If i wanted the compiler to generate code for `return 0;` I'd write it that way. If i wanted to use c as my macro assembler i very much expect the compiler to _not_ make this transformation.

If I wanted language with clever optimisations I'd go for something way higher level like lisp, haskell. Most optimisations are often only useful to make sure you can program generically without a headache. Optimizing code generated by macros or monomorphisation/type specialisation/template instantiation/however you call it is really useful. Is it worth it otherwise? For me the difference between gcc -O1 and -O3 suggest this is not worth it in the domain where c is typically used.

Optimizations don't really matter here. Optimization can't cause non-conforming code to be emitted (for a non-buggy compiler). A non-optimizing compiler could emit exactly the same code. If they don't, that's just inefficient code gen by the non-optimizing compiler.