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

[Francisco Jerez]

Alexander von Gluck IV <kallisti5 at unixzen.com> writes:

> On Fri, 11 Oct 2013 09:50:08 -0700
> Francisco Jerez <currojerez at riseup.net> 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 fil
>> 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.
>
> That would solve my issues.  Do we need to do any kind of impact testing? I
> haven't done much Mesa Linux development, so I'm not sure of the process.
>
Just send a git-send-email or git-format-patch formatted patch to the
mailing list, and make sure to CC the original author of that commit so
he can comment on that.

Thanks.

>  -- Alex
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