One of the great advantages of manual memory management is that it does not require a runtime that can identify all live references. A pointer may be cast in an int, NaN-boxed, etc. - no problem.
In this scheme the "promoting live objects" phase works by copying objects to a new allocation. Crucially, this avoids the ABA problem by always walking forwards in memory. Second, it does not attempt to eagerly update all references - instead it permits references to become dangling. So let's say we have a dangling NaN-boxed pointer.
Now later, our NaN-boxed dangler is unpacked and dereferenced. This causes a SIGSEGV since the page is no longer readable. The signal handler notices it and attempts to fix it up.
IMO this is where things go off the rails:
> If the object was promoted, the handler scans all registers and the current stack frame, and patches all stale references to promoted objects.
We were so close, but now the runtime needs to be able to distinguish references to promoted objects and values that happen to share their address. We almost got away without stack maps.
> Therefore, we modify the compiler to emit meta-data describing the location of heap references in registers and on the stack for every load and store instruction (instead of just gc safe points)
Ok I give up. Now we're just building a garbage collector.
This does not help our NaN-boxing example. There's no way to easily inform the NaN-boxed value that it needs to update; we could hack it by comparing the register's value after the dereference, but that solution is hard to love.
IMO more interesting would be to modify the compiler to require all dereferences to be updatable, and propagate that back through to the original reference. In C++ speak we could imagine this:
void *dereference(void *&ptr) {...}
Anyways seems interesting but could be bolder?