Hi,
On Thu, 14 Jul 2011, Mikael Morin wrote:
> Couldn't we simulate the desired behaviour with more than one decl, one of
> them const qualified? Like so:
>
> void
> sub (int *restrict non_aliasing_var)
> {
> *non_aliasing_var = 5;
> {
> const int *non_aliasing_var_tmp = non_aliasing_var;
> some_function ();
> if (*non_aliasing_var_tmp != 5)
> foobar_();
> }
> }
Nope, const qualification is mostly ignored by the middle end. The reason
is that const-ness implies only not changing the pointed-to memory _via
this very pointer_. It doesn't mean the pointed-to memory isn't reachable
via other means (e.g. other pointers or direct access to global memory)
that could be used to change it. restrict gives you some of that, but
still doesn't rule out reachability via global memory.
We need something like "this pointer points to memory that isn't reachable
via other mean including global memory". non-global mostly is that but
requires some adjustments to escaping at function exit (otherwise many
stores into it will be dead).
Ciao,
Michael.
