Re: Unjustified optimization due to restricted struct members?

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Nov 30, 2023 at 1:50 PM Ties Klappe <tg.klappe@gmail.com> wrote:
>
> 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.

It's not implemented.  I think there's even a bugreport about this,
basically restrict
gets only "one layer deep".

Richard.

> 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.