llvm dev - May 2017 - Removing "fno-rtti" flag from llvm-config --cxxflags

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.


Is there any way to remove the "fno-rtti" flag from the llvm-config (
seen on running "llvm-config --cxxflags" ) ?


Thanks ,

Malhar

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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). 
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

Well, i removed the vtable from Value and Instruction recently, so that
shouldn't be a problem anymore.

On May 23, 2017 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).  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
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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