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);
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/>
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