Re: Nested restrict pointers: missed optimization & client with UB?

[Ties Klappe <tg.klappe@gmail.com>]

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Thank you both for your quick replies.

@Joseph, thank you for linking me to the other issue. If I understand
correctly what the point is, would you then agree that the program main
when calling foo2 has *defined* behavior?
What surprises me is that *p and *q might be the same restricted pointer:
the *xp* object itself is not declared as *int** *restrict* but as *int**.

By passing *xp* as argument to foo1, is the type of the object *xp* then
implicitly converted to (or merely interpreted as) *int* restrict *(because
of the argument type)*, i.e.* xp corresponds to the object *P *the standard
refers to?

int main() {
    int x = 0;
    int* xp = &x;

    int res = foo2(&xp, &xp);

    return 0;
}

---

@Richard, thank you for the alternative implementation. Is foo3 meant to be
optimized by GCC currently (I didn't manage to get GCC13.2 to do it)? Or is
it a hypothetical example that would allow GCC to optimize it?

int foo3(int *restrict * p, int *restrict * q)
{
    int a;
    *p = &a;
    **q = 11;
    **p = 12;
    return **q;
}

Kind regards,
Ties


Op di 13 feb 2024 om 15:29 schreef Joseph Myers <josmyers@redhat.com>:

> On Tue, 13 Feb 2024, Ties Klappe via Gcc wrote:
>
> > int foo2(int *restrict *p, int *restrict *q)
> > {
> >     **p = 10;
> >     **q = 11;
> >     return **p;
> > }
>
> In this case, *p and *q might be the same restricted pointer object.  See
> the more detailed explanation at
> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14192#c8>.
>
> --
> Joseph S. Myers
> josmyers@redhat.com
>
>