[ARM32] Empty structs are ignored when passing in C++

[tomeksowi]

Problem similar to #97285, opening new issue since the RISC-V one is closed. I'm less familiar with ARM32 so I can't point to the ABI wording that specifies empty struct handling.

struct Empty {};

int after_empty(int i0, Empty e, int i1)
{
	return i1;
}

int after_empty_on_stack(
    int i0, int i1, int i2, int i3, int i4, int i5, int i6, int i7, Empty e, int i8)
{
	return i8;
}

after_empty(int, Empty, int):
        mov     r0, r1
        bx      lr
after_empty_on_stack(int, int, int, int, int, int, int, int, Empty, int):
        ldr     r0, [sp, #16]
        bx      lr

GCC passes i1 and i8 in r2 and [sp, #20] respectively. Comparison on Godbolt
ARM64 on Clang looks ok, it matches GCC and passes i1 in w2

Clang version: 18.1.0 and Godbolt "trunk" at the time of filing this issue (2024-07-09).

cc @svs-quic @efriedma-quic
Part of dotnet/runtime#84834

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: Tomasz Sowiński (tomeksowi)

Problem similar to #97285, opening new issue since the RISC-V one is closed. I'm less familiar with ARM32 so I can't point to the ABI wording that specifies empty struct handling.

struct Empty {};

int after_empty(int i0, Empty e, int i1)
{
	return i1;
}

int after_empty_on_stack(
    int i0, int i1, int i2, int i3, int i4, int i5, int i6, int i7, Empty e, int i8)
{
	return i8;
}

after_empty(int, Empty, int):
        mov     r0, r1
        bx      lr
after_empty_on_stack(int, int, int, int, int, int, int, int, Empty, int):
        ldr     r0, [sp, #<!-- -->16]
        bx      lr

GCC passes i1 and i8 in r2 and [sp, #<!-- -->20] respectively. Comparison on Godbolt
ARM64 on Clang looks ok, it matches GCC and passes i1 in w2

Clang version: 18.1.0 and Godbolt "trunk" at the time of filing this issue (2024-07-09).

cc @svs-quic @efriedma-quic
Part of dotnet/runtime#84834

[ostannard]

For AArch32, this is defined by CPPABI32:

The GC++ABI defines the way in which empty class types are laid out. For the purposes of parameter passing in AAPCS32, a parameter whose type is an empty class shall be treated as if its type were an aggregate with a single member of type unsigned byte.

https://github.com/ARM-software/abi-aa/blob/main/cppabi32/cppabi32.rst#summary-of-differences-from-and-additions-to-the-generic-c-abi

[ostannard]

Fixed by #124762.