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

[Eli Zaretskii]

> Cc: monnier@iro.umontreal.ca, pipcet@gmail.com, 41321@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sat, 30 May 2020 10:52:07 -0700
> 
> On 5/29/20 10:54 PM, Eli Zaretskii wrote:
> >>    (USE_LSB_TAG
> >>     ? (uintptr_t) word % GCALIGNMENT == Lisp_Symbol
> >>     : (uintptr_t) word % alignof (struct Lisp_Symbol) == 0)
> > I don't understand how this will work, given that Lisp object on the
> > stack can be pushed as 2 non-contiguous 32-bit words.  Can you
> > explain?
> 
> On a --with-wide-int host where !USE_LSB_TAG, the above test will work 
> correctly on the low-order word of a Lisp object that is a symbol, 
> because ((uintptr_t) word % alignof (struct Lisp_Symbol) == 0) must be 
> true on such a word.
> 
> The test is only for symbols; it's not for other Lisp objects.

So any pointer whose alignment is the same as 'struct Lisp_Symbol'
will pass the test, regardless of the tag bits?  That's basically most
of the struct pointers on those architectures, no?