Hell, the internal complexities of scheduling/branch-prediction etc inside the processor may strictly be defined as "deterministic" but in reality cannot be fully kept in the head of a software engineer at this point, too.
It's been a long time since I've worked on a machine I could describe as "understandable."
Aside: One of the niceties of using agentic LLMs to do optimization work is letting them do the drudgery of creating a pile of microbenchmarks for different scenarios (scalar popcount on a this-shaped vector, or SIMD on another? which wins? sometimes the answer is surprising!). I've built all sorts of bespoke tools / harnesses for forcing them to evaluate their findings empirically, and it's amazing what can be done that would take me weeks by hand.
It's been a long time since I've worked on a machine I could describe as "understandable."
Aside: One of the niceties of using agentic LLMs to do optimization work is letting them do the drudgery of creating a pile of microbenchmarks for different scenarios (scalar popcount on a this-shaped vector, or SIMD on another? which wins? sometimes the answer is surprising!). I've built all sorts of bespoke tools / harnesses for forcing them to evaluate their findings empirically, and it's amazing what can be done that would take me weeks by hand.