[LLVMdev] Structure returns to-do or not-to-do?

[John McCall]

On Oct 20, 2010, at 2:56 PM, roger roger wrote:
> I notice that using the online demo, functions which return structures are converted to be void, and instead take an extra argument which is the a pointer to the returned structure, and the allocation is made by the caller.
> Even declared, but undefined functions are converted like this, presumably this doesn't cause a problem during linking?

Right now, conformance with the platform ABI calling convention is an intricate dance between the frontend (i.e. that which emits the IR) and the backend.  So if the ABI says that a particular structure return is implemented by passing a buffer pointer as an implicit first argument to the function, then the front-end is responsible for generating IR that follows that pattern if it wants ABI conformance.  The optimization pass you point out is potentially breaking ABI conformance, which means it (hopefully) only fires for functions with internal linkage.

If you're writing a frontend that doesn't care about conformance with the platform ABI — generally because your functions aren't going to be called from C — then you're limited only by what the backends you're targeting actually support.  Unfortunately, LLVM provides pretty limited promises here, and I wouldn't want to mis-state them.

John.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20101020/30735089/attachment.html>