[swift-dev] Validating ABI consistency between runtime C++ and standard library Swift code

[John McCall]

> On Oct 11, 2016, at 8:10 PM, Joe Groff via swift-dev <swift-dev at swift.org> wrote:
> I just tracked down a bug due to C++ code in the Swift runtime code trying to interface with standard library code written in Swift, but getting the ABI slightly wrong and leading to some nasty hard-to-reproduce heisenbugs. While I've narrowed down the bug in this specific case, it seems like this is a potentially continuing source of maintenance bugs, especially as we try to bring up the Swift calling convention in the coming year. I'm wondering if there's anything we could do to help validate that C++ and Swift code in the runtime is correctly interfacing. We could at least check that we're passing the right number of arguments by comparing the LLVM IR of the runtime and stdlib, perhaps, though this would have to be a fuzzy type-level match since Clang and Swift aren't normally going to agree on the exact pointer types of arguments.

One potential approach:  make a def file with the names and (somehow abstracted) expected prototypes of Swift functions defined by the standard library but used by the runtime.  In the runtime, use that to autogenerate a header.  In IRGen, add a +Asserts check before finalizing the module that, if there are any functions with those names, they do match the abstract prototypes.  Hope that all such interactions use only portable IRGen call-lowering patterns.

John.