Optimizations are enabled by the semantics of the language. If the language mandated that generated code must conform exactly to specific sequences of operation

Optimizations are enabled by the semantics of the language. If the language mandated that generated code must conform exactly to specific sequences of operations dictated by the source code, instead of merely respecting the observable behavior of the abstract machine, it would be impossible to optimize C code in the same way that it's impossible to optimize Assembly code.