Where do you get 24 bytes of overhead from? A mark-sweep collector only needs a bitmap of 1 bit per alignment granule.
edit: oh, that's what the article uses - the tracing function and size function can be per-heap and the forwarding pointers can overwrite the start of objects, to just have enough header to support tracing and sizing, which could be one word.
Hi Hayley, could you explain how the tracing function and size info can be packaged into one word?
Since the GC Arena is meant as a default heap, the size should be allowed to be pretty big (> 32 bits). And doesn't the tracing function need to support a full pointer size? since they're not allocated but rather stored in the executable I guess they'll be placed in some predictable place in the address space, so I could take advantage of that.
We're going to put just a type ID in the word - you could probably get away with much smaller than a 64-bit ID, but padding will round it up to 64 bits in any case. The heap has exactly one tracing function and size function for all objects, and the functions check the type ID to determine what type they're looking at.
Ah I get what you mean now. Yeah that would totally work. Having a custom tracing function per type lets the host language (C in this case) also store its own types in the GC Heap if so desired quite easily though, since all the programmer has to do is call gc_move(...) once per struct field in that function.
Writing out a typeinfo struct of some sort by hand is not as simple.
Maybe a good middle ground would be to register the tracing function separately with the GC Heap, which would return an ID for later use with gc_alloc.
I could also make the size just be the return value of the same function. The question is then what's worse, an extra 8 bytes in the header of every allocation, or the time spent in the function calculating the size. Since the function has to be called anyway, I'm guessing there's no point in storing it.
That would mean even the ID is not necessary, since an 8byte header is about as good as you can get due to padding.
edit: oh, that's what the article uses - the tracing function and size function can be per-heap and the forwarding pointers can overwrite the start of objects, to just have enough header to support tracing and sizing, which could be one word.