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

Sean Silva chisophugis at gmail.com
Fri Oct 31 21:13:06 PDT 2014

On Fri, Oct 31, 2014 at 6:21 PM, Sean Silva <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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