[djray]

They aren't equal, as the faster version does an unconditional memory write instead of only writing to the array if the condition is satisfied. The compiler is strictly forbidden from turning a conditional write into an unconditional one.

[peterfirefly]

The Linux kernel had problems years ago when gcc started to do exactly that in certain cases (because it screwed things up with task switches, interrupts, and SMP). It fairly quickly afterwards either stopped doing it entirely or got a switch that would stop it from doing it. Don't remember which.