Hello,
I'm try to figure out the contents od the base class instance in D, LDC Windows 64bit.
I'm sure that there is a 8 byte VMT pointer.
But unable to fund information about the remaining 8 byte. I guess it must be a unique 8byte (void* ?) identifier that is useful for Monitor. In a video someone mentioned this as 'magic'.

Is there a documentation about that 'magic' field? I'd love to use some bits in it if I'm sure about the consequences.

I have a really small object, only 32 bytes. At this point if I want to add a flag bit I have 3 choices:
- add a new field to the class -> effectively the instanceSize will grow by 50% (16bytes with alignment)
- compress the current 16bytes of data. -> eats more cpu
- Hide it somewhere 'magically'.

Thank You in advance!

On Thursday, 16 January 2020 at 14:30:04 UTC, realhet wrote:
> Is there a documentation about that 'magic' field?

I'm pretty sure the only fields in there are pointer to vtable and pointer to monitor object...

> I have a really small object, only 32 bytes. At this point if I want to add a flag bit I have 3 choices:

Do you need virtual functions? If not, you could probably just make a struct instead.

On Thursday, 16 January 2020 at 14:32:24 UTC, Adam D. Ruppe wrote:
> On Thursday, 16 January 2020 at 14:30:04 UTC, realhet wrote:
>> Is there a documentation about that 'magic' field?
>
> I'm pretty sure the only fields in there are pointer to vtable and pointer to monitor object...
>
>> I have a really small object, only 32 bytes. At this point if I want to add a flag bit I have 3 choices:
>
> Do you need virtual functions? If not, you could probably just make a struct instead.

Alternatively, the class can be marked as extern(C++).

On Thursday, 16 January 2020 at 14:32:24 UTC, Adam D. Ruppe wrote:
> On Thursday, 16 January 2020 at 14:30:04 UTC, realhet wrote:
>> Is there a documentation about that 'magic' field?
>
> I'm pretty sure the only fields in there are pointer to vtable and pointer to monitor object...
>
>> I have a really small object, only 32 bytes. At this point if I want to add a flag bit I have 3 choices:
>
> Do you need virtual functions? If not, you could probably just make a struct instead.

Thank you both for the hints!

Yes I need virtual functions (this is the base class of my layout system: it can be a letter, a picture, or a paragraph of text.).

I've tried extern(C++) and it went down 8 bytes.

But the I tried the following:
synchronized(obj){ ... }
Compiles without a problem. Hmm...

I think I will use extern(C++) and later when I will have a big system to test, I will benchmark it. I'm sure that Monitor functionality is not needed for these objects, so that extra 8 bytes will worth it.

Update:
- All of the child classes needed to be marked with extern(C++)
- static class members are not supported, only __gshared static.
- passing a delegate to a constructor of this class expects a (extern(C++) delegate) too.
- Internal compiler error: string[string] can not be mapped to C++

So extern(C++) is not good in the current case.

I will try to use that 8 byte magic illegally, and will see if it is unstable or not.
I will not do any synchronization, but I think the GC will crash upon releasing these objects.

Thx for the help!

On Thursday, 16 January 2020 at 15:28:06 UTC, realhet wrote:
> Update:
> - All of the child classes needed to be marked with extern(C++)
> - static class members are not supported, only __gshared static.
> - passing a delegate to a constructor of this class expects a (extern(C++) delegate) too.
> - Internal compiler error: string[string] can not be mapped to C++
>
> So extern(C++) is not good in the current case.

The 3 latter points can be trivially worked around via extern(D):

extern(C++) class C
{
    extern(D):
    static int tlsGlobal;
    this(void delegate()) {}
    void foo(string[string] aa) {}
}

void main()
{
    C.tlsGlobal = 123;
    auto c = new C(() {});
    c.foo(null);
}

> I will not do any synchronization, but I think the GC will crash upon releasing these objects.

Very likely.