An alias occurs when different variables point directly or indirectly to a single area of storage. Aliasing refers to assumptions made during optimization about which variables can point to or occupy the same storage area. When an alias exists, or the potential for an alias occurs during the optimization process, pessimistic aliasing occurs. This can inhibit optimizations like dead store elimination and loop transformations on aliased variables. Also, pessimistic aliasing can generate additional loads and stores as the compiler must ensure that any changes to the variable that occur through the alias are not lost.
When aliasing occurs there is less opportunity for optimization transformations to occur on and around aliased variables than variables where no aliasing has taken place. For example, if variables A, B, and C are all aliased, any optimization must assume that a store into or a use of A is also a store or a use of B and C, even if that is not the case. Some of the highest optimization levels can improve alias analysis and remove some pessimistic aliases. However, in all cases, when it is not proven during an optimization transformation that an alias can be removed that alias must be left in place.
Where possible, avoid programming techniques that lead to pessimistic aliasing assumptions. These aliasing assumptions are the single most limiting factor to optimization transformations. The following situations can lead to pessimistic aliasing:
Some compiler options like -qalias can affect aliasing directly. For more information on how to tune the aliasing behavior in your application, see Options for providing application characteristic.