Precise GC

[Steven Schveighoffer]

On Sat, 07 Apr 2012 21:56:09 -0400, Walter Bright  
<newshound2 at digitalmars.com> wrote:

> Of course, many of us have been thinking about this for a looong time,  
> and what is the best way to go about it. The usual technique is for the  
> compiler to emit some sort of table for each TypeInfo giving the layout  
> of the object, i.e. where the pointers are.
>
> The general problem with these is the table is non-trivial, as it will  
> require things like iterated data blocks, etc. It has to be compressed  
> to save space, and the gc then has to execute a fair amount of code to  
> decode it.
>
> It also requires some significant work on the compiler end, leading of  
> course to complexity, rigidity, development bottlenecks, and the usual  
> bugs.
>
> An alternative Andrei and I have been talking about is to put in the  
> TypeInfo a pointer to a function. That function will contain customized  
> code to mark the pointers in an instance of that type. That custom code  
> will be generated by a template defined by the library. All the compiler  
> has to do is stupidly instantiate the template for the type, and insert  
> an address to the generated function.
>
> The compiler need know NOTHING about how the marking works.
>
> Even better, as ctRegex has demonstrated, the custom generated code can  
> be very, very fast compared with a runtime table-driven approach. (The  
> slow part will be calling the function indirectly.)
>
> And best of all, the design is pushed out of the compiler into the  
> library, so various schemes can be tried out without needing compiler  
> work.
>
> I think this is an exciting idea, it will enable us to get a precise gc  
> by enabling people to work on it in parallel rather than serially  
> waiting for me.

I think this is a really good idea.

I would like to go further and propose that there be an arbitrary way to  
add members to the TypeInfo types using templates.  Not sure how it would  
be implemented, but I don't see why this has to be specific to GCs.  Some  
way to signify "hey compiler, please initialize this member with template  
X given the type being compiled".

This could be a huge bridge between compile-time and runtime type  
information.

-Steve