GCC 9 Compiler Tuning Benchmarks On Intel Skylake AVX-512

In applications that are already hand-tuned and optimized for particular microarchitectures, there isn't much to gain out of the compiler flags.

Google's libvpx vpxenc encoder did get boosted by the Skylake-AVX512 targeting.

LTO did help the performance with Himeno.

Understandably, the more optimization passes that need to run, the longer the compilation process.