volatile int blackhole;
blackhole = 1;
const int X = blackhole;
const int Y = blackhole;

Compiler is forbidden to assume that X == 1 would be true. It's also forbidden to assume that X == Y. const just means the address and/or the data at the address is read only. const volatile int* const hwreg; -> "read only volatile value at read only address hwreg". Compiler can assume the hwreg address won't magically change, but can't assume the value read from that address won't.