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

[Aaron Ballman via cfe-dev]

On Mon, Oct 12, 2015 at 6:04 PM, Nico Weber via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> 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;".

I think that's a reasonable approach, but #3 is a good fallback position IMO.

~Aaron

>
>>
>>
>>>
>>>
>>> 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
>>>>
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>>>>
>>>
>>>
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>>
>
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