(I'll assume int is 32 bits, the hardware generates zero on shl overflow, and we don't care if a[31] is negative, since "everything working fine" wouldn't be tr

(I'll assume int is 32 bits, the hardware generates zero on shl overflow, and we don't care if a[31] is negative, since "everything working fine" wouldn't be true otherwise.) The compiler is perfectly capable of seeing 1<<index and reasoning: if index >= 32, then x is 0 (on this hardware), so the two branches of the conditional are the same, and we can just use the first one. On the other hand, if index < 32, then index < ARRAYSIZE (transitive property), the conditional always picks the first branch, and we can just use that one. By exhaustivity, we can just use the first branch: `int x = 1<<index; a[index] = x;`. Further optimization produces `a[index] = 1 << index`. Note that that did not make any reference to undefined behaviour.