[llvm-dev] Why Clang is unpacking my StructType Function arguments

[Tim Northover via llvm-dev]

Hi Zhang,

On Wed, 18 Jul 2018 at 09:06, Zhang via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> It would be great if someone could tell me how to tell clang from unpacking my arguments and if that's not possible , the correct way to handle this diffrence.

There's no way to control this I'm afraid. The issue is that each
platform has an ABI document that specifies in detail where arguments
have to be passed in registers and on the stack. LLVM IR isn't really
detailed enough to represent all of these nuances properly.
Especially, you can't always just use the struct type itself or you'll
be incompatible with other compilers and libraries.

So there is unfortunately a hidden contract between Clang and each
backend: Clang (in lib/CodeGen/TargetInfo.cpp) knows how each backend
will treat simple parameters and does things like the splitting up
you've seen, and adding unused padding arguments to make sure the
arguments go where they should. And because this kind of effort is
sometimes needed, it's viewed as a pretty low priority to preserve the
type where it would be sufficient. Correctness is the most important
metric, after all.

It's not an ideal situation and there's been talk in the past of
moving some of that logic into a utility library in LLVM itself, or
even enhancing LLVM IR to handle it more elegantly, but nothing has
come of it yet I'm afraid so at the moment you have to either
duplicate that kind of logic yourself or use Clang as some kind of
library to do it for you (I'm not sure of the details, but I believe
Swift takes this approach).

Cheers.

Tim.