[LLVMdev] Using the unused "version" field in the bitcode wrapper (redux)

[Yung, Douglas]

Hi,

The conversation has drifted slightly, so I wanted to bring it back to the version field in the bitcode wrapper.

Currently in the toolchain which we ship and support, we use a proprietary linker. That linker is unable to read bitcode files and we do not have any plans to enable it to as far as I’m aware. Because of this, we need a way of identifying the version of a bitcode file without needing to read/understand the bitcode itself. The bitcode wrapper is perfectly suited for this task as it is simple to parse without the linker needing to understand bitcode, is already defined and already includes a version field.

If we could get the compiler to use that version field that is already present, it would be a simple solution to our problem. So I guess my question is whether there is any objection to actually using a field which is already allocated for this kind of information?

Douglas Yung

From: Sean Silva [mailto:chisophugis at gmail.com]
Sent: Friday, October 31, 2014 21:13
To: llvmdev at cs.uiuc.edu
Cc: Yung, Douglas
Subject: Re: Using the unused "version" field in the bitcode wrapper (redux)



On Fri, Oct 31, 2014 at 6:21 PM, Sean Silva <chisophugis at gmail.com<mailto:chisophugis at gmail.com>> wrote:
Hi all,

Doug Yung started a discussion earlier (http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-September/077227.html) about using the unused "version" field in the bitcode wrapper, and I think there was some misunderstanding. I'd like to clarify the motivation.

The reason we want to add the version field is to easily identify "old" bitcode. It is only LLVM version granularity, but that is good enough for us. The obvious thing is that we offer compatibility, so why do we need to know the bitcode version? There are really two reasons:

1. In the short term, LLVM does theoretically provide compatibility. However, we at Sony are under a much stronger commitment to our customers than the open source project here, so until the test infrastructure is beefed up quite a bit to improve this confidence in the backwards compatibility promise, the version field is a quick unobtrusive way for us to behave correctly in the short term. Beefing up the compatibility testing is a separate discussion that everybody realizes is a much larger long-term endeavor; we at Sony are glad to help with that. As we prepare for our first official SDK release with LTO, where our customers are officially sanctioned to feed bitcode to our tools, a solution is needed though. I think that existing toolchains with LTO will also benefit from this in the near-term.

2. In the long term, it *will* break across major releases. Currently I don't think we have a way to identify this. This is a bit longer-term perspective, but it will eventually come up and this fixes it.


Also, is there a reason why the bitcode wrapper is Darwin-specific?

Rafael gave me some of the backstory on this. Basically it is to work around some buggy behavior in the Darwin ar. Adding that on the front of the bitcode file just to get a version doesn't seem like a very clean thing to do.

Doug, what other alternatives did you guys consider before settling on this?

As for #2 above, the non-universality of the wrapper makes using the wrapper as a version indicator sort of a non-starter.

Looks like I've taken my second U-turn on this proposal :/


Can we just always use it and simplify the code path?

-- Sean Silva

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