compiling clang with rtti

Hi @llvm,

I've already asked the following questions to the clang dev list but got no response. Maybe there is a wider audience at llvmdev and someone here can help me. The question actually boils down to: How can I compile clang with rtti enabled?

And this was my original mail (with some points now better explained) to clang:

<--BEGIN-->
Hi @clang,

I'm somewhat puzzled about using rtti when building clang under gcc (gcc 4.3.3, linux/ubuntu).
(There is no problem under MSVC since rtti seems to be active there anyway).
The appropriate line 348 in llvm/makefile.rules is commented out meaning that llvm is usually compiled using rtti. However every clang lib adds -fno-rtti to CXXFLAGS by hand.
This is not filtered out by makefile.rules even if REQUIRES_RTTI is defined (IMHO -fno-rtti should be filtered out in that case).
Now I'm two-way bewildered.
First: why is line 348 commented out? This renders REQUIRES_RTTI useless. Does rtti (llvm) and non-rtti (clang) code work together at all? I had serious trouble when trying this with my own application and eventually resigned .

I'm forced to use rtti in my application as it uses some clang libs but also some tr1 stuff (which apparently needs rtti) and boost. This leads me to the second question:
How can I compile the clang libs with rtti enabled? Eventually I got it by editing every single Makefile in the "lib" sub-directories and commenting out the appropriate line "CXXFLAGS = -fno-rtti".

However this can't be a proper solution. As stated earlier if REQUIRES_RTTI would be evaluated similiar to ENABLE_EXPENSIVE_CHECKS regarding rtti then it would be easy to enable rtti for clang.
OTOH I'm not a makefile expert (to be honest I'm very far away from it). Thus maybe I'm just overlooking the right way to enable rtti for the clang libs.
<--END-->

Thanks in advance for any help...

Best regards
Olaf Krzikalla

Hi Olaf,

Hi @clang,

I'm somewhat puzzled about using rtti when building clang under gcc (gcc
4.3.3, linux/ubuntu).
(There is no problem under MSVC since rtti seems to be active there anyway).
The appropriate line 348 in llvm/makefile.rules is commented out meaning
that llvm is usually compiled using rtti. However every clang lib adds
-fno-rtti to CXXFLAGS by hand.
This is not filtered out by makefile.rules even if REQUIRES_RTTI is
defined (IMHO -fno-rtti should be filtered out in that case).
Now I'm two-way bewildered.
First: why is line 348 commented out? This renders REQUIRES_RTTI
useless. Does rtti (llvm) and non-rtti (clang) code work together at
all? I had serious trouble when trying this with my own application and
eventually resigned .

Dunno, but I think its fine to fix REQUIRES_RTTI, I'll do so right now.

I'm forced to use rtti in my application as it uses some clang libs but
also some tr1 stuff (which apparently needs rtti) and boost. This leads
me to the second question:

That doesn't necessarily explain why you are forced to use RTTI (just
as clang doesn't use RTTI but LLVM does, and only the LLVM parts build
with RTTI).

- Daniel

Daniel Dunbar schrieb:

Hi @clang,
First: why is line 348 commented out? This renders REQUIRES_RTTI
useless. Does rtti (llvm) and non-rtti (clang) code work together at
all? I had serious trouble when trying this with my own application and
eventually resigned .
    
Dunno, but I think its fine to fix REQUIRES_RTTI, I'll do so right now
  

Thank you, this should fix my actual problem.

I'm forced to use rtti in my application as it uses some clang libs but
also some tr1 stuff (which apparently needs rtti) and boost. This leads
me to the second question:
    
That doesn't necessarily explain why you are forced to use RTTI (just
as clang doesn't use RTTI but LLVM does, and only the LLVM parts build
with RTTI).
  

Digging deeper into this yields two issues.
First boost function (and gcc's tr1::function implementation) needs
rtti. Two months ago there was a discussion at the boost mailing list
about this requirement (Douglas Gregor was involved) and the conclusion
was that it needs to be working without rtti. However at the moment I
have to go with what we have.
Secondly gcc produces 'missing typeinfo' linker errors if I create a
object (of a class which was defined in a clang lib, e.g. AST) in my own
lib (where rtti is activated). The gcc strategy for creating these
typeinfo objects explains why there is no typeinfo object if the clang
lib is compiled without rtti. I'm not sure if the linker error is
necessary anyway but on the other hand mixing rtti and non-rtti this is
nothing covered by the standard. Thus I have to live with that too.
Eventually my only solution for both points was to turn on rtti for
clang libs.

Best regards
Olaf Krzikalla