3 ms·

This is also fairly standard as a calling convention as well? The Itanium C++ ABI (the standard for x64 on Linux) has the same text: https://itanium-cxx-abi.git

This is also fairly standard as a calling convention as well? The Itanium C++ ABI (the standard for x64 on Linux) has the same text: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#non-trivial-return-values https://itanium-cxx-abi.github.io/cxx-abi/abi.html#non-trivi... So either there's some extra COM consideration I'm not aware of, or there's a serious confusion in how calling conventions work. If someone returns an aggregate, what option do you have other than a pointer? If it's a callee-allocated pointer, what are the lifetime rules and how do you free it? Passing caller-allocated storage is the obvious answer and is far from unusual, or undocumented. The difference I can tell between the MSVC and Itanium C++ ABIs is just that MSVC uses "this, ret" order for methods, and Itanium uses "ret, this". There's probably some subtle differences in what constitutes "non-trivial" between the two ABIs as well.

Const-me 5y ago
Apparently, the OP’s issue was ABI incompatibility between VC++ (also used by Windows SDK and everything built on top, including Direct3D 12), and System V (used on Linux, also MinGW on Windows). VC++ documentation says structures are returned through a caller-allocated pointer. Pretty sure modern VC++ optimizes that thing when it can, especially with link time code generation enabled, but for functions and COM methods imported from DLLs that’s not an option. System V http://c9x.me/compile/doc/abi.html http://c9x.me/compile/doc/abi.html only classifies values as MEMORY when they exceed 16 bytes. The D3D12_CPU_DESCRIPTOR_HANDLE only has a single size_t field, and therefore returned in RAX register in that convention. Moreover, a structure of two size_t fields would be returned in RAX + RDX registers.
- Jasper_ 5y ago
  System V is only a C ABI, not a C++ ABI, so of course it wouldn't cover C++-only details. What C++ ABI does MinGW use on Windows?
  - Const-me 5y ago
    That thing is COM, which is a small subset of C++ ABI. Technically it’s about the same as on Windows, i.e. C ABI with extra first argument for this pointer. Once upon a time I made this library https://github.com/const-me/comlightInterop/ https://github.com/const-me/comlightInterop/ The native side of the interop is idiomatic C++, here’s an example https://github.com/Const-me/ComLightInterop/blob/master/Demos/HelloWorldCpp/HelloWorld.cpp https://github.com/Const-me/ComLightInterop/blob/master/Demo... The C# side of the interop is implemented through the built-in C interop, here’s the relevant part of the library https://github.com/Const-me/ComLightInterop/blob/master/ComLight/Emit/NativeDelegates.cs#L39-L128 https://github.com/Const-me/ComLightInterop/blob/master/ComL... I’ve tested Linux version of that library on AMD64, ARMv7, and ARM64 CPUs, but only with gcc compiler on the native side.

Const-me 5y ago

Apparently, the OP’s issue was ABI incompatibility between VC++ (also used by Windows SDK and everything built on top, including Direct3D 12), and System V (used on Linux, also MinGW on Windows). VC++ documentation says structures are returned through a caller-allocated pointer. Pretty sure modern VC++ optimizes that thing when it can, especially with link time code generation enabled, but for functions and COM methods imported from DLLs that’s not an option. System V http://c9x.me/compile/doc/abi.html http://c9x.me/compile/doc/abi.html only classifies values as MEMORY when they exceed 16 bytes. The D3D12_CPU_DESCRIPTOR_HANDLE only has a single size_t field, and therefore returned in RAX register in that convention. Moreover, a structure of two size_t fields would be returned in RAX + RDX registers.

Jasper_ 5y ago
System V is only a C ABI, not a C++ ABI, so of course it wouldn't cover C++-only details. What C++ ABI does MinGW use on Windows?
- Const-me 5y ago
  That thing is COM, which is a small subset of C++ ABI. Technically it’s about the same as on Windows, i.e. C ABI with extra first argument for this pointer. Once upon a time I made this library https://github.com/const-me/comlightInterop/ https://github.com/const-me/comlightInterop/ The native side of the interop is idiomatic C++, here’s an example https://github.com/Const-me/ComLightInterop/blob/master/Demos/HelloWorldCpp/HelloWorld.cpp https://github.com/Const-me/ComLightInterop/blob/master/Demo... The C# side of the interop is implemented through the built-in C interop, here’s the relevant part of the library https://github.com/Const-me/ComLightInterop/blob/master/ComLight/Emit/NativeDelegates.cs#L39-L128 https://github.com/Const-me/ComLightInterop/blob/master/ComL... I’ve tested Linux version of that library on AMD64, ARMv7, and ARM64 CPUs, but only with gcc compiler on the native side.