bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Andrea Corallo]

Pip Cet <pipcet@gmail.com> writes:

> I believe this isn't the problem we're looking for, but it might be
> related anyway.
>
> I'm seeing this in the assembler source code for insdel.c produced
> with the mingw cross compiler (i686-w64-mingw32-gcc-win32):
>
>     movl    60(%esp), %eax
>     movl    %eax, (%esp)
>     movl    72(%esp), %eax
>     movl    %eax, 4(%esp)
>     call    _Fmarker_position
> If I'm reading this correctly, it's of some concern for wide-int
> builds: the two 32-bit halves of a Lisp_Object are stored
> non-consecutively.
>
> Our stack marking doesn't catch that; at least, it doesn't for
> symbols, where the less-significant half isn't a valid pointer. For
> pseudovectors, things should still work...
>
> So I think we have a problem with such --wide-int builds in cases
> where a stack temporary holds an unpinned uninterned symbol while GC
> is called. Something like
>
> (prog1
>   (gensym)
>   (garbage-collect))
>
> might trigger it. No problem with gcc -m32 on GNU/Linux, for some reason.

Very interesting.  AFAIK there's no guarantees for the compiler to spill
a DI reg in adjacent memory.  Also reading the GC code your observation
seems correct to me.

-- 
akrl@sdf.org