AdrianV / nimbackend

nim backend for delphi

nimbackend

WARNING this a WIP - many bugs expected ! Currently only tested with Delphi 2007 on Win32 !

nim backend for delphi. We have to go into the deep because nim compiles to C or C++ and the interop between Delphi and nim and therefore C / C++ is difficult because of many reasons:

• Delphi uses its own calling convention register. So calling from C into Delphi is almost impossible.
• One can use the the bcc32c (C++ Compiler from Embarcadero), but it is still not very compliant to Standard C and the overall experience together with nim is frustrating.
• nim does automatic memory management with GC or the --newruntime. Delphi has manual memory management together with some automatic management for some types like strings and dynamic arrays or Variant
• Sometimes the size of Delphi integer types is inconsistent over different version - NativeInt for example.
• and many more...

After many tests with different approaches, clean and dirty, I have chosen to get my hands dirty. My approach does all, what is recommended not to do ! So be careful and don't trust me.

• As C compiler currently only a actual mingw32 GCC (8.1 +) will work, since it has a calling convention which is compatible with the Delphi register calling convention. For this to work nim must be convinced to use this as its fastcall convention. For this all nim code must be compiled with --include:nbcommon
• Be aware that you will get problems when you include nim code which needs the default fastcall convention. In reality this should be a rare case, but keep it in mind.
• nim and Delphi must use the same memory manager - at least for the shared data. We have the option to use the nim memory manager in Delphi or a custom Delphi memory manager in nim. Both is supported.
• When a thread is created in Delphi this thread must be registered in nim. This is done with a hook procedure on the Delphi side.
• For both reasons you must include the nimbackend unit into your Delphi project at a very early stage. If you choose to use the nim memory manager, it must be the first unit in your project. If you use a custom Delphi memory mananger like FastMM or BrainMM, nimbackend should be imported right after them.
• The behavior of the managed Delphi data types must be mimicked in nim. I am currently working on AnsiString and Dynamic Arrays
• Since it is not possible to static link C (compiled with GCC) and Delphi Code, the nim part of an application is compiled into a DLL. But this is not a normal DLL since it's imports and exports are application specific - for two different Delphi applications, you will need two differently named backend DLLs.
• Since a Delphi application can not export symbols (why not ???) we must use a manual generated export table for the Delphi code called from nim. I am looking into an (semi) automatic generation for this.
• Care has to be taken how function results are returned. In Delphi for some types like strings, records or dynamic arrays a hidden pointer is passed as the last parameter which receives the result.
• Delphi methods pass the Self as the first parameter, which matches nicely to nim
• Same for Delphi Callbacks procedure (..) of object