Re: Conservative GC isn't safe

[Eli Zaretskii]

> From: Ken Raeburn <address@hidden>
> Date: Sun, 27 Nov 2016 01:17:54 -0500
> Cc: address@hidden
> 
> > Indeed.  Hans Boehm's done a fair bit of research in this issue,
> > including discussing the underlying assumptions and arguing that
> > compilers should (and usually do) guarantee those assumptions.
> 
> I’d be surprised if that held reliably when the last use of a Lisp_Object in 
> some function extracts an object pointer and then never references the 
> Lisp_Object as such ever again.
> 
> Lisp_Object foo (Lisp_Object obj)
> {
>   …
>   return mumble (XSYMBOL (obj));
> }
> 
> It’s got no reason to specifically obfuscate the value, but it may also have 
> no reason to keep a copy of the Lisp_Object value around when it’s no longer 
> needed.  It’s not so much that the compiler has decided to start using an 
> interior pointer on its own, but instead just doing what we told it to do.  
> If “mumble” triggers GC, stack marking may well find only the pointer and not 
> the original “obj” value in this function, especially if the compiler 
> optimizes away the stack frame of “foo” completely.

IOW, you envision the possibility that the object identified by 'obj'
is not held in any stack-based memory cell in any of the 'foo's
callers, and instead all of them hold that object only in registers?
Is that a reasonable assumption?