Alias analysis

Alias analysis is a technique in compiler theory, used to determine if a storage location may be accessed in more than one way. Two pointers are said to be aliased if they point to the same location. Alias analysis techniques are usually classified by flow-sensitivity and context-sensitivity. They may determine may-alias or must-alias information. The term alias analysis is often used interchangeably with points-to analysis, a specific case. Alias analysers intend to make and compute useful information for understanding aliasing in programs. 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: p.foo = 1; q.foo = 2; i = p.foo + 3; There are three possible alias cases here: The variables p and q cannot alias (i.e., they never point to the same memory location). The variables p and q must alias (i.e., they always point to the same memory location). 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 because p.foo + 3 = q.foo + 3. In both cases, we are able to perform optimizations from the alias knowledge (assuming that no other thread updating the same locations can interleave with the current thread, or that the language memory model permits those updates to be not immediately visible to the current thread in absence of explicit synchronization constructs). 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. In alias analysis, we divide the program's memory into alias classes.