakage cycle (major
> versions), since that would give the impression that it is "ok" to break
> compatibility since it can be detected through the version field.
>
>
>
> -- Sean Silva
>
>
>
>
>
> Since the original wrapper is only emitted for Darwin targets, I ran an
> experiment where I took bitcode files produced by the official Apple tools
> and then modified the version field to be non-zero. From my simple tests,
> there seemed to be no problems with existing tools when the version field
> is non-zero.
>
>
>
> Douglas Yung
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Sunday, September 28, 2014 1:02
> *To:* Alex Rosenberg
> *Cc:* Greg Bedwell; Yung, Douglas; cfe-dev at cs.uiuc.edu;
> llvmdev at cs.uiuc.edu
> *Subject:* Re: [cfe-dev] [LLVMdev] Proposal to add Bitcode version field
> to bitcode file wrapper
>
>
>
>
>
>
>
> On Sat, Sep 27, 2014 at 11:35 PM, Alex Rosenberg <alexr at leftfield.org>
> wrote:
>
> How is this use case different from the LTO-supported toolchains shipped
> by other vendors such as Apple? Do they have this theoretical problem too?
>
>
>
> If the issue is solely constrained to debug info metadata, then why not
> use metadata to describe the format/version of the debug info?
>
>
>
> FWIW (I haven't followed the rest of this thread) - that's what we/Apple
> have done. There's a module flag metadata that specifies the debug info
> metadata schema version, then the verifier (or some other pass, I forget
> how it's phrased) can check if the version matches the current LLVM's debug
> info metadata version, and if it doesn't match, it strips out all the debug
> info metadata.
>
>
>
>
> Alex
>
>
> On Sep 27, 2014, at 3:19 AM, Greg Bedwell <gregbedwell at gmail.com> wrote:
>
>  As I understand it, the bitcode compatibility promise doesn't extend as
> far as debug info metadata (happy to be wrong here!).  I think we have a
> usecase where need to guarantee that debug information from any two
> arbitrary bitcode files going into an LTO link will result in the
> expected/correct debug information going into the resulting ELF file;
> unless we can be sure that this will always work between bitcode files
> generated by different versions we'd need some way of flagging up an
> incompatibility and providing useful information on the reason to the user.
>
>
>
> --Greg Bedwell
>
> SN Systems - Sony Computer Entertainment Group
>
>
>
> On 27 September 2014 02:24, Sean Silva <chisophugis at gmail.com> wrote:
>
>
>
>
>
> On Fri, Sep 26, 2014 at 5:18 PM, Yung, Douglas <
> douglas_yung at playstation.sony.com> wrote:
>
> Sorry if I was unclear. There are currently no “known incompatibilities”
> that I am aware of, although I fully admit to not being an expert on the
> topic. The idea is that we add versioning information to the bitcode so
> that if an issue were discovered, it could be easily detected and dealt
> with.
>
>
>
> It sounds like time would be better invested in improving the testing of
> our bitcode compatibility promise.
>
>
>
> -- Sean Silva
>
>
>
>
>
> Douglas Yung
>
>
>
> *From:* Bob Wilson [mailto:bob.wilson at apple.com]
> *Sent:* Friday, September 26, 2014 16:39
> *To:* Yung, Douglas
> *Cc:* llvmdev at cs.uiuc.edu; cfe-dev at cs.uiuc.edu
> *Subject:* Re: [LLVMdev] Proposal to add Bitcode version field to bitcode
> file wrapper
>
>
>
> Bitcode backward compatibility, at least for the current major version, is
> supposed to make this unnecessary. Can you provide more information about
> what “known incompatibilities” you’re seeing?
>
>
>
>  On Sep 26, 2014, at 2:40 PM, Yung, Douglas <
> douglas_yung at playstation.sony.com> wrote:
>
>
>
> Hi,
>
>
>
> We would like to add a version number to the bitcode wrapper. This feature
> would allow easier identification of what compiler produced the bitcode so
> that known incompatibilities with LTO compilation could be detected.
> Roughly speaking, this version number would consist of the major, minor and
> optionally the patch version of the compiler used to produce the bitcode.
> The version information would be encoded in 4 bytes, with the first byte
> representing the major version number, the second byte the minor version
> number, and the third and fourth bytes optionally encoding the patch
> version or other information. As to where to place this information, we are
> considering two different possibilities for updating the bitcode wrapper
> specification.
>
>
>
> The first is to simply add a single 32bit wide field at the end of the
> existing bitcode wrapper format field. This would result in the new
> structure looking like this:
>
> [Magic_{32}, Version_{32}, Offset_{32}, Size_{32}, CPUType_{32},
> BitcodeVersion_{32}]
>
> All of the existing fields would keep their current meanings, and the new
> field BitcodeVersion is simply appended with the format described in the
> first paragraph.
>
> A second idea was to use the existing Version field in the bitcode wrapper
> format to store the bitcode version information. According to the
> documentation (
> http://llvm.org/docs/BitCodeFormat.html#bitcode-wrapper-format) this
> field is currently always set to 0. This would allow us to make use of what
> is (presumably) an unused field.
>
>
>
> As this is a feature that we feel would be beneficial to the community, we
> wanted to get feedback on the design for our upcoming patches. Any thoughts
> or opinions on this would be greatly appreciated.
>
>
>
> Thanks!
>
>
>
> Douglas Yung
>
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