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

[Pip Cet]

On Fri, May 29, 2020 at 7:32 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
> 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.

You mean it shouldn't be that bad with the existing code? You're probably right.

It would have been very bad with the code I posted though, so best ignore that.

> > 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'.

Oh! You're right, of course. How silly of me not to realize.