Baby's Second Garbage Collector

Post by LeonTing1010 » Mon, Apr 06, 2026, 4:16 AM UTC

matheusmoreira wrote: post #19

mananaysiempre wrote: post #18

Earlier quoted context omitted.

One recent interesting partly-conservative, moving design I remember reading about is Whippet[1] by Andy Wingo (Guile maintainer, SpiderMonkey contributor). He’s been partly forced into it, though, by the fact that Guile’s existing external API (unlike, say, Lua’s) exposes the fact that it uses a non-moving collector (by allowing the user to keep hold of raw pointers to Scheme objects), so I’m not sure if this should…

I really like his blog! I emailed him when I published my delimited continuations article because it addresses the overlapping native/lisp stacks problem he wrote about. Sadly I don't think he's seen it. > One way is to inform the garbage collector of the locations of all roots [...] implicitly, in the form of a side table generated by the compiler associating code locations with root locations. I wonder if there's a…

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Post by viega » Mon, Apr 06, 2026, 5:52 PM UTC

mananaysiempre wrote: post #5

TL;DR: Conservative collector. Not where I would have taken things, but valid. Re forcing a register spill: assuming the GC is invoked via an ABI-compliant function call, you don’t actually need to save all the scalar registers manually, only the callee-save ones (as setjmp does). Alternatively, you can make the compiler do the spilling by inserting a function preserving no registers into the call chain—this is spell…

Interesting; while I knew about `preserve_all` and `preserve_most` attributes, I didn't have an inkling that they've added `preserve_none`. But that's much cleaner that the traditional `setjmp()` or custom asm (which I've never needed-- `setjmp()` has always seemed to get the job done portably).

As for stack bounds, address of a local is easy enough to make work well, but if you care about scanning thread stacks, I'm not aware of a good way to do that with compiler intrinsics only. If you're willing to assume pthreads, it's doable, but even that requires platform-specific code. The only viable portable approaches for dealing w/ thread stacks I'm aware of aren't awesome:

1. Assume your control of thread creation isn't circumventable, and add your own sentinel at the other end of the stack. 2. Probe the stack for the first access violation.

No option is particularly satisfying there; wonder if I missed some other intrinsic added along the way to solve that problem.

Re: Baby's Second Garbage Collector

Re: Baby's Second Garbage Collector