Re: Conservative GC isn't safe

[Daniel Colascione]

On 11/26/2016 12:30 AM, Paul Eggert wrote:
> On 11/26/2016 12:11 AM, Daniel Colascione wrote:
> >   1) mark_maybe_pointer looks only for exact matches on object start.
> > It's perfectly legal for the compiler to keep an interior object
> > pointer and discard the pointer to the object start.
>
> Yes, just as it's perfectly legal for the compiler to subtract 42 from
> every pointer before putting it in a register or storing it into memory.
> In practice, though, compilers don't do this around calls to the garbage
> collector. (True, this assumption should be documented better.)

I can imagine a compiler having a legitimate reason to use an interior 
pointer, but I can't see why it'd subtract 42, XOR it with 0xDEADBEEF, 
or make other opaque transformations. We already search the memory tree, 
and each tree node has both start and end information for each 
allocation. We should be able to cope with interior pointers.

> >   2) INTERVAL is GCed, but it's not represented in the memory tree:
> > struct interval isn't a real lisp object and it's allocated as
> > MEM_TYPE_NON_LISP. Even a direct pointer to the start of an interval
> > won't protect it from GC. Shouldn't we treat intervals like conses?
>
> Does the code ever create an interval that is accessible only via locals
> when a GC occurs? If not, Emacs should be OK. (This should also be
> documented better.)

Anywhere in the code? Forever? I wouldn't be confident saying so.