[dmd-internals] x86_64 ABI questions

[Walter Bright]

On 6/7/2012 4:42 PM, David Nadlinger wrote:
> 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?

My intention is to have it work exactly as if it were an equivalent 
struct, i.e. int[3] would behave as if it were struct S { int a,b,c;}.

>
>  - 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.

I agree with you, but when I did that I was just trying to get something 
that worked at all.

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 By the way, http://dlang.org/abi.html seems to be quite outdated, as 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

Yes, more work needs to be done on that.

>
> 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
>
>