bug#54532: [PATCH] sorting

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Wed, 23 Mar 2022 21:24:16 +0100
> Cc: Andrew Cohen <acohen@ust.hk>, 54532@debbugs.gnu.org
> 
> > Can you tell more why this was needed? Emacs has conservative stack 
> > marking, so any Lisp object that is referred by some stack-based variable 
> > should be protected from GC. Why isn't that enough in this case?
> 
> Because Lisp values are temporarily moved out to a heap allocated buffer 
> which isn't traversed by the stack scanner. `record_unwind_protect_ptr_mark` 
> can be seen as a generalisation of `record_unwind_protect_ptr` (because 
> that's what it is).

I understand that these objects are on the heap, but AFAIU a reference
to them is on the stack, isn't it?

> > Is this really eassume, or is eassert better here?
> 
> No, eassume is appropriate here. It provides more optimisation opportunities.
> In fact, most uses of eassert are better or just as good as eassume as long 
> as there are no function calls or tricky memory dereferences.

I asked about a specific instance, not in general.  That instance was
at the end of the function, right before it returns, and I wonder what
kind of optimization opportunities that could present.