[LLVMdev] bit code file incompatibility due to debug info changes

[Chandler Carruth]

I agree that this really needs to be fixed before 3.4...
On Mon, Nov 18, 2013 at 9:56 AM, Bob Wilson <bob.wilson at apple.com> wrote:

> Sometime over the last year, and I'm fairly certain it has been since we
> released 3.3, the debug info metadata format has changed in incompatible
> ways.  If you try to use an older bit code file with the current compiler,
> it is not uncommon for the compiler to crash.  This goes against our goals
> for keeping backward compatibility for bit code files and is especially
> problematic for LTO since you can't reliably mix objects from different
> versions of clang.
>

I don't think that you need LTO for this to be a problem. The bitcode
really has to be readable by future versions of LLVM.


>
> At a minimum, it seems like we need a version number in the debug info
> metadata so we can detect this situation and avoid crashing.
>

Or to put it in the terms of the IR: we need to autoupgrade the debug info
metadata just like we do intrinsics. With debug info this might (at the
worst) involve dropping old metadata.


>
> I don't have a test case but we should be able to come up with one if
> needed.
>

I think doing so would be useful.
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