Re: [PATCH 1/1] OvmfPkg/IoMmuDxe: shut up "unused-const-variable" gcc-6 warning in RELEASE

[Laszlo Ersek <lersek@redhat.com>]

On 09/06/17 19:12, Ard Biesheuvel wrote:
> On 6 September 2017 at 18:09, Laszlo Ersek <lersek@redhat.com> wrote:
>> On 09/06/17 18:56, Ard Biesheuvel wrote:
>>> On 6 September 2017 at 17:48, Laszlo Ersek <lersek@redhat.com> wrote:
>>>> Starting with gcc-6, a new warning option called "-Wunused-const-variable"
>>>> has become available (and enabled by default under our build settings):
>>>>
>>>>   https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Warning-Options.html
>>>>
>>>> We should give it the same treatment as Ard gave "unused-but-set-variable"
>>>> in commit 20d00edf21d2 ("BaseTools/GCC: set -Wno-unused-but-set-variables
>>>> only on RELEASE builds", 2016-03-24); i.e., we should restrict the warning
>>>> to the DEBUG build target.
>>>>
>>>> However, because the new warning is gcc-6+ only, we cannot add
>>>> "-Wno-unused-const-variable" to any GCC5 macros in
>>>> "BaseTools/Conf/tools_def.template". While the GCC6 toolchain and/or the
>>>> desired handling of the new warning are investigated in
>>>> <https://bugzilla.tianocore.org/show_bug.cgi?id=700>, suppress the warning
>>>> for gcc-6+ (in RELEASE builds) as follows:
>>>>
>>>> - Replace the "mBusMasterOperationName" array with the
>>>>   BUS_MASTER_OPERATION_NAME(Expression, ShortName) macro that compares
>>>>   Expression against EdkiiIoMmuOperationBusMaster<ShortName>, and in case
>>>>   of a match, evaluates to "ShortName", stringified,
>>>>
>>>> - when composing the DEBUG message -- which the preprocessor might
>>>>   eliminate from RELEASE builds, thereby removing its references to
>>>>   variables --, build a ladder of comparisons with
>>>>   BUS_MASTER_OPERATION_NAME().
>>>>
>>>
>>> I wonder if we should put this in a header file somewhere. I really
>>> hate that we need to create new GCCx versions just for small
>>> deviations between versions of the compiler.
>>>
>>> I.e., add this to MdePkg/Include/Base.h
>>>
>>> #ifdef MDEPKG_NDEBUG
>>>   //
>>>   // Don't warn about unused but set variables: they may only be
>>>   // referenced by DEBUG code.
>>>   //
>>>   #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
>>>   #if __GNUC__ >= 6
>>>     #pragma GCC diagnostic ignored "-Wunused-const-variable"
>>>   #endif
>>> #endif
>>
>> The option in question is "-Wno-unused-const-variable". But, that's a
>> side question.
>>
> 
> No. The option is "-Wunused-const-variable", and adding the 'no-'
> prefix is the same as ignoring it.

The best place to commit a typographical error is in the correction for
an earlier typographical error! :)

> 
>> If the above is acceptable to the MdePkg maintainers, it is certainly
>> good enough for me. In that case, I will reassign TianoCore BZ#700 to
>> MdePkg. (I was careful to write "desired handling of the new warning" in
>> the commit message, because I expected that the introduction of the GCC6
>> toolchain just for this might not be universally liked.)
>>
> 
> No. I think we should have fewer rather than more. I think the current
> collection in tools_def is ridiculous, but that is my personal
> opinion.
> 
>> Personally I wouldn't like to submit the MdePkg patch myself, as GCC
>> pragmas are another delicacy that doesn't fit on my plate right now. (If
>> I happen to break something with it, I get to fix it too... Like I'm
>> fixing it now within OvmfPkg.)
>>
>> The point of this patch is to get OVMF back to a building state ASAP.
>> Once we fix BZ#700, I'm more than happy to revert the patch. I sought to
>> write the patch in a way that would be tolerable both short-term and
>> mid/long-term.
>>
> 
> Fair enough.
> 

Thanks.
Laszlo