I remember a letter from some time ago saying that LLVM IR format is private to specific version of LLVM, and is not guaranteed to be compatible with other versions in the future or in the past. Does this announcement mean that is not the case anymore, and LLVM IR is backwards compatible now?
There's IR assembly (the human readable format) and the bitcode format. There are no compatability guarantees for the assembly but new LLVM versions can read the old bitcode inside the same major version.
LLVM is a great tool, but I've had trouble in the past with version compatibility. I wrote a compiler that used LLVM as a backend (I used LLVM versions 3.4-3.6). The problem was that each minor version of LLVM slightly changed the API. It was things like removing parameters from methods, renaming methods, removing the need for some methods completely, or adding/removing some static-link libraries. If you only wish your tool to be compatible with a single version of LLVM, its not a problem, but attempting to support a selection of minor versions ended up being a pain. These minor versions would come out 3-4 times a year and I would need to find what broke each time, and if there was even an equivalent solution in the new version.
I didn't work on the level of IR, so I didn't come across any problems there, but I wouldn't be surprised if the IR syntax changed slightly across minor versions.
All of LLVM is open source, and the format isn't quite so fluid as GP makes it sound. It should be relatively trivial for one versed in the LLVM codebase to grok the current features of the bitcode.
That said, as long as you keep an indication of the LLVM version that your bitcode was generated with, I really don't see a problem with fluid bitcode specs.
