[ping * 4] PR35503 - warn for restrict

[Prathamesh Kulkarni]

On 2 November 2016 at 23:17, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
> On 2 November 2016 at 23:07, Jason Merrill <jason@redhat.com> wrote:
>> 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.
> Ah, indeed, it's just printing address of buf, not using the contents.
>>
>>> 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.
> Yes, that's a limitation of the patch, it just looks at the prototype, and
> not how the arguments are used in the function.
>>
>>> 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.
> Should I commit the patch by keeping Wrestrict "standalone",
> ie, not including it in either Wall or Wextra ?
Hi,
After Joseph and Jason's approval, I have committed a rebased version
of patch as r242366
after bootstrap+test on x86_64-unknown-linux-gnu, cross-test on
arm*-*-*, aarch64*-*-* and
verifying no warning is triggered on kernel build with make
allmodconfig && make all.
Because the patch only looks at function prototype, there could be
false positives with the warning (restrict example 3 in C11 std),
and hence the warning isn't enabled by default, and neither by Wall or Wextra.
The warning is only enabled with -Wrestrict option.

Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
>>
>> Jason