Editing Optimizing compiler (section)

== Common themes ==
Optimization includes the following, sometimes conflicting themes.

;Optimize the common case: The common case may have unique properties that allow a ''[[fast path]]'' at the expense of a ''slow path''. If the fast path is taken more often, the result is better overall performance.
;Avoid redundancy: Reuse results that are already computed and store them for later use, instead of recomputing them.
;Less code: Remove unnecessary computations and intermediate values. Less work for the CPU, cache, and memory usually results in faster execution. Alternatively, in [[embedded systems]], less code brings a lower product cost.
;Fewer jumps by using ''straight line code'', also called ''[[branch-free code]]'': Less complicated code. Jumps (conditional or [[unconditional branch]]es) interfere with the prefetching of instructions, thus slowing down code. Using inlining or loop unrolling can reduce branching, at the cost of increasing [[binary file]] size by the length of the repeated code. This tends to merge several [[basic block]]s into one.
;Locality: Code and data that are accessed closely together in time should be placed close together in memory to increase spatial [[locality of reference]].
;Exploit the memory hierarchy: Accesses to memory are increasingly more expensive for each level of the [[memory hierarchy]], so place the most commonly used items in registers first, then caches, then main memory, before going to disk.
;Parallelize: Reorder operations to allow multiple computations to happen in parallel, either at the instruction, memory, or thread level.
;More precise information is better: The more precise the information the compiler has, the better it can employ any or all of these optimization techniques.
;Runtime metrics can help: Information gathered during a test run can be used in [[profile-guided optimization]]. Information gathered at runtime, ideally with minimal [[computational overhead|overhead]], can be used by a [[Just-in-time compilation|JIT]] compiler to dynamically improve optimization.
;Strength reduction: Replace complex, difficult, or expensive operations with simpler ones. For example, replacing division by a constant with multiplication by its reciprocal, or using [[induction variable analysis]] to replace multiplication by a loop index with addition.