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

[Paul Eggert]

On 5/29/20 11:37 AM, Pip Cet wrote:
> if you have a very large vector, and a symbol block
> aliasing it as symbol offsets goes away, you have to search for other
> symbol blocks with that property, which might take a long time.

It shouldn't be that bad, because when you are worrying about symbols offset by
'lispsym', you need to look only for symbol blocks; it won't matter if these
values appear to point into a vector because you won't follow them in that case.

> However, I wonder what you mean by "what appear to be symbol-tagged
> pointers"? Surely we need to look up all pointers twice, no matter
> what their tag is, since they might be a reference to something inside
> the struct Lisp_Symbol.

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'.