[cfe-dev] Clang builds Qt

[Tor Arne Vestbø]

Hey all,

I’m happy to report that as of Qt revision 15f52e00 and Clang revision 
116737 Qt builds with Clang on both Linux and Mac, including support for 
precompiled headers.

You’ll find the makespecs in Qt’s master branch [1]:

   - unsupported/macx-clang
   - unsupported/linux-clang

The mkspec is currently unsupported, but we've started running a few 
configurations with Clang in our internal CI system to catch build 
regressions.

Numbers-wise we’re seeing faster build times on Mac, 1.39x and 1.69x for 
debug and release respectively. On Linux debug builds are a bit slower, 
0.99x as fast, but release builds are 1.23x faster [2].

Binary size is also reduced on Mac, 13% for debug and  5% for release, 
but on Linux the binaries grow an average of 117% in debug, and 5% in 
release. Performance is still under investigation, but initial results 
seem to indicate a 5% slowdown on Mac and a 16% slowdown on Linux.

We’re seeing a handful of regressions in the unit-test suite, most 
notably QMutex::tryLock(), but these will be investigated further.

Thanks!

Tor Arne

[1] http://qt.gitorious.org/qt/qt/commits/master
[2] http://labs.qt.nokia.com/2010/10/29/compiling-qt-with-clang/