If you write something like

    int a;
    ...
    int b = a
    if (a + 1 > b){
       foo();
    }

Then compilers will use the fact that integer overflow is undefined behavior. This allows the compiler to assume the `if` statement will always be true. And hence it removes this check.

This isn't just the compiler removing a check the programmer wanted. This could occur because of constants, where other constants make the check meaningful. It could also occur after in-lining a specific function call.

Really, the only case where this optimization would be bad is if the author was trying to detect overflow. Sadly, detecting if something would cause undefined behavior in C code is hard. Because you cannot try the thing.