[dmd-internals] x86_64 ABI questions

[David Nadlinger]

I'm currently working on a few codegen issues in LDC, where I'm not 
quite sure if I understood the intended ABI for extern(D) on x86_64 
correctly:

  - What is the exact specification for returning static arrays by 
value?

  - What is the ABI for D vararg functions? (extern(D) void foo(…);)

  - What was the reason of explicitly exposing __va_argsave_t instead of 
going for compiler magic in vararg functions? The latter is what we do 
in LDC (LLVM provides va_* intrinsics), and it seems to work fine. We 
obviously try to be compatible to DMD, but I don't quite see why we 
should move towards an (at least subjectively) worse design: With the 
DMD implementation, the API is different from C _and_ requires annoying 
special casing of code for x86_64. Sure, code that relied on directly 
manipulating the argument pointer on x86 obviously can't work, but a 
program just using va_start/va_arg/va_end should compile fine on either 
platform without changes. With DMD, it doesn't.

By the way, http://dlang.org/abi.html seems to be quite outdated, as it 
still mentions static arrays being passed by reference.

Also related to my current LDC pre-release work, name mangling on OS X: 
http://d.puremagic.com/issues/show_bug.cgi?id=8207

Thanks,
David