[cfe-dev] [cfe-commits] False positive for -Wunreachable-code

[Abramo Bagnara]

Il 30/10/2012 19:19, David Blaikie ha scritto:
>>>
>>> Your definition of "false positive" differs from mine/ours. Hopefully
>>> my description above helps describe the motivation here.
>>
>> Yes, my definition of false positive is different, see:
>>
>> http://en.wikipedia.org/wiki/Type_I_and_type_II_errors#False_positive_error
>>
>> "is the default unreachable"? ("is there a wolf near the herd?")
>>
>> If the message says "warning: will never be executed
>> [-Wunreachable-code]" ("Wolf, wolf!") then we have a false positive.
>>
>> The idea that a warning is a false positive if it has not found a bug in
>> the code is rather weird... do you know any warning that is able to
>> always find a bug without knowing the programmer intentions?
> 
> Generally, no. They make local assumptions so to make a best effort at
> finding bugs. Part of that is also to avoid finding non-bugs because
> doing so creates a burden on developers that may eventually eclipse
> the benefit they gain from the warning.
> 
> We're not trying to write warnings to teach people about the semantics
> of their code - they can read books about that. They're meant to be
> actionable for a good reason, not just to indicate that the developer
> read & understood the diagnostic message & then went on their merry
> way.
> 
>> That apart, of course if we want a compilation switch that deviates from
>> the standard and says that an enum typed expression cannot have any
>> value different from enum constants specified we can do that, but really
>> we want that?
> 
> Not really, no - the compiler implements the standard. For the purpose
> of diagnostics we might be interested in adding an attribute to enum
> types or variables that could indicate whether that variable or
> variables of that type are intended to contain an/all values in the
> representable range (to cause us to do things like diagnose the code
> after the switch or in the default of a covered switch as reachable,
> etc).

In the long term this seems indeed the right way to handle that, but I
believe that more useful (and in our work of software verifier tools
writer indeed mandatory) is to add a global option (disabled by default)
to avoid statistical assumption about programmer intention.

(i.e. the option says (among other things): "as the standard does not
limit enum typed values to enum constants listed in enum declaration,
then this assumption, although if true for 90% of cases, is not taken
for granted)

This would not harm in any way the use of clang warnings as a way to
identify locations that are bug with a probability > 90%, while in the
same time makes clang a valid tool to find code weakness according to
language standard semantics and to suggest proper defensive programming
technique whenever they are needed.

Is this a viable path to get the best of the two worlds?

-- 
Abramo Bagnara

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