Actually everything is quite simple. Config files for Phoronix test suites are using gcc by default and -O2 is provided by default in this config files. I think they just compiled these tests by gcc with the default config (which already includes -O2) while for clang they had to specify some custom options (like "-fopenmp -lgomp", though the second one is not needed), but forget to add any optimization options.
This wouldn’t be the first time that Phoronix has gotten sloppy with using a common set of optimization flags in their gcc vs clang benchmarks. Although, it is getting harder to make direct comparisons. Clang now autovectorizes at -O2 while, as you can see from the my prior posting of the assembly from-fverbose-asm -O2, FSF gcc 4.9.0 doesn’t.
Yes, this is true, but I don't think that it is a good idea to compare performance between code compiled with -O2 and compiled with -O0.