Array Slices and Interior Pointers

[Rainer Schuetze]

On 11.12.2012 22:08, Alex Rønne Petersen wrote:
> On 11-12-2012 21:24, Rainer Schuetze wrote:
>>
>> This stores the type info with the reference, not with the memory block,
>> but it does not make a big difference. (Actually it does: if the
>> reference only is a reference a base class of the actual instance, type
>> info is lost.)
>
> This got me thinking a bit.
>
> In the current on-going precise GC work, what is type info actually used
> for? It seems to me that given the current GC semantics, the only thing
> it's useful for is figuring out what parts of memory do /not/ contain
> pointers, and nothing else.

Yes, it is only interested in pointers. The current implementation 
creates a bitmap from type introspection at compile time, where each bit 
specifies whether the respective word of an instance is a pointer. When 
a memory block is allocated, the bitmap is copied (with some 
complications) from the TypeInfo object into a memory bitmap that is 
used for GC scanning later. This seems slightly inefficient with respect 
to memory usage, but it allows to scan faster, as the complications have 
to be dealt with only once, not every time when scanning. It also allows 
changing the scanning information of only a part later e.g. to integrate 
emplace!T with precise scanning (though this isn't implemented yet).

>>
>> I understand that the CLI forbids interior pointers, but that seems an
>> implementation detail of its GC.
>
> It's standardized: Ecma 335, I.8.2.1.1.

I still read that restriction as driven by an implementation detail 
("For performance reasons..."), not by some design necessity.

>
> And it's something we need to deal with if we care about D on the CLI.
>