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

[Chad Versace]

On 08/10/2012 11:40 AM, Andy Furniss wrote:
> Kenneth Graunke wrote:
>> On 08/10/2012 07:51 AM, Chad Versace wrote:
>>> 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.
>>
>> Oh cool! :D
>>
>> I had no idea it did that.
> 
> Neither does my old gcc 4.2.3
> make[2]: Entering directory `/home/andy/Src/Mesa-git/mesa/src/glsl'
> g++ -DPACKAGE_NAME=\"Mesa\" -DPACKAGE_TARNAME=\"mesa\"
> -DPACKAGE_VERSION=\"8.1.0\" -DPACKAGE_STRING=\"Mesa\ 8.1.0\"
> -DPACKAGE_BUGREPORT=\"https://bugs.freedesktop.org/enter_bug.cgi\?product=Mesa\"
> -DPACKAGE_URL=\"\" -DPACKAGE=\"mesa\" -DVERSION=\"8.1.0\" -DSTDC_HEADERS=1
> -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1
> -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1
> -DHAVE_UNISTD_H=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=\".libs/\" -DYYTEXT_POINTER=1
> -DHAVE_LIBEXPAT=1 -I. -I../../include -I../../src/mapi -I../../src/mesa/ 
> -D_GNU_SOURCE -DPTHREADS -DDEBUG -DTEXTURE_FLOAT_ENABLED -DUSE_X86_ASM
> -DUSE_MMX_ASM -DUSE_3DNOW_ASM -DUSE_SSE_ASM -DHAVE_POSIX_MEMALIGN
> -DIN_DRI_DRIVER -DUSE_XCB -DGLX_INDIRECT_RENDERING -DGLX_DIRECT_RENDERING
> -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER -DHAVE_ALIAS -DHAVE_LLVM=0x0301 
> -DFEATURE_GL=1 -g -O2 -Wall -fno-strict-aliasing -fno-builtin-memcmp
> -Wno-narrowing -g  -MT standalone_scaffolding.o -MD -MP -MF
> .deps/standalone_scaffolding.Tpo -c -o standalone_scaffolding.o `test -f
> '../../src/glsl/standalone_scaffolding.cpp' || echo
> './'`../../src/glsl/standalone_scaffolding.cpp
> cc1plus: error: unrecognized command line option "-Wno-narrowing"
> make[2]: *** [standalone_scaffolding.o] Error 1
> make[2]: Leaving directory `/home/andy/Src/Mesa-git/mesa/src/glsl'
> make[1]: *** [all-recursive] Error 1
> make[1]: Leaving directory `/home/andy/Src/Mesa-git/mesa/src'
> make: *** [all-recursive] Error 1

Andy, I just reverted the patch.

This opens a larger question. gcc 4.2 was released in May 2007, over 5 years
ago, and received its last update in May 2008. How old of a gcc should new Mesa
releases support?

Before 2007 I had not even been introduced to Linux, so I'm not in a position to
answer that question.