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

[Laszlo Ersek <lersek@redhat.com>]

On 09/07/17 07:47, Thomas Lamprecht wrote:
> On 09/07/2017 05:38 AM, Gao, Liming wrote:
>> Laszlo:
>>    I add -Wno-unused-const-variable option in GCC5 RELEASE option, and
>> build OvmfPkg with GCC5.3. I don't meet any warning or error. Do you
>> meet any issue? So, you think we can't add this option in GCC5 tool
>> chain.
>>
> 
> Yes, he is right, looking at:
> https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#Warning-Options
> 
>> When an unrecognized warning option is requested (e.g.,
>> -Wunknown-warning), GCC emits a diagnostic stating that the option is
>> not recognized. However, if the -Wno- form is used, the behavior is
>> slightly different: no diagnostic is produced for -Wno-unknown-warning
>> unless other diagnostics are being produced. This allows the use of new
>> -Wno- options with old compilers, but if something goes wrong, the
>> compiler warns that an unrecognized option is present.
> 
> Also the older ones support it:
> https://gcc.gnu.org/onlinedocs/gcc-4.9.4/gcc/Warning-Options.html#Warning-Options
> 
> https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/Warning-Options.html#Warning-Options
> 
> 
> I verified this behavior on gcc 5.4 and on gcc version 4.4.5 (Debian
> 4.4.5-8)
> (spun up an squeeze container, fun times)
> 
> This seems like an easy solution, hopefully also a valid one in the edk2
> context.
> The gcc rationale seems good, if there is such a warning suppress it, if
> not we
> wouldn't output any warning anyhow, so there is nothing to suppress.
> 
> The only drawback seems that if something goes wrong the user could
> wonder if
> those "unrecognized command line options" have something to do with his
> problem.
> 
> It looks like this (gcc 4.4.5):
> 
> tom@olddeb:~# gcc -Wall -Wno-foo-bar test.c
> test.c: In function 'main':
> test.c:4: warning: unused variable 'test'
> At top level:
> cc1: warning: unrecognized command line option "-Wno-foo-bar"
> 
> And very similar on gcc 6:
> 
> tom@plain-stretch:~# gcc -Wall -Wno-foo-bar test.c
> test.c: In function ‘main’:
> test.c:7:9: warning: unused variable ‘test’ [-Wunused-variable]
>      int test;
>          ^~~~
> test.c: At top level:
> cc1: warning: unrecognized command line option ‘-Wno-foo-bar’

Sounds great, thank you guys for discovering this. I believe this would
be a simple solution to BZ#700. Please feel free to update the subject
of that bug as appropriate.

Correspondingly, this patch can be ignored.

Thanks!
Laszlo