Investigate what to do with complex types

[ZacWalk]

In MSVC complex numbers are represented in a non-standard way using a struct. In MSVC cexp is defined:

_Dcomplex cexp( _Dcomplex z );

Gcc uses the standard approach with a complex type builtin. In GCC cexp is defined as:

double complex cexp( double complex z );

These two approaches are incompatible but normally only a problem when you call a complex method. However lately we noticed some optimisations might replace cos/sin with cexp causing a crash.

My recommendation would be to disable optimisations that add cexp like this.

[Blackhex]

The _Complex double type itself seems to work OK accross the DLL boundary, the problem is with msvcrt.dll bulitin functions like cexp having different signature than GCC expects, as https://github.com/Windows-on-ARM-Experiments/mingw-woarm64-build/issues/102 tests demonstrates.

[apinski-quic]

I looked into the ABI and _Complex double should be treated the same as composite types which passed via d0/d1 and returned in d0/d1.

That is passing _Complex double should have the same ABI as passing struct { double real; double imag;}. I tested clang to see that is what is implemented there even. So this would mean there is some ABI mistake with respect of _Complex double not being treated the same as the struct.

[apinski-quic]

So I looked into using arm64 MSVC and the definition of the struct is basically:

struct dc { double val[2]; };

Someone to check the implementation of the ABI in the GCC for arm64 windows but I can say the standard elf ABI that GCC implements both _Complex double and the above structs are passed exactly the same. So there is some ABI issues and I suspect there might be others. Both should be considered HFAs in terms of the both ABIs.