@pinned classes

[div0]

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bearophile wrote:
> div0:
>> The trouble with a moving garbage collector is that you have to be able
>> to accurately scan the stacks of all threads. This is difficult to do
>> with a language that can arbitrarily call into functions provided by
>> different languages for which the compiler won't and can't have stack
>> layout info.
> 
> All pointers are to pinned memory. You can't give or receive an "unpinned reference"
>to an external function written in other languages. Only the D GC
manages references
> to unpinned classes. So, can you explain me what problems you see?
> 
> Bye,
> bearophile

Well on windows, in a single threaded application your stack usually
looks like this:

==== Stack start
D runtime start up
- -
- -
- -
void main(...) - your application stack
- -
- -
- -
DispatchMessage - win32 call
*
*
*
*
windowProc - your D application window callback proc
- -
- -
- -
- -
SendMessage - win32 call
*
*
*
*
windowProc - another D application window callback proc
- -
- -
- -
- -

Where - is D stack and * is unknown stack (usually win32 functions)
Any part of the stack used by D code might and probably will have
pointers to unpinned objects.

In order to move an object you must be able to update all pointers to
it, otherwise obviously your application is going to crash.

There is no reasonable way to known anything about the parts of the
stack indicated by *, including really where they start or end once you
start talking about nested callbacks.

You can't easily or reasonably find the bits of the stack used by D
code, so you can't scan the stack, and therefore you can't move any object.

Throw threading into the mix and your stacks get even more complicated.

This isn't strictly a windows problem though. There are quite a few C
libraries which use a callback mechanism and they'll all suffer from the
same problem. This is part of the reason why you have to jump through
hoops to call native code from Java or C#.

- --
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk
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