Would APEs work with D?

[Raimondo Mancino]

Hello,

assuming that I have little experience with assembly, so I 
couldn't check if the ape.S is actually compatible, and that I 
also still have little experience with D:

I was reading [this very interesting 
article](https://justine.lol/ape.html) and while reading it I had 
the impression that it would be possible to produce D APEs.

It does require some ABI changes as far as I can tell, but I 
think that excluding Phobos (since it would require more work to 
get started) a simple hello world using 
[Cosmopolitan](https://github.com/jart/cosmopolitan/) would do 
the job (maybe we could have more complex code compiling with 
`@pragma(mangle)` for ABI compatibility and interop).

If we look at Cosmopolitan's hello.c:

```c
/* Compile with:
gcc -g -O -static \
   -fno-pie -no-pie -mno-red-zone -nostdlib -nostdinc \
   -o hello.com hello.c \
   -Wl,--oformat=binary -Wl,--gc-sections 
-Wl,-z,max-page-size=0x1000 -fuse-ld=bfd \
   -Wl,-T,ape.lds \
   -include cosmopolitan.h \
   crt.o ape.o cosmopolitan.a
*/

#include "libc/stdio/stdio.h"

int main() {
   printf("%s\n", "hello world");
   return 0;
}
```

My guess is that we could produce a portable executable with 
Cosmopolitan and thus write something like this in our hello.d:

```d
/* I am not sure how would we compile this, but
  * I guess it's going to be something like:
gdc -g -O -static \
   -fno-pie -fuse-ld=bfd -mno-red-zone
   -fno-druntime
   -o hello.com hello.d
   -Wl,--oformat=binary -Wl,--gc-sections 
-Wl,-z,max-page-size=0x1000 -fuse-ld=bfd \
   -Wl,-T,ape.lds
   crt.o ape.o cosmopolitan.a
  */

@pragma(lib, "cosmopolitan"); // or in dub.sdl/.json

@nogc: // I am not sure whether the default GC would work

extern(C) int printf(const char*, ...) @trusted nothrow;

extern(C) int main() {
   printf("%s\n", "hello world");

   return 0;
}
```

If I understand correctly, the needed steps would be:

1. Build the binary
2. Write PE magic number
3. Write BIOS boot sector (if needed)
4. Write sh compatible execution script (load executable code 
through qemu)
5. Write platform-specific sections and tables (ELF, *BSD, ...)
6. Put executable code and data sections
7. PKZIP magic (if needed)

What do you think about it?