llvm dev - Oct 2020 - Calling conventions on x86

Implementing C ABI calling conventions turns out to be difficult – not that all
platforms are hard, it’s just that it’s difficult making sure you’re actually
covering all the cases.
I would love for it to be some good and concise reading on what I need to do in
the frontend to lower the calls to conform to the C ABI (and variants). I’ve dug
a bit into AARCH64 and System V AMD 64, but it’s really really hard to get some
solid information. I was going to do x86, but then I discovered that unlike what
the documentation on x85 tells me, Clang is actually lowering structs to its
components – in certain cases – but if I understand correctly x86 would always
place these things on the stack anyway, so there’s strictly no different to the
x86 ABI if you pass a struct with two ints, or if you pass two int parameters.
And yet Clang lowers them differently, which results in different LLVM IR and –
without optimization – pretty different assembler, but with -O1 they’re
identical.

In addition to that I’ve been looking at the alignment across platforms for
various types. Some things seem odd and I’d like to know whether those are LLVM
bugs or actually correct:

1. On AARCH64, all OS’s except Apples and Microsoft has a minimum preferred
alignment of 32 bits. For Microsoft and Apple they instead match the ABI
alignment
2. On AARCH64_32, all OS’s have 64 bit pointers, except for Apple & MS that
has 32.
3. On 32 bit ARM, netbsd & Apple OS’s have a max required alignment of 32,
for the rest it is the same as the type size, so 64 bit ints are 64 bit aligned
and so on.
4. On 32 bit ARMEB, only netbsd has the behaviour mentioned in (3)
5. For PowerPC64, only the PS3 (Lv2) use 32 bit pointers.
6. For x64, elfiamcu surprisingly has a 32 bit alignment and preferred alignment
of 128 bit floats. However, 64 bit floats are 64 bit aligned…
7. I was unable to find any working triple for a bunch of architectures: arc,
avr, le32, le64 etc etc.

Are the above correct – or results of processor X not in use for other operating
systems etc?

Christoffer
AEGIK / www.aegik.se