[LLVMdev] How to make sure the compatibility of LLVM IR

[Tim Northover]

Hi,

> I am wondering how to know the exhausted incompatibility of LLVM IR?

Unfortunately, you're probably going to have a very hard time of it.
Compatibility in that direction isn't something LLVM either tries to
maintain or document. We try to make sure older bitcode files will
compile with newer versions of LLVM, though even that only works to an
extent.

The IR itself doesn't change *very* regularly (running "git log
docs/LanguageReference.rst" would give some idea of what's happened
since 3.2 -- I see ~150 since the 3.2 era). But how it's interpreted
and what backends are expected to cope with is more in flux. And the
API is almost certainly very different, so doing it all in a single
process would lead to a world of pain.

Cheers

Tim.