[cfe-dev] : Analyzer ignoring the effects of a function call?

[Don Quixote de la Mancha]

Inside your if statement, a is always 0.  So the expression can be
simplified to "c = b;".

I've found clang's static analyzer of immense help with my iPhone App,
but the messages it presents are sometimes rather obscure in their
meaning.

Don Quixote de la Mancha
quixote at dulcineatech.com

On Sun, Oct 2, 2011 at 5:48 PM, Arjun Singri <arjunsingri at gmail.com> wrote:
> But how did it arrive at the conclusion that 'a' was '0' when it is clearly
> unknown? If I don't provide an intial value for 'a', it still reports the
> same warning. Does that mean that it is interpreting that 'a' is set to zero
> inside the function?
>
> On Sun, Oct 2, 2011 at 5:35 PM, John McCall <rjmccall at apple.com> wrote:
>>
>> On Oct 2, 2011, at 4:59 PM, Arjun Singri wrote:
>>
>> I think I am getting a false positive for IdempotentOperationChecker which
>> is affecting another checker I am writing. Here is the program I am running
>> the clang analyzer on:
>> #include<stdio.h>
>>   int main(void)
>>   {
>>       int a = 5;
>>       int b = 4;
>>       int c = 10;
>>       scanf("%d", &a);
>>       if (a + b == 4) {
>>           c = a + b;
>>       }
>>       return c;
>>   }
>> I get:
>> $clang --analyze d.cpp
>>
>>
>> d.cpp:12:15: warning: The left operand to '+' is always 0
>>         c = a + b;
>>                         ~  ^
>> Why is it ignoring the scanf function call? Is this expected because the
>> analyzer doesn't do inter-procedural analysis? But even then I think it
>> shouldn't ignore the effects of the function call. Now this is happening
>> because the LHSVal.isConstant(0) call inside the IdempotentOperationChecker
>> is evaluating to true.
>>
>> It's not ignoring the scanf call:  if it were, it would think that 'a' was
>> 5, not 0.  No, here it's presumably analyzing the dominating if condition
>> and doing basic algebra.
>> John.
>
>
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>



-- 
Don Quixote de la Mancha
quixote at dulcineatech.com

   Custom Software Development for the iPhone and Mac OS X
   http://www.dulcineatech.com/custom-software-development/