Re: Conservative GC isn't safe

[Eli Zaretskii <eliz@gnu.org>]

> From: Ken Raeburn <raeburn@raeburn.org>
> Date: Mon, 28 Nov 2016 04:36:56 -0500
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, emacs-devel@gnu.org
> 
> But we do use interior pointers sometimes; looking at Fsubstring’s handling of a vector object:
> 
>   else
>     res = Fvector (ito - ifrom, aref_addr (string, ifrom));
> 
>   return res;
> }
> 
> … here we pass Fvector a pointer to somewhere within the “contents” array of the vector passed as argument “string”; it’s neither the Lisp_Object value, nor the start of the allocated structure.  Now, I don’t think this is a case that can trigger GC at the critical time.  But clearly we’ve got at least one case where we keep an interior pointer and — locally, at least; the caller could be another matter — don’t keep a live handle on the object itself.

But 'string' still references the contents array of the vector, so GC
will mark it when it comes to 'string'.

> And the compiler can do it too.  For example, if we did something like this:
> 
>   DEFUN (“frob-array-elts", Ffrob_array_elts, Sfrob_array_elts, 1, 1, 0,
>          doc: /* Blah */ )
>     (Lisp_Object obj)
>   {
>     int i;
>     for (i = 0; i < 30; i += 3)
>       {
>         frob (AREF (obj, i));
>       }
>     return Qnil;
>   }
> 
> I tried compiling this (“gcc version 4.9.2 (Debian 4.9.2-10)” on x86-64).  The generated code computes obj+3 (vector tag is 5, contents array starts at offset 8) and obj+0xf3 (end of the iteration), and overwrites the register containing the original “obj” value with the argument to be passed to “frob”.
> 
> If, in this case, “frob” were something that could trigger GC, then stack scanning would not see “obj”, at least not in this stack frame.  And if the caller is doing something like:
> 
>   Ffrob_array_elts (get_vector_of_stuff ());
> 
> then the caller needn’t retain any other references to “obj” either.

Again, 'obj' will be somewhere on the stack up the call frames.
Otherwise, how did it wind up in your frob-array-elts function?