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

[Bob Wilson]

I just filed this as pr17982, but I'd like to call attention to it, since it seems like something we should try to fix in the 3.4 release.

Here's the description from the PR:

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.

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.

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