Wasm: druntime with full GC

[H. S. Teoh]

So I've been working on my minimal druntime for wasm, and discovered
that LLVM actually uses a shadow stack for wasm, not the native
non-addressable wasm stack. Which means that it's possible to scan the
(shadow) stack for GC roots. Which means, in theory, that the full
druntime *could* be ported to wasm, GC and all.

Of course, GC parameters would have to be tweaked in order to fit the
memory constraints of your typical wasm environment. And probably lots
of performance tweaking would be needed. But it'd work.

Which also means that we don't have to wait for WasmGC support to land
in llvm before we can target wasm. In an ideal world the host GC would
take care of providing GC services, but even today, what we have ought
to be enough to port the current D GC to wasm(!).

//

Porting the entirety of druntime to wasm is kinda way out of my scope
right now. I'll probably continue working on gradually expanding my
current minimal druntime to cover more and more of the language. It
might be a good opportunity to experiment with alternative GC schemes
too. I've started with a dumb bump-the-pointer allocator, and planning
to gradually expand it to full GC in the not-so-near future.  Maybe I'll
experiment with write barriers and incremental GCs as well, once I get
to that point...


T

-- 
Tell me and I forget. Teach me and I remember. Involve me and I understand. -- Benjamin Franklin