Precise GC

[Sean Kelly]

On Apr 7, 2012, at 6:56 PM, 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.

With __traits and such, I kind of always figured we'd go this way. There's simply no reason to have the compiler generate a map. Glad to see it's working out.