Thursday, 28 February 2013

A bit of fun on the side: Compiling with Clang

In the other bit of spare time I have considered looking at compiling FreeCAD using Clang. The end result is that only a few changes are needed to be made to the FreeCAD source so that it compiles with Clang.It doesn't seem to affect building using g++ either. I haven't measured the performance yet, but noticeably compiling takes less time.

I still need to collect these changes and post a patch for this and allow other people to test it out for themselves. I also have to find how I can debug FreeCAD after compiling with Clang - it appears to be incompatible with GNU's gdb.

Other Progress (slightly off-topic):

Workload seems to have increased once again at University. I spent a bit more time at the weekend on the Drawing Module: the majority of the work is looking at projecting faces onto the drawing plane. This is relatively similar to before but requires further work to sort edges into order, store these in a general format and then drawing using Qt's painter system. There's still work to do, but when I get that complete can hopefully share it.

What is Clang?

Clang is an alternative compiler that use LLVM to compile a variety of programming langaguesusing runtime optimisation methods. LLVM is used heavily in Mac OS X to improve graphics performance applications, but is also used in a handful of open source projects - in particular Open Source graphics drivers. LLVM provides the possibility to compile many different programming languages from Python, Fotran, OpenCL and importantly for this post c / c++.

Clang provides the infrastructure for compiling c++ and aims to offer a better alternative to gnu's gcc in terms of performance, better usability to developers and other advanced features such as static code checking. I won't say much more, but it seems a good alternative and therefore I decided to look at compiling FreeCAD using Clang.

What I did?

  1. Install Clang
  2. Change default compiler for make tool chain
  3. Change some of the check inside CMakeList.txt file
  4. (Ubuntu) Add a symlink for libgfortran for the linker to correctly work

Selecting the default compiler for your system (Ubuntu)

Most importantly install clang on your system. It seems that with Ubuntu installing clang introduces a an unwanted dependency of  gcc 4.6. Once installed, depending on your system you can inform Cmake explicitly to use clang or alternatively you can change the system wide default for make to use the clang compiler.The first option is obviously the most sensible, but I am not sure of the command.

The only help I have seen on setting the compiler is the following stackoverflow question:

In Ubuntu, it is straighforward to update the system wide compiler use for c++, simply run

sudo update-alternatives --config c++

This will bring up an interface to select the compiler you wish to use:

I need to post the patches to CMake, hopefully soon and these can be verified by other users. 

Following this, configure using cmake and then attempt to build using clang. You may notice that there is a linking error for lgfortran and then the build system fails. I think this is an issue with Ubuntu, but to solve this I had to create a symlink to gfortran library which isn't very convenient.

sudo ln -s /usr/lib/gcc/x86_64-linux-gnu/4.7/

Hopefully FreeCAD should happily compile very fast now at blinding speed!


I have spent some time last week to look at some further details.

Compiling using Clang per project:

There is an advanced option within CMake to specify what compiler to use for c and c++ per project build. Fortunately KDevelop makes this rather easy: Open Project Configuration by right clicking on the project to open the following dialog:

Click 'Show Advanced Values' and then set the following variable:

  • CMAKE_CXX_COMPILER  = /usr/bin/clang++ 
  • CMAKE_C_COMPILER to /usr/bin/clang

Compiler flags can be conveniently set in CMAKE_CXX_FLAGS, where for both gnu compiler and clang -g is for full debug.

Specifying Variables in the command line

It can also be done in the command line - I found really straightforward Guide for setting Variables in Cmake from (

Variables are set quite easily - simply prepend -D before the variable name and assign the value like so
cmake -DVARIABLE=value
Alternatively, you can specify the type of the variable below:
cmake -DVARIABLE:TYPE=value

Debugging with Clang:

Unfortunatly there is currently a bug that is preventing gdb being used with clang, which is apparently fixed in the latest release. I tried to get this too compile but I encountered errors. Hopefully with the next release of LLVM and Clang 3.3, the issue will be resolved. 


  1. With some of my projects I've noticed that Clang gives a lot more warnings than gcc. sometimes errors also.
    Sometimes the warnings just promote better coding-style, sometimes they are actual bugs or almost-bugs that are worth fixing.
    Did you try turning up the warning level and seeing what happens?

  2. Hi Andy,

    I haven't yet tried that. Clang has picked up on a few areas errors that gcc has missed, but I think by default the cmake options for gcc in FreeCAD are only for severe errors.

    Later I will look at these errors and static code analysis which is featured in clang. I have recently found that gdb can be used with clang, but a bug prevents this except in the development branch of clang.

    There are also some useful run-time checks that can be used: see

    Obviously this will be of little use to the majority of users, but may be helpful during development. Therefore a further investigation is needed