Let me take a shift operation as an example. On architectures where it's no problem to "overshift" (e.g. the operation always results in 0), the compiler could

Let me take a shift operation as an example. On architectures where it's no problem to "overshift" (e.g. the operation always results in 0), the compiler could be just fine and leave shifts alone. Maybe a 0 is exactly what the user wanted! Or, the compiler could assume that the programmer made sure that overshifting never happens, since it's UB. Based on that assumption, it could make more aggressive (and possibly totally insignificant) optimizations by inferring other assumptions that don't align with the programmer's idea anymore. I never encountered such a sitation myself, but there are stories like that on the internet, where for example the Linux kernel was broken. Another scenario, the user might always do an operation that sometimes results in a random value (which is UB), but in the end only use the resulting value if it was not UB. The compiler instead could assume the operation can never happen in the first place. Again, there is a misalignment here in how the compiler reasoned and how the programmer reasoned (closer to the actual hardware). So, the difficult problem is probably defining the UB configurations as narrowly as possible. It might be a good idea to specify UB per architecture in some cases, but then it would lead to a lot of complexity. In any case a guideline for compilers should be to minimize surprises, which means not applying many optimizations around UB. That's easier said than done I guess.