Re: PR35503 - warn for restrict pointer

[Fabien Chêne <fabien dot chene at gmail dot com>]

2016-08-30 17:34 GMT+02:00 Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>:
> On 30 August 2016 at 20:24, Tom de Vries <Tom_deVries@mentor.com> wrote:
>> On 26/08/16 13:39, Prathamesh Kulkarni wrote:
>>>
>>> Hi,
>>> The following patch adds option -Wrestrict that warns when an argument
>>> is passed to a restrict qualified parameter and it aliases with
>>> another argument.
>>>
>>> eg:
>>> int foo (const char *__restrict buf, const char *__restrict fmt, ...);
>>>
>>> void f(void)
>>> {
>>>   char buf[100] = "hello";
>>>   foo (buf, "%s-%s", buf, "world");
>>> }
>>
>>
>> Does -Wrestrict generate a warning for this example?
>>
>> ...
>> 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];
>> }
>>
>> h (100, a, b, b)
>> ...
>>
>> [ Note that this is valid C, and does not violate the restrict definition. ]
>>
>> If -Wrestrict indeed generates a warning, then we should be explicit in the
>> documentation that while the warning triggers on this type of example, the
>> code is correct.
> I am afraid it would warn for the above case. The patch just checks if
> the parameter is qualified
> with restrict, and if the corresponding argument has aliases in the
> call (by calling operand_equal_p).
> Is such code common in practice ? If it is, I wonder if we should keep
> the warning in -Wall ?
>
> To avoid such false positives, we would need to track which arguments
> are modified by the call.
> I suppose that could be done with ipa mod/ref analysis (and moving the
> warning to middle-end) ?
> I tried looking into gcc sources but couldn't find where it's implemented.

I don't know either which pass can be used, but I guess it should be doable.
If so, a first step would be to determine useless restrict-qualified
args (and possibly warn for them), and then perform the current
-Wrestrict analysis only on non-useless restrict-qualified args ?

--
Fabien