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

Francisco Jerez currojerez at riseup.net
Fri Oct 11 18:50:08 CEST 2013

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.


> --Ken
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