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

[Paul Eggert <eggert@cs.ucla.edu>]

On 2024-07-16 06:38, Eli Zaretskii wrote:
>> From: Gerd Möllmann<gerd.moellmann@gmail.com>
>> Cc: Emacs Devel<emacs-devel@gnu.org>,  Helmut Eller<eller.helmut@gmail.com>
>> The function scan_ambig assumes that that references are aligned on word
>> boundaries (8 bytes here). I haven't checked (and I'm too lazy to check
>> :-)), but that assumption doesn't have to be true.
> I think it_is_ true.  At least the original allocation code in
> alloc.c made sure it was true.  Paul, am I right?

You're right. In lisp.h, struct Lisp_Symbol is declared with a 
GCALIGNED_UNION_MEMBER, which means that struct Lisp_Symbol must be on 
an address that is a multiple of 8. This is verified statically by 
lisp.h's line "verify (GCALIGNED (struct Lisp_Symbol));". Although there 
could be a bug in Emacs that would use pointer arithmetic to misalign a 
struct Lisp_Symbol, that would be the same class of bug that would 
misalign any other type.

GCALIGNED_UNION_MEMBER is implemented via GNU C and/or C11 primitives if 
available. Although this could be ineffective on older compilers, the 
"verify (GCALIGNED (struct Lisp_Symbol));" would catch any older 
compiler that happened to not align struct Lisp_Symbol properly. In 
practice we've never run into any such compiler and are never likely to 
in the future, so long as we insist only on an alignment of 8.