[RFC] GCC caret diagnostics

[Chris Lattner]

On Mar 12, 2008, at 9:49 PM, Manuel López-Ibáñez wrote:
>> The clang front-end generates these warnings.  This means that the  
>> set
>> of warnings produced by the compiler doesn't change as the optimizer
>> evolves, are generally less mystifying to the user, and have perfect
>> location info as a side effect.  People who use -Werror tend to  
>> prefer
>> when they don't get new random warnings due to a compiler upgrade.
>> This approach is similar to what Java compilers do and frontends like
>> EDG do (afaik).
>
> But then you don't have constant propagation and other optimisations
> that remove fail positives, do you? Well, we have discussed already
> doing all uninitialized warnings  before optimisation but users do
> complain about false positives.

There is no right answer, and this topic has been the subject of much  
debate on the GCC list in the past.  I really don't care to debate the  
merits of one approach vs the other with you, I just answered your  
question about what clang does.

Amusingly, GCC 4.0 emitted a false positive about this example despite  
using the techniques you discuss, and no GCC emits any of these  
warnings at -O0 (which is a very common mode used when doing a  
majority of development for some people).

>> The Clang project is also growing a static analysis engine which is
>> very adept at solving path sensitive versions of these problems,  
>> which
>> is useful for finding deeper bugs.
>>
>
> I guess that must take considerably more time and resources and be
> redundant with some of the work of the optimisers. I guess we cannot
> use such approach in GCC.

I have no opinion about the approach that you take in GCC.  In  
practice, we have been able to do this analysis very quickly and get  
good results, and will continue to refine them as clang continues to  
mature.

I personally think that it is a major problem that GCC doesn't produce  
these diagnostics unless optimizations are enabled, and I continue to  
think that having diagnostics change depending on what optimization  
level is enabled is bad.

-Chris