Well you are likely thinking of Raphael Espindola's tests of using Clang to build Firefox, and those tests showed that Clang/LLVM was better in some instances b

Well you are likely thinking of Raphael Espindola's tests of using Clang to build Firefox, and those tests showed that Clang/LLVM was better in some instances but overall GCC was faster. HOWEVER the GCC he tested against was GCC 4.2, a ~5 year old version of GCC which is the version OSX ships with (he was using OSX) so the tests as comparative between GCC and Clang/LLVM of today were pointless. It should also be noted that Firefox as used on people's machines today gets a large performance boost from profile-guided-optimization (remember back when people noticed that win32 Firefox under wine was faster than native Firefox on Linux? This was because of the Linux versions not being compiled with profile-guided-optimization at the time), and this optimization is not available for Clang/LLVM (though iirc there's a GSOC project for it this summer).