Re: PR80806

[Martin Sebor <msebor@gmail.com>]

On 06/29/2017 12:05 PM, Jeff Law wrote:
> On 06/29/2017 11:57 AM, Jeff Law wrote:
>> On 05/23/2017 09:58 AM, Martin Sebor wrote:
>>> On 05/18/2017 12:55 PM, Prathamesh Kulkarni wrote:
>>>> Hi,
>>>> The attached patch tries to fix PR80806 by warning when a variable is
>>>> set using memset (and friends) but not used. I chose to warn in dse
>>>> pass since dse would detect if the variable passed as 1st argument is
>>>> a dead store. Does this approach look OK ?
>>>
>>> Detecting -Wunused-but-set-variable in the optimizer means that
>>> the warning will not be issued without optimization.  It also
>>> means that the warning will trigger in cases where the variable
>>> is used conditionally and the condition is subject to constant
>>> propagation.  For instance:
>> Yea.  There's definitely tradeoffs for implementing warnings early vs
>> late.  There's little doubt we could construct testcases where an early
>> warning would miss cases that could be caught by a late warning.
>>
>>
>>>
>>>   void sink (void*);
>>>
>>>   void test (int i)
>>>   {
>>>       char buf[10];   // -Wunused-but-set-variable
>>>       memset (buf, 0, sizeof(buf));
>>>
>>>       if (i)
>>>         sink (buf);
>>>   }
>>>
>>>   void f (void)
>>>   {
>>>       test (0);
>>>   }
>>>
>>> I suspect this would be considered a false positive by most users.
>>> In my view, it would be more in line with the design of the warning
>>> to enhance the front end to detect this case, and it would avoid
>>> these issues.
>> Given no knowledge of sink() here, don't we have to assume that buf is
>> used?  So, yea, I'd probably consider that a false positive.
> Oh, wait, I missed the constant propagation.  That makes this one less
> clear cut in my mind -- it means its context sensitive.  I could easily
> argue either way on this one.

Suppose buf were the small buffer in std::string and sink() some
conditional use of the class, like in this more complicated code:

   void foo (std::string&);
   void bar (char*);

   void test (int i)
   {
     std::string s;   // calls memset (s.buf, 0, sizeof s.buf)

     char array[100] = "";

     if (i > 100)
       foo (s);       // needs a large buffer
     else
       bar (array);   // works with a small buffer
   }

Variations on this idiom aren't uncommon.

Martin