Re: PR80806

[Jeff Law <law at redhat dot com>]

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.

Jeff