Re: Unjustified optimization due to restricted struct members?

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

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Thank you Richard.

Similar to the struct example, I was also wondering about why the following
code does *not* get optimized (e.g. https://godbolt.org/z/9eGrjjK81):

int f(int* restrict a[restrict 2]) {
*(a[0]) = 10;
*(a[1]) = 11;
return *(a[0]);
}

Do you happen to know why a reload via a[0] is required? I would have
expected to see the same optimization as is performed for the struct
example.

Kind regards,
Ties

Op do 30 nov 2023 om 13:16 schreef Richard Biener <
richard.guenther@gmail.com>:

> On Thu, Nov 30, 2023 at 12:07 PM Ties Klappe via Gcc <gcc@gcc.gnu.org>
> wrote:
> >
> > When reading section 6.7.3.1 of the C standard (quoted below) about
> > the *restrict
> > *type qualifier, the first section talks about *ordinary identifiers*.
> > These are defined in section 6.2.3, and exclude members of structures.
> >
> > Let D be a declaration of an ordinary identifier that provides a means of
> > > designating an object P as a restrict-qualified pointer to type T.
> >
> >
> > I would assume that this means that in the code excerpt below the
> function
> > *h* cannot be optimized by substituting the load of *b.p *for *10*, as
> the
> > standard does not specify what it means for a struct member to be
> restrict
> > qualified. However, the code is still optimized by gcc (but not Clang),
> as
> > can be seen here: https://godbolt.org/z/hEnKKoaae
> >
> > struct bar {
> > int* restrict p;
> > int* restrict q;
> > };
> >
> > int h(struct bar b) {
> > *b.p = 10;
> > *b.q = 11;
> > return *b.p;
> > }
> >
> > Was this a deliberate choice, or does it simply follow from how restrict
> is
> > supported in gcc (and could this be considered a bug w.r.t. the
> standard)?
>
> Hmm, this was a deliberate choice (it also works for global 'b'), I didn't
> think
> the standard would exclude that.  Note GCCs C++ standard library makes
> use of restrict qualified pointers as structure members for example.
>
> Richard.
>