[cfe-dev] rfc: winnt.h's UNREFERENCED_PARAMETER() vs clang's -Wunused-value

[Nico Weber via cfe-dev]

On Mon, Oct 12, 2015 at 3:02 PM, David Majnemer <david.majnemer at gmail.com>
wrote:

>
>
> On Mon, Oct 12, 2015 at 2:18 PM, Reid Kleckner via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> I'd really rather not do 2. Fighting with MSVC CRT headers over va_start
>> has been pretty awkward. Users have reported issues with our header
>> override fails. I'd rather not start fighting with winnt.h.
>>
>> I'm OK with 3, but I'm desensitized to having to do this kind of pattern
>> matching for __uuidof, etc. Maybe we should wait for another +1. :)
>>
>
> A +1 from me for option #3 as well.  I'd feel more comfortable doing #2 if
> it was for correctness (along the lines of our  va_start/va_arg hack).
>
> On what basis does MSVC decide that UNREFERENCED_PARAMETER should silence
> an unused parameter?  IIRC, it expands to wrapping it's argument in
> parenthesis.  Could we teach clang to do the same?  Sounds like it could be
> a more general fix.
>

That sounds like a really good idea to me – teach sema that "(x);" has the
same meaning as "(void)x;".


>
>
>>
>> On Sat, Oct 10, 2015 at 2:50 PM, Nico Weber via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Hi,
>>>
>>> clang has a useful warning -Wunused-value that warns on things such as
>>> `4;` (this probably meant to say `return 4;`).
>>>
>>> winnt.h has a macro UNREFERENCED_PARAMETER(x) that's used to mark unused
>>> function parameters. It's somewhat commonly used on Windows, for example
>>> the default project generated by MSVC uses it a few times. The macro is
>>> defined as `#define UNREFERENCED_PARAMETER(x) (x)`, which means if you use
>>> it like it's intended:
>>>
>>>   void f(int x) {
>>>     UNREFERENCED_PARAMETER(x);
>>>   }
>>>
>>> then clang will emit a -Wunused-value warning for it. How do we want to
>>> fix this? Ideas:
>>>
>>> 1. Don't. Tell people to not use UNREFERENCED_PARAMETER().
>>> 2. Have a winnt.h wrapper in lib/Headers that undefines this macro,
>>> include_next's regular winnt.h, undefines it again, and then defines it to
>>> something that doesn't trigger -Wunused-value (say, `(void)p`, or `do {
>>> (void) P; } while (0)` or similar).
>>> 3. Change clang's Wunused-value to check if the unused value is expanded
>>> from a macro that was defined in a system header that's spelled
>>> UNREFERENCED_PARAMETER.
>>>
>>> 2 sounds like it has the highest chance of unintended side effects. 3 is
>>> a bit hacky, but seems nicest from a user's perspective and isn't worse
>>> than other things we do really.
>>>
>>> Other ideas? Opinions?
>>>
>>> Thanks,
>>> Nico
>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>>
>>
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>
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