Default initializers (or: win32 bindings' struct sizes)

[Frits van Bommel]

Lionello Lunesu wrote:
> What's more, the there are no linker errors for the default built-in initializers:
> 
> struct S { int s; }    // no linker error
> struct D { int d=0; }  // linker error!
> 
> The linking error occurs even with the -O -release -inline flags.
> 
> Why does DMD (I don't know about GDC) require that __init symbol for 
> custom initializers and not for the default ones? Is there any chance 
> that this requirement will be dropped?

Actually, it's not (only) about custom initializers:
*****
urxae at urxae:~/tmp$ cat test.d
import test2;
import std.stdio;

void main()
{
     S s;
     writefln(cast(int)s.c);
}
urxae at urxae:~/tmp$ cat test2.d
struct S { char c; }
urxae at urxae:~/tmp$ dmd test.d
gcc test.o -o test -m32 -lphobos -lpthread -lm -Xlinker 
-L/home/urxae/opt/dmd/lib
test.o: In function `_Dmain':
test.d:(.gnu.linkonce.t_Dmain+0x7): undefined reference to 
`_D5test21S6__initZ'
collect2: ld returned 1 exit status
--- errorlevel 1
*****

(Similarly fails for wchar, dchar, float, double and real)
It looks to be about everything but *all-zero* default initializers. 
Those seem to be the only ones not to need explicit initializers to be 
generated if the type is used as a local variable. Their initialization 
is inlined.

For some reason the inlining doesn't happen for explicitly 
zero-initialized variables, nor for non-zero or non-default initializers.

This despite the fact that global/static variables seem to get the 
initializer "inlined" into the '.data' or (if all-zero) '.bss' section[1].
So it's not that the compiler doesn't know what the initializers should 
be, it's that the code generated to initialize local variables at 
run-time depends on the initializer data being generated by compilation 
of the imported module if it's either non-default or not all-zero.

Preventing your link error would be a simple matter of treating 
initializer symbols similar to templates: put them in link-once sections 
in every object file where they're needed. Not that I'm saying this 
should necessarily be done; I'm just saying it should be relatively 
simple to implement.


[1]: At least, those are the names on Linux (and anywhere else that uses 
ELF binaries, for that matter). Not sure what they are on Windows, but 
the principle should be similar.