6.3 Scheduling

The lowest level of optimization is scheduling, in which the compiler determines the best ordering of individual instructions. Most CPUs allow one or more new instructions to start executing before others have finished. Many CPUs also support pipelining, where multiple instructions execute in parallel on the same CPU.

When scheduling is enabled, instructions must be arranged so that their results become available to later instructions at the right time, and to allow for maximum parallel execution. Scheduling improves the speed of an executable without increasing its size, but requires additional memory and time in the compilation process itself (due to its complexity).