GCC is the open-source compiler that is used all over. Not every AMD EPYC system people are using AOCC2.0 on. Likewise, people do not only compile code on ICC that is used on Intel Xeons. Arm has focused efforts on getting optimizations in GCC because it is so popular.

Generally, that is why we prefer to publish "compiler optimized" as best-case performance as well as "GCC" as more of the least common denominator. Both sets of data points are important.

Official SPECint published numbers will not use GCC because the organizations that submit them always want to see the best performance. Ampere used a scaling factor off of published numbers.

If you want to see the impact, we have some numbers from my ThunderX2 review: https://www.servethehome.com/cavium-thunderx2-review-benchma...

You can see the impact clearly there even though that was from a few years ago. Cray has a better performing compiler for ThunderX2 but we did not get to use it due to licensing restrictions.

I hope that helps. The bigger need is for more data since this is one view of performance. There are other needs as well such as FP performance.