[Mesa-dev] RFC: LLVM, -fno-rtti, and Haiku

[Kenneth Graunke]

On 10/11/2013 09:50 AM, Francisco Jerez wrote:
> Kenneth Graunke <kenneth at whitecape.org> writes:
> 
>> On 10/10/2013 04:27 PM, Alexander von Gluck IV wrote:
>>>
>>> In llvm.py -fno-rtti is always a build flag if LLVM present >= 3.2
>>>
>>> This breaks everything on our end (missing rtti related symbols) in our
>>> C++ libGL.so as Haiku uses dynamic casts.
>>>
>>> We build our LLVM packages with rtti (REQUIRES_RTTI=1).
>>>
>>> Not 100% sure why we're forcing no-rtti if LLVM >= 3.2.
>>> "llvm-config --cxxflags" should always show "-fno-rtti" if REQUIRES_RTTI=1
>>> wasn't set at build time. If REQUIRES_RTTI was set, -fno-rtti is removed
>>> from the llvm-config cxxflags.
>>>
>>> It was originally added here:
>>> http://cgit.freedesktop.org/mesa/mesa/commit/scons/llvm.py?id=d37ae642034bcaca39492c1eb75b029fb27ceffb
>>>
>>> My solutions are either removing the forced -fno-rtti, or wrapping it
>>> with a platform != 'Haiku'
>>>
>>> Thoughts?
>>>
>>>  -- Alex
>>
>> I would love to see us build with -fno-rtti for all Linux builds.  I've
>> been meaning to try that and measure the impact.
>>
> The -fno-rtti option is evil, it changes the C++ ABI in an incompatible
> way.  As you may have noticed from the build error, in some cases it's
> impossible to link normal C++ object files with -fno-rtti object files
> if the interface between them exposes polymorphic types.
> 
> That's the reason why some LLVM versions require us to build the
> interfacing module with -fno-rtti, and the same versions require us to
> build *without* -fno-rtti if RTTI was enabled in the LLVM build, as
> might be the case in Haiku and some Linux distributions.
> 
> AFAICT the 'if' statement in scons/llvm.py:198 and the automake
> conditional in configure.ac:1953 are broken and should probably be
> removed.  LLVM doesn't require -fno-rtti unless llvm-config says
> otherwise, and if it still does in some case it's an llvm-config bug
> that can probably be addressed differently.
> 
> I don't think it's a good idea to enable -fno-rtti except for isolated
> modules that can be guaranteed not to expose or use any C++ API.  There
> are legitimate uses of RTTI, and enabling -fno-rtti means that modules
> that use it cannot talk to modules that don't.
> 
> Thanks.

Which would be fine for Mesa (except maybe on Haiku), since all of our
usage of C++ is internal and we don't expose /any/ C++ API.  Nor should we.

It looks like Clover uses RTTI.  Nothing else does, and I'd like to keep
it that way.

--Ken