[LLVMdev] RFC: Timeline for deprecating the autoconf build system?

[Tom Stellard]

On Fri, Oct 31, 2014 at 04:31:47PM -0700, Bob Wilson wrote:
> 
> > On Oct 31, 2014, at 4:19 PM, Eric Christopher <echristo at gmail.com> wrote:
> > 
> > 
> > 
> > On Fri Oct 31 2014 at 3:11:22 PM Tom Stellard <tom at stellard.net <mailto:tom at stellard.net>> wrote:
> > Hi,
> > 
> > I would like to propose deprecating the autoconf build system at some
> > point in the future.  Maintaining two build systems is a hassle not
> > only for this project, but also for other projects that use LLVM
> > and have to deal with the slight differences in output between the two
> > build systems.
> > 
> > It seems like most people are using CMake at this point, so my questions
> > to the community are:
> > 
> > - Is there any technical reason why the remaining autoconf users can't switch
> >   to CMake?
> > 
> > 
> > I think Bob was the lead on keeping the autoconf system last year when this came up, there is a PR somewhere in the system about the blocking things that need to work in cmake to get it to happen. I don't know where we are on that list or what features people still need.
> 
> I’ve come around to the point of accepting the inevitability of moving to cmake, but I think there’s quite a bit of work to be done to get everything to work. The compiler-rt build in particular is problematic.
> 
> > 
> > Personally I still use the autoconf system, but am willing to remove it if we can get to a single system, but all of the requirements need to be handled first.
> > 
> > -eric
> >  
> > For example, I personally use automake, and the only reason I don't
> > use CMake is because it doesn't produce a single shared object
> > (e.g. libLLVM-3.6.0svn.so <http://libllvm-3.6.0svn.so/>).
> > 
> > - What is a reasonable timeframe to allow the remaining autoconf users
> >   a chance to migrate to CMake?
> 
> I don’t know how to answer that. Someone will need to do the work to first identify all the problems and then to get them fixed.
> 
> Converting everything to cmake will take quite a lot of work. In comparison, the minor hassle of keeping the makefiles working for a bit longer seems pretty insignificant.

The main problem is testing.  It doubles the testing load to have two
build systems, because we need to make sure both build systems work
on all platforms.

Even if we are able to test both build systems on all platforms, we
are still creating more work for projects depending on LLVM, because
they also need to do build testing with LLVM libraries from both build
systems, since their outputs are so different.

Would it be possible for you to attempt a CMake ToT build in your
configuration and come up with a list of CMake deficiencies?  It seems
like there may have been a few CMake improvements since you last tried,
and this may help us figure out what the scope of this change would be.

One other idea I had was that maybe we could start deprecating autoconf
one platform at a time, maybe start with Linux and then do others.  This
might make the transition a little smoother.

Thanks,
Tom