^ Exactly. What would you do then if the OS did not come with a compiler?
Besides, what if there was a updated version of the compiler available that improves the run time performance? What would you conclude then about the performance comparison between OSes? In other words, it's still exactly the same OS -- you touched nothing of the kernel, system commands or libraries -- and yet, after updating the compiler the benchmarks "magically" got better? How can such benchmarks tell us anything at all about the OS if the OS did not change?