Alias Analysis - Overview

Overview

In general, alias analysis determines whether or not separate memory references point to the same area of memory. This allows the compiler to determine what variables in the program will be affected by a statement. For example, consider the following section of code that accesses members of structures:

There are three possible alias cases here:

1. The variables p and q cannot alias.
2. The variables p and q must alias.
3. It cannot be conclusively determined at compile time if p and q alias or not.

If p and q cannot alias, then i = p.foo + 3; can be changed to i = 4. If p and q must alias, then i = p.foo + 3; can be changed to i = 5. In both cases, we are able to perform optimizations from the alias knowledge. On the other hand, if it is not known if p and q alias or not, then no optimizations can be performed and the whole of the code must be executed to get the result. Two memory references are said to have a may-alias relation if their aliasing is unknown.