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

[Stefan Monnier]

> What I was trying to say is that if a pointer lacks the symbol tag, then we
> needn't worry about it being offset by 'lispsym'. These pointers need to be
> looked up only once, even if they happen to be pointers into a struct
> Lisp_Symbol. We can safely assume that a compiler won't take a Lisp_Object 
> that
> is a symbol, and add a small offset to it without also adding 'lispsym'.

I don't think that true.

The original problematic case is for wide-int where a 64bit Lisp_Object
containing a symbol is split into a 32bit tag saying "this is a symbol"
and a 32bit pointer to which an offset has been added.

So when we encounter a 32bit word on the stack, it may be a "plain
pointer" or it may be the 32bit of a pointer to a symbol with an
offset applied but we can't tell which it is because we don't have the
tag at that point.


        Stefan "looking forward to bignums replacing wide-ints"