[llvm-dev] Removing "fno-rtti" flag from llvm-config --cxxflags

[David Blaikie via llvm-dev]

On Tue, May 23, 2017 at 5:30 AM David Chisnall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 23 May 2017, at 13:18, Jajoo, Malhar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi everyone,
> >
> > I just had a question -
> >
> > I am aware that LLVM supports it's own form of RTTI ( using dyn_cast<>()
> ,etc) but
> > I wish to use C++ RTTI currently.
> >
> > I have tried building with  "cmake -G "Unix Makefiles"
> -LLVM_ENABLE_RTTI=ON"
> > but that doesnt seem to remove the "fno-rtti" flag from llvm-config
> > and I still get an error when I try using "dynamic_cast<>" from C++ RTTI.
>
> You missed the D.  All CMake definition flags need this.  Try:
>
>  -DLLVM_ENABLE_RTTI=ON
>
> Also, I’d recommend using Ninja not Unix Makefiles.
>
> Note that LLVM did work quite well with programs using RTTI without
> enabling RTTI for LLVM until someone broke IR/Instructions.h about a year
> ago to remove the out-of-line definitions and force the vtable for several
> instructions to be emitted in every compilation unit including this file
> (i.e. a lot of them).


FWIW that is inconsistent with the LLVM coding conventions (which ask that
every class with a vtable have a non-inline anchor function) so patches I
think would be welcome to fix any case where it comes up - though LLVM's
far from -Wweak-vtable clean (but if it were made clean, and then the
warning were enabled - it'd probably stay fairly clean, I think).


>   This means that any file using RTTI that includes this header tries to
> emit a vtable with type info that refers to superclass objects defined
> elsewhere.  Fixing this would make LLVM significantly more useful as a
> library.
>
> David
>
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