On Thu, Nov 30, 2023 at 1:50 PM Ties Klappe <tg.kla...@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.guent...@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.
