Re: Conservative GC isn't safe

[Paul Eggert]

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.)

>   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.)