Comment by O_nlogn

Neither, they found that 6.2% of the programs that SILO generated performed better on their benchmarks (based on a sum of the expected and actual execution latency of the assembly on some test data) than the gcc -O3 baseline (i.e. 93.8% of the programs SILO generated did not perform better, or were not correct). AFAIK they don't state how much better the super optimized programs were.