Grep the code for rand_reset. The code uses a fixed seed, fixed sizes for all programs, stable nop baseline, etc. also the pointer indirection blocks compiler optimizations.
I guess that you are referring to other projects because the benchmarks in this repo use a stable seed, turbo disabled, physical machine, both random and stable sizes, etc.
Danny, you mentioned loop and SLP vectorization. One thing that bothered me while watching the development of the SLP and Loop vectorizers over the last two years was that developers who cared about SPEC added functionality that increased compile time. I remember one change that added a second phase that scans the entire IR in the SLP vectorizer to get a 1.4% win in one of the SPEC programs. I hoped the FTL project would be able to enable the vectorizers, but to my disappointment the vectorizers are too slow to justify the boost on javascript workloads.