[ping * 4] PR35503 - warn for restrict

[Jason Merrill]

On Wed, Nov 2, 2016 at 1:08 PM, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
> On 2 November 2016 at 18:29, Jason Merrill <jason@redhat.com> wrote:
>> Then I'll approve the whole patch.
> Thanks!
> Trying the patch on kernel build (allmodconfig) reveals the following
> couple of warnings:
> http://pastebin.com/Sv2HFDUv
>
> I think warning for str_error_r() is correct

It's accurate, but unhelpful; snprintf isn't going to use the contents
of buf via the variadic argument, so this warning is just noise.

> however I am not sure if
> warning for pager_preexec() is legit or a false positive:
>
> pager.c: In function 'pager_preexec':
> pager.c:35:12: warning: passing argument 2 to restrict-qualified
> parameter aliases with argument 4 [-Wrestrict]
>   select(1, &in, NULL, &in, NULL);
>                  ^~~            ~~~
> Is the warning correct for  the above call to select() syscall ?

The warning looks correct based on the prototype

extern int select (int __nfds, fd_set *__restrict __readfds,
                   fd_set *__restrict __writefds,
                   fd_set *__restrict __exceptfds,
                   struct timeval *__restrict __timeout);

But passing the same fd_set to both readfds and exceptfds seems
reasonable to me, so this also seems like a false positive.

Looking at C11, I see this example:

EXAMPLE 3 The function parameter declarations
void h(int n, int * restrict p, int * restrict q, int * restrict r)
{
  int i;
  for (i = 0; i < n; i++)
    p[i] = q[i] + r[i];
}

illustrate how an unmodified object can be aliased through two
restricted pointers. In particular, if a and b
are disjoint arrays, a call of the form h(100, a, b, b) has defined
behavior, because array b is not
modified within function h.

This is is another example of well-defined code that your warning will
complain about.

> Should we instead keep it in Wextra, or continue keeping it in Wall ?

It seems that it doesn't belong in -Wall.  I don't feel strongly about -Wextra.

Jason