Re: [1/2] PR 78736: New warning -Wenum-conversion

[Martin Sebor <msebor@gmail.com>]

On 05/10/2017 06:19 AM, Prathamesh Kulkarni wrote:
> On 9 May 2017 at 23:34, Martin Sebor <msebor@gmail.com> wrote:
>> On 05/09/2017 07:24 AM, Prathamesh Kulkarni wrote:
>>>
>>> ping https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00161.html
>>>
>>> Thanks,
>>> Prathamesh
>>>
>>> On 3 May 2017 at 11:30, Prathamesh Kulkarni
>>> <prathamesh.kulkarni@linaro.org> wrote:
>>>>
>>>> On 3 May 2017 at 03:28, Martin Sebor <msebor@gmail.com> wrote:
>>>>>
>>>>> On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:
>>>>>>
>>>>>>
>>>>>> Hi,
>>>>>> The attached patch attempts to add option -Wenum-conversion for C and
>>>>>> objective-C similar to clang, which warns when an enum value of a type
>>>>>> is implicitly converted to enum value of another type and is enabled
>>>>>> by Wall.
>>>>>
>>>>>
>>>>>
>>>>> It seems quite useful.  My only high-level concern is with
>>>>> the growing number of specialized warnings and options for each
>>>>> and their interaction.
>>>>>
>>>>> I've been working on -Wenum-assign patch that complains about
>>>>> assigning to an enum variables an integer constants that doesn't
>>>>> match any of the enumerators of the type.  Testing revealed that
>>>>> the -Wenum-assign duplicated a subset of warnings already issued
>>>>> by -Wconversion enabled with -Wpedantic.  I'm debating whether
>>>>> to suppress that part of -Wenum-assign altogether or only when
>>>>> -Wconversion and -Wpedantic are enabled.
>>>>>
>>>>> My point is that these dependencies tend to be hard to discover
>>>>> and understand, and the interactions tricky to get right (e.g.,
>>>>> avoid duplicate warnings for similar but distinct problems).
>>>>>
>>>>> This is not meant to be a negative comment on your patch, but
>>>>> rather a comment about a general problem that might be worth
>>>>> starting to think about.
>>>>>
>>>>> One comment on the patch itself:
>>>>>
>>>>> +         warning_at_rich_loc (&loc, 0, "implicit conversion from"
>>>>> +                              " enum type of %qT to %qT", checktype,
>>>>> type);
>>>>>
>>>>> Unlike C++, the C front end formats an enumerated type E using
>>>>> %qT as 'enum E' so the warning prints 'enum type of 'enum E'),
>>>>> duplicating the "enum" part.
>>>>>
>>>>> I would suggest to simplify that to:
>>>>>
>>>>>   warning_at_rich_loc (&loc, 0, "implicit conversion from "
>>>>>                        "%qT to %qT", checktype, ...
>>>>>
>>>> Thanks for the suggestions. I have updated the patch accordingly.
>>>> Hmm the issue you pointed out of warnings interaction is indeed of
>>>> concern.
>>>> I was wondering then if we should merge this warning with -Wconversion
>>>> instead of having a separate option -Wenum-conversion ? Although that
>>>> will not
>>>> really help with your example below.
>>>>>
>>>>> Martin
>>>>>
>>>>> PS As an example to illustrate my concern above, consider this:
>>>>>
>>>>>   enum __attribute__ ((packed)) E { e1 = 1 };
>>>>>   enum F { f256 = 256 };
>>>>>
>>>>>   enum E e = f256;
>>>>>
>>>>> It triggers -Woverflow:
>>>>>
>>>>> warning: large integer implicitly truncated to unsigned type
>>>>> [-Woverflow]
>>>>>    enum E e = f256;
>>>>>               ^~~~
>>>>>
>>>>> also my -Wenum-assign:
>>>>>
>>>>> warning: integer constant ‘256’ converted to ‘0’ due to limited range
>>>>> [0,
>>>>> 255] of type ‘‘enum E’’ [-Wassign-enum]
>>>>>    enum E e = f256;
>>>>>               ^~~~
>>>>>
>>>>> and (IIUC) will trigger your new -Wenum-conversion.
>>>>
>>>> Yep, on my branch it triggered -Woverflow and -Wenum-conversion.
>>>> Running the example on clang shows a single warning, which they call
>>>> as -Wconstant-conversion, which
>>>> I suppose is similar to your -Wassign-enum.
>>
>>
>> -Wassign-enum is a Clang warning too, it just isn't included in
>> either -Wall or -Wextra.  It warns when a constant is assigned
>> to a variable of an enumerated type and is not representable in
>> it.  I enhanced it for GCC to also warn when the constant doesn't
>> correspond to an enumerator in the type, but I'm starting to think
>> that rather than adding yet another option to GCC it might be better
>> to extend your -Wenum-conversion once it's committed to cover those
>> cases (and also to avoid issuing multiple warnings for essentially
>> the same problem).  Let me ponder that some more.
>>
>> I can't approve patches but it looks good to me for the most part.
>> There is one minor issue that needs to be corrected:
>>
>> +         gcc_rich_location loc (location);
>> +         warning_at_rich_loc (&loc, 0, "implicit conversion from"
>> +                              " %qT to %qT", checktype, type);
>>
>> Here the zero should be replaced with OPT_Wenum_conversion,
>> otherwise the warning option won't be included in the message.
> Oops, sorry about that, updated in the attached patch.
> In the patch, I have left the warning in Wall, however I was wondering
> whether it should be
> in Wextra instead ?
> The warning triggered for icv.c in libgomp for following assignment:
> icv->run_sched_var = kind;
>
> because icv->run_sched_var was of type enum gomp_schedule_type and
> 'kind' was of type enum omp_sched_t.
> However although these enums have different names, they are
> structurally identical (same values),
> so the warning in this case, although not a false positive, seems a
> bit artificial ?

I'd say the warning is justified in this case, even if the two
enums are clearly designed to be interchangeable.  It will be
a reminder to review code like it to make sure it is, in fact
intended and correct.  If it is, it's easy to suppress by
an explicit cast.  So based on this example alone I wouldn't
feel compelled to remove it from -Wall just yet.

FWIW, when I'm concerned about the impact of a new warning I
build a few packages with it (usually Binutils, Glibc, the
Linux kernel, GDB, Bash, and Busybox).  That gives me some
sense of what to expect.  In the end it's a judgment call as
different people have varying degrees of tolerance for these
kinds of warnings.

Martin

>
> Thanks,
> Prathamesh
>>
>> Martin
>>
>>
>>>>
>>>> test-eg.c:3:12: warning: implicit conversion from 'int' to 'enum E'
>>>> changes value from 256 to 0 [-Wconstant-conversion]
>>>> enum E e = f256;
>>>>        ~   ^~~~
>>>>
>>>> Thanks,
>>>> Prathamesh
>>>>>
>>>>>
>>>>> Martin
>>
>>