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Chapter 5: Efficient C Programming > 5.6 Pointer Aliasing


Two pointers are said to alias when they point to the same address. If you write to one pointer, it will affect the value you read from the other pointer. In a function, the compiler often doesn’t know which pointers can alias and which pointers can’t. The compiler must be very pessimistic and assume that any write to a pointer may affect the value read from any other pointer, which can significantly reduce code efficiency.

Let’s start with a very simple example. The following function increments two timer values by a step amount:


Note that the compiler loads from step twice. Usually a compiler optimization called common subexpression elimination would kick in so that *step was only evaluated once, and the value reused for the second occurrence. However, the compiler can’t use this optimization here. The pointers timer1 and step might alias one another. In other words, the compiler cannot be sure that the write to timer1 doesn’t affect the read from step. In this case the second value of *step is different from the first and has the value *timer1. This forces the compiler to insert an extra load instruction.


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