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

[Robinson, Paul]

| Bitcode backward compatibility, at least for the current major version, is supposed to make this unnecessary.

I think the "at least for the current major version" part is one thing that concerns us.  LLVM 4.0 will promise to read LLVM 3.4 bitcode, but LLVM 4.1 will not, according to my understanding of the current promise.  Smoothly identifying that point and being able to provide an intelligent diagnostic seems like goodness. Hard to distinguish "old" bitcode from "broken" bitcode without recording version info of some kind, and the sooner we start recording the version number the more completely we're able to diagnose the situation properly when the time comes.
--paulr

From: cfe-dev-bounces at cs.uiuc.edu [mailto:cfe-dev-bounces at cs.uiuc.edu] On Behalf Of Greg Bedwell
Sent: Saturday, September 27, 2014 3:20 AM
To: Sean Silva
Cc: 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

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<mailto:chisophugis at gmail.com>> wrote:


On Fri, Sep 26, 2014 at 5:18 PM, Yung, Douglas <douglas_yung at playstation.sony.com<mailto: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<mailto:bob.wilson at apple.com>]
Sent: Friday, September 26, 2014 16:39
To: Yung, Douglas
Cc: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>; cfe-dev at cs.uiuc.edu<mailto: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<mailto: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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