On Tue, Jul 09, 2024 at 12:28:18PM GMT, Alejandro Colomar wrote:
> Hi Jakub,
>
> On Tue, Jul 09, 2024 at 11:18:11AM GMT, Jakub Jelinek wrote:
> > On Tue, Jul 09, 2024 at 11:07:59AM +0200, Alejandro Colomar wrote:
> > > Yup, I was thinking that maybe noalias is a better name.
> >
> > Name is one thing, but you'd also need to clearly define what it means.
> > When restrict is access based, it is clear what it means.
> >
> > If you want something else which is not based on accesses and which should
> > allow warnings in the callers, I suppose you need to specify not just the
> > pointer but the extent as well (and maybe stride) or that it is an '\0'
>
> Agree. Here's how I'd define it as an attribute:
>
> noalias
>
> The noalias function attribute specifies that the pointer to
> which it applies is the only reference to the array object that
> it points to (except that a pointer to one past the last
> element may overlap another object).
>
> If the number of elements is specified with array notation, the
> array object to be considered is a subobject of the original
> array object, which is limited to the number of elements
> specified in the function prototype.
>
> Example:
>
> [[alx::noalias(1)]] [[alx::noalias(2)]]
> [[gnu::access(read_write, 1)]] [[gnu::access(read_only, 2)]]
> void add_inplace(int a[n], const int b[n], size_t n);
Ooops, I meant 'n' to be the first parameter.
>
> char arr[100] = ...;
>
> add_inplace(arr, arr + 50, 50);
>
> In the example above, the parameters a and b don't alias inside
> the function, since the subobjects of 50 elements do not overlap
> eachother, even though they are one single array object to the
> outer function.
>
> It may need some adjustment, to avoid conflicts with other parts of
> ISO C, but this is the idea I have in mind.
>
> > terminated string, because if you want to say that for
> > void foo (char *, const char *, int);
> > the 2 pointers don't really alias, the size information is missing. So,
> > shall the new warning warn on
> > struct S { char a[1024]; char b[1024]; } s;
> > foo (s.a, s.b, 512);
>
> This does not need clarification of bounds. You're passing separate
> objects, and thus cannot alias (except that maybe you're able to cast
> to the struct type, and then access s.b from a pointer derived from
> s.a; I never know that rule too well).
>
> > or not? Or foo (s.a, s.a + 512, 512);
>
> According to the definition I provide in this email, the above is just
> fine.
>
> Thanks!
>
> Have a lovely day!
> Alex
>
> >
> > Jakub
> >
> >
>
> --
> <https://www.alejandro-colomar.es/>
--
<https://www.alejandro-colomar.es/>
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