> Second, you can define "trivially compiled C" for every platform as a set of substitution rules that transform every C operation into machine instructions wit

> Second, you can define "trivially compiled C" for every platform as a set of substitution rules that transform every C operation into machine instructions without any optimization. Such a trivial compilation would have every variable write correspond to a memory write, keep no variables in registers, always compile the same expression to the exact same machine code (except for memory addresses of variables involved), never inline functions etc. But it would still be allowed to e.g. put 32-bit integers in 64-bit registers on the basis that int operations are assumed not to overflow. The committee should define that for every supported platform as a reference C compiler, then require that C compilation produce machine code with observable side-effects for single-threaded execution equivalent to the reference trivially compiled C. Such an approach would preclude putting any variable in a register if there is any not-provably-valid pointer store anywhere in the program. Feel free to try, but I guarantee you that for most non trivial optimization it's going to be easy to find a piece of code that is UB today and where the optimization is observably different from the reference compilation.