The problem with the D GC

[Frits van Bommel]

Ralf Schneider wrote:
> It dosen't seem so hard for me to let the compiler set such an attribute on 
> arrays without pointers...

It's pretty hard if you can't modify the compiler ;).
Sean can't modify what DMD does (at least, not directly). He _can_ 
(directly) modify what the runtime library does by replacing it, which 
is what he's done.
He (or anyone else, for that matter) might be able to implement this in 
GDC though...
And Walter might be convinced to implement it in DMD (or, if it's 
front-end only code, accept a patch that implements it).

Of course, that still leaves arrays of structs, which may contain both 
pointers and non-pointers.
What we really need is a way for the GC to know what the type of the 
memory is, or at least where the pointers are. This may be possible by 
adding this info to TypeInfo & subclasses[1]. But then every memory 
block would need a pointer to the relevant TypeInfo (or some condensed 
form of this information, like flags for "only pointers" and "no 
pointers", with TypeInfo pointer only if both are false).
This would definitely require some sort of compiler support though; it 
would need to generate appropriate type information for structs, objects 
(the actual memory, not the references) and unions.
It would then need to supply this information to the GC in the runtime, 
requiring extra code generation.


[1]: This could be as simple as supplying a member function that 
performs a callback for every offset containing a pointer.