Re: MPS: assertion failed: header_type (h) != IGC_OBJ_FWD

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: eggert@cs.ucla.edu,  pipcet@protonmail.com,  emacs-devel@gnu.org,
>   eller.helmut@gmail.com
> Date: Tue, 16 Jul 2024 16:39:46 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Example with sizeof(int) = 4, and sizeof(void *) = 8
> >> 
> >>   struct x
> >>   {
> >>     int x;
> >>     struct Lisp_Symbol *s;
> >>   };
> >> 
> >> What about the offset of x::s?
> >
> > The pointer or the Lisp_Symbol struct it points to?
> 
> What is offsetof (struct x, s)? Is it guaranteed to be 8 in this case?

Yes, AFAIK.

> But it's not limited to struct members. The question is similar for
> control stacks, and anything allocated via malloc. Given an address
> range [start, end) to scan with scan_ambig, where can references be
> found?

AFAIU, each memory block allocated by malloc is guaranteed to have the
alignment of the largest fundamental data type on the platform.  So
yes, 8-byte alignment is guaranteed in this case.  (See also the
max_align_t data type introduced by C11.)