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

[Sean Silva]

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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