The GC scans the heap and stack and it would be hard for the GC to scan the heap while maintaining perfect knowledge about all the types.
Go has this problem too but they choose to use a "conservative collector" which will just assume that if something is a word that contains something that happens to be a valid location in memory it considers that location to be alive even if it's actually garbage.
The main drawback is that if you're not sure if something is really a pointer, you cannot relocate objects and compact the heap. Ocaml has a generational collector with a nursery implemented with a semi space collector. A semi space collector performs a collection which involves moving live objects and leaving the garbage behind. This is very efficient because young objects are less likely to survive and thus the amount of work is proportional to the amount of surviving objects and not the garbage.
It is still not clear to me why it is necessary to store the type information inline. I'm absolutely not an expert, but I thought that some GC'd languages had stack layout metadata[1], to figure out the type of each stack slot. Similarly heap allocated data should have an associated type layout info.
[1] possibly static and indexed by the instruction pointer, not unlike DWARF unwind info.
It's not necessary. Indeed there are many GCs that use type information (as a sibling comment says, Go GC is now precise). But it's more complicated, so it's a tradeoff. You may pay some little cost in dealing with tagged arithmetic but you gain a simpler GC codebase, one perhaps that can work with both the AOT and bytecode compilers (just speculating)
Go has this problem too but they choose to use a "conservative collector" which will just assume that if something is a word that contains something that happens to be a valid location in memory it considers that location to be alive even if it's actually garbage.
The main drawback is that if you're not sure if something is really a pointer, you cannot relocate objects and compact the heap. Ocaml has a generational collector with a nursery implemented with a semi space collector. A semi space collector performs a collection which involves moving live objects and leaving the garbage behind. This is very efficient because young objects are less likely to survive and thus the amount of work is proportional to the amount of surviving objects and not the garbage.