Here lies madness. What optimizations are not allowed exactly? Strict aliasing? Assuming dereferenced pointers are not null? Assuming all array accesses are always in bound? Assuming that integers don't wrap? Assuming that allocated memory is not accessed after a free? Reusing function frame slots of variables with non overlapping lifetimes? Not laying down local variables in the most obvious way? Caching and reordering non-volatile memory accesses? Accessing local variables of functions that have returned?

I'm sure you will say obviously not allowed to the first few and obviously allowed to the last few, but at some point all of these optimizations (and more) did break some code as some point.