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

Chris Lattner wrote:
>> 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.
> 
> I'm dubious that doing that would actually work anyway.  Since LLVM
doesn't
> provide a stable API, being able to reuse a previously installed version of
> llvm is not very likely to work.
Yes, we as an external project using LLVM as a library are painfully
familiar with the unstable API by now :)

Anyhow, people might have LLVM (a supported version) installed (compiled
with the default compilation flags or installed from a binary package
of their distro) for other reasons than compiling TCE.

 From now on these users need to recompile LLVM from sources with non-default
flags to make TCE (or other external projects using both LLVM and RTTI) to
be able to link with it. I think this should matter something in comparison to
a 5% disk size reduction which can be still achieved with disabling RTTI
when compiling LLVM for low disk space devices or during LLVM development
that requires frequent recompiles.

As far as I understood, enabling RTTI by default does not break any
clients, but disabling does. So I guess it's a matter how much value
the LLVM project gives to external projects using LLVM as a library.

How much is the compile time speedup BTW, has it been measured?

-- 
--Pekka

From now on these users need to recompile LLVM from sources with 
non-default
> flags to make TCE (or other external projects using both LLVM and RTTI) to
> be able to link with it. I think this should matter something in comparison
to
> a 5% disk size reduction which can be still achieved with disabling RTTI
> when compiling LLVM for low disk space devices or during LLVM development
> that requires frequent recompiles.
>    
I think Chris means 5% runtime speed, which matters

regards,

On Thu, Feb 18, 2010 at 3:42 PM, Pekka Jääskeläinen
<pekka.jaaskelainen at tut.fi> wrote:
>  From now on these users need to recompile LLVM from sources with
non-default
> flags to make TCE (or other external projects using both LLVM and RTTI) to
> be able to link with it. I think this should matter something in comparison
to
> a 5% disk size reduction which can be still achieved with disabling RTTI
> when compiling LLVM for low disk space devices or during LLVM development
> that requires frequent recompiles.
IMO, if you wish to preserve the ability to ever turn off RTTI, you
must turn it off by default for the developers' builds so that any
code which might depend on RTTI breaks immediately. Otherwise, it
seems only a matter of time before dependencies slowly creep back into
the code base. This has certainly proven true with GCC warnings. =/ I
think Nick has it right: developers should build with the most
restrictive settings expected to work, packagers should build with the
most forgiving settings they can tolerate.

On 02/19/2010 12:54 PM, Chandler Carruth wrote:
> IMO, if you wish to preserve the ability to ever turn off RTTI, you
> must turn it off by default for the developers' builds so that any
> code which might depend on RTTI breaks immediately. Otherwise, it
> seems only a matter of time before dependencies slowly creep back into
> the code base. This has certainly proven true with GCC warnings. =/ I
> think Nick has it right: developers should build with the most
> restrictive settings expected to work, packagers should build with the
> most forgiving settings they can tolerate.
Yes, I see the point and benefits in having RTTI disabled by default
during LLVM development.

Releases are a different story. That is, what should the distributed
tar balls configure with by default. IMHO end users should get RTTI
enabled by default as it is less likely to break stuff for them later.

I mean, one doesn't usually think things like this when one
installs a piece of software from sources: "Maybe I should disable
this default flag because maybe a software I'm going to try
in 3 weeks breaks unless I do it".

As far as I know, RTTI enabled should not increase execution time in
case RTTI features (typeinfo, dynamic_cast) are not used. Please
correct me if I'm wrong.

-- 
Pekka