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

[Kenneth Uildriks]

On Wed, Oct 20, 2010 at 4:56 PM, roger roger <roger83 at hotmail.co.uk> 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?
>
> I had a look back through the previous releases and saw that this was
> required before version 2.3 because return types could not be structures
> (structures weren't considered first class). However, since then it is legal
> to return a structure, and that there is an optimization pass which looks
> specifically for the type of functions which have an 'sret' argument and
> tries to return the structure in registers if it fits.
> I can imagine that the above behavior could be an artifact of the old
> system, but obviously, when the structure is too large, it's going to have
> to be return via pointer anyway.
>
> So my question is, should I be following the demo's example for IR
> generation or just generate IR which returns structures, even if they could
> be too large to fit in registers?
>
> Thanks,
> Roger

The X86 platform should handle structure returns that don't fit in
registers (by converting it to an sret argument at codegen time), but
as far as I know, that hasn't been implemented on any other platform.