llvm dev - Feb 2010 - [LLVMdev] Disabling rtti on default build - could it be reverted/re-enabled?

We at Tampere University of technology are using LLVM for high-level 
optimizations in the compiler for our TTA processor toolkit(TCE)

We have a llvm backend which creates our own intermediate code, and our 
own instruction selector after that (as LLVM's machine code cannot 
represent the actual machine codes of our processors).

This makes our llvm backend quite different from the ordinary llvm 
backends, and we have to link lots of code to same binaries which also 
link to llvm libraries.

LLVM 2.7 is dropping rtti on default build, which is a problem to us.
We are using Boost libraries on many places in our code, and Boost has 
some heavy usage of type info tricks, which cannot be compiled without 
rtti, so we have to keep rtti on for our code.

But our code also implements the llvm backend framework classes, which 
are compiled without rtti , which causes problems; we cannot derive from 
the non-rtti enabled base classes with derived classes that are compiled 
with rtti on.

So could rtti be turned on for default builds of llvm so that we could 
continue using (non-customized) llvm libraries for our compiler?

Just having rtti on should not cause you any performance deficits, just 
causes some extra typeinfo methods to be created to your binaries, and 
these should be quite small so they should not make them much larger.

On Feb 17, 2010, at 3:15 AM, Heikki Kultala wrote:
> LLVM 2.7 is dropping rtti on default build, which is a problem to us.
> We are using Boost libraries on many places in our code, and Boost has 
> some heavy usage of type info tricks, which cannot be compiled without 
> rtti, so we have to keep rtti on for our code.
> 
> But our code also implements the llvm backend framework classes, which 
> are compiled without rtti , which causes problems; we cannot derive from 
> the non-rtti enabled base classes with derived classes that are compiled 
> with rtti on.
> 
> So could rtti be turned on for default builds of llvm so that we could 
> continue using (non-customized) llvm libraries for our compiler?
Hi Heikki,

You should be able to get this by doing "make REQUIRES_RTTI=1" when
building LLVM.

-Chris

On 02/17/2010 09:58 PM, Chris Lattner wrote:
> You should be able to get this by doing "make REQUIRES_RTTI=1"
when building LLVM.
What is the main reason RTTI is now disabled by default? We discussed
this briefly in IRC but could not reach a proper conclusion and I was
directed to ask here for the proper reason against it.

Having RTTI support in should not have execution speed penalties, only
using the typeinfo does, and binary size benefit is also very small.

Seems this has been discussed a while ago, but I'd like to ask to
reconsider:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2010-January/028970.html

It would be better to have the support on by default as having it on should
not break the majority of clients (RTTI on is the default for g++ at least) to
the LLVM lib. It now breaks for us, and I have a feeling we are not the only
ones. Especially now that the shared lib generation has been fixed, I predict
use cases like ours won't be all that uncommon (clients to libLLVM*.so that
use also libs like Boost that require RTTI).

Mainly this is about a nuisance to need to ask users of TCE to recompile
LLVM (which might be even installed from a precompiled binary for their 
distro) to be able to compile TCE.

It's not easy to get rid of RTTI requiring code for us as we use Boost
heavily in our derived MachineFunctionPasses.

-- 
Pekka