[Mesa-dev] [PATCH] mesa: Remove C++11 narrowing warnings

[Chad Versace]

On 08/09/2012 01:22 PM, Kenneth Graunke wrote:
> On 08/09/2012 01:10 PM, Chad Versace wrote:
>> Add -Wno-narrowing to CXXFLAGS for gcc.
>>
>> This removes warnings of the form
>>     warning: narrowing conversion of X from 'int' to 'float' inside { } is
>>     ill-formed in C++11 [-Wnarrowing]
>> in ff_fragment_shader.cpp and gen6_blorp.cpp of the form.  When building
>> i965, I observed no other difference in the build output.
>>
>> CC: Kenneth Graunke <kenneth at whitecape.org>
>> Signed-off-by: Chad Versace <chad.versace at linux.intel.com>
>> ---
>>  configure.ac | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/configure.ac b/configure.ac
>> index 89686b4..fa4fd71 100644
>> --- a/configure.ac
>> +++ b/configure.ac
>> @@ -228,6 +228,9 @@ if test "x$GXX" = xyes; then
>>      # gcc's builtin memcmp is slower than glibc's
>>      # http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43052
>>      CXXFLAGS="$CXXFLAGS -fno-builtin-memcmp"
>> +
>> +    # Silence C++11 warnings that we don't care about.
>> +    CXXFLAGS="$CXXFLAGS -Wno-narrowing"
>>  fi
>>  
>>  dnl even if the compiler appears to support it, using visibility attributes isn't
> 
> Nak.  I believe this will break the build if using a g++ too old to
> recognize -Wno-narrowing.  You'd need to do the whole rigamarole used in
> the compiler flag checks a bit earlier.  Or import the handy m4 macro
> from xorg-util-macros which does all the rigamarole in one line.

It's safe to do this. If an old gcc encounters a -Wno option that it does not
recognize, then it will usually silently ignore it. I verified this by building
with -Wno-whatever, and the option was silently ignored, as expected.

>From the GCC Manual [http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html],
emphasis mine:

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.