> ignoring the fact that an array access is out of bounds and performing the array access This doesn't mean anything. The standard imposes no requirements that

> ignoring the fact that an array access is out of bounds and performing the array access This doesn't mean anything. The standard imposes no requirements that an array access is translated into a memory access by the compiler. And that's a good thing because it enables optimizations that can be critical to achieve decent performance. Assume for example that the compiler is able to prove (after inlining) that all elements of an array are equal to zero. This array is then used in a loop where in each iteration an index into this array is calculated and the corresponding array element is added to an accumulator. If the standard were to impose a rule that array accesses have to correspond to memory accesses, this code could not be optimized because one of the computed indices might lie outside of the array and the compiler is then forced to produce assembly that performs the memory access. With UB, however, the compiler is allowed to assume that all accesses to the array are valid and will be able to completely remove the (very cache-unfriendly) loop.