[pjmlp]

> On top of that, all runtimes that use a tracing GC require tons of spare memory at all times unless programs are very carefully written to avoid memory allocation.

Only if the said language doesn't allow for stack or static globals.

Quite a few languages do allow it.

[fauigerzigerk]

Doesn't that effectively amount to manual memory management? What particular languages are you referring to?

[pjmlp]

> Doesn't that effectively amount to manual memory management?

Not really, example in Active Oberon:

    TYPE
      point = RECORD x, y : INTEGER; END;

    VAR
      staticPoint : point; (* On the stack or global *)
      gcPoint     : POINTER TO point; (* GC pointer *)
      noGCPoint   : POINTER(UNTRACED) TO point; (* pointer not traced by the GC *)

> What particular languages are you referring to?

Mesa/Cedar, Oberon, Oberon-2, Active Oberon, Component Pascal, Modula-2+, Modula-3, D, Oberon-07, Eiffel, BETA.

There are probably a few other ones.

[fauigerzigerk]

>noGCPoint : POINTER(UNTRACED) TO point;

That's fine for one point. How about N points where N varies at runtime?

If I allocate memory dynamically outside the GC's remit, I'm going to have to release that memory somehow.

[pjmlp]

Depends on the specifics of the language.

On Active Oberon's case, those pointers are still safe. They can only point to valid memory regions, think of them as weak pointers that can also point to data on the stack or global memory.

This in safe code.

If the package imports SYSTEM, it becomes an unsafe package, and then just like e.g. Rust's unsafe, the rules are bended a bit and usage with SYSTEM.NEW() SYSTEM.DISPOSE() is allowed.

Just like any safe systems programming language, it is up to the programmer to ensure this pointer doesn't escape the unsafe package.

[fauigerzigerk]

I still don't get how you can say that this memory is not under the GC's control but it's "not really" manual memory management either. Is it reference counting then? How does that memory get released?

[pjmlp]

It doesn't get released, unless you are doing manual memory management inside an unsafe package.

In a safe package it can only point to existing data, there isn't anything to release.

If the pointee is something that lives on the heap, it is similar to weak references. Points to GC data, but doesn't count as yet another GC root.

If the pointee is on the stack or global memory (data segment in C), then there is also nothing to release. Global memory only goes away when program dies, stack gets released on return. Memory that was allocated by the compiler due to VAR declarations, it is static.

Usually the idea is that you use untraced pointer to navigate statically allocated data structures, they are not to be exposed across modules.