[llvm-dev] Current preferred approach for handling 'byval' struct arguments

[Reid Kleckner via llvm-dev]

Today, the vast majority of target in Clang coerce aggregates passed this
way into appropriate word-sized types. They all use their own custom
heuristics to compute the LLVM types used for the coercions. It's terrible,
but this is the current consensus.

I would like to improve the situation so that passing LLVM aggregates
directly does the right thing when the LLVM struct type and C struct type
are effectively the same, so that custom frontend lowering is required for
hard cases involving things like _Complex and unions.

On Tue, Mar 7, 2017 at 9:42 AM, Alex Bradbury via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> As many of you will know, handling of struct arguments in LLVM is
> actually slightly more fiddly than you might expect. For targets where
> aggregates are always passed on the stack it's easy enough, the Clang
> ABI code marks these arguments as byval and the call lowering code in
> LLVM will copy the object to the stack when needed. There are more
> options for when the target has more complex ABI rules, e.g. that as
> much of a struct that will fit should be passed in argument registers.
> One option is to let Clang (or the frontend of your choice) mark the
> argument as byval, but then have the LLVM call lowering code assign
> registers where possible. Alternatively, you can alter the frontend's
> ABI handling so it decomposes the struct for you - the challenge being
> ensuring that the appropriate packing/alignment is maintained. Because
> of this for some targets byval does mean "pass on the stack", but not
> others.
>
> There's been some discussion of this issue previously:
> * In the context of PowerPC, which will coerce structs below a certain
> size to an integer array
> <http://lists.llvm.org/pipermail/llvm-dev/2015-March/083554.html>
> * Previous suggestions that we really want an "onstack" attribute
> <http://lists.llvm.org/pipermail/llvm-dev/2012-May/049406.html>
>
> I'm working with a calling convention
> (https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md)
> where structs of up to two words in length should be passed in
> registers, but otherwise on the stack (EXCEPT in the case where there
> is only one argument register left, in which case the struct is split
> between that register and the stack).
>
> Is there a consensus now on how this should be handled?
>
> Thanks,
>
> Alex
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