Well, the author also wrote: > Moreover GCC -O2 defaults are (in my opinion unfortunately) still not enabling vectorization and unrolling which may have notice

Well, the author also wrote: > Moreover GCC -O2 defaults are (in my opinion unfortunately) still not enabling vectorization and unrolling which may have noticeable effects on benchmarks. ... which makes it sound like Clang does a better job at this with Firefox code.