Re: Concurrency has landed

[Paul Eggert]

Stefan Monnier wrote:
> I really
> can't imagine an architecture where "p1 - p2" is valid C code (i.e. both
> p1 and p2 point inside the same object) and works before `free` yet it
> doesn't give the same answer after `free`.

That's not what the code in question does. p1 addresses a newly allocated 
object, and p2 is a dangling pointer that addresses the old (freed) object.

The problem with -fcheck-pointer-bounds does not occur when the program computes 
p1 - p2, as the program still gets a valid integer. The problem occurs later, 
when Emacs computes q + (p1 - p2), where q is a dangling pointer into the old 
object. On conventional architectures this has the same value as (q - p2) + p1, 
but that's not true with -fcheck-pointer-bounds, which attaches q's bounds 
(which are now invalid) to the resulting pointer.