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:
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. Alias classes are disjoint sets of locations that cannot alias to one another. For the discussion here, it is assumed that the optimizations done here occur on a low-level intermediate representation of the program. This is to say that the program has been compiled into binary operations, jumps, moves between registers, moves from registers to memory, moves from memory to registers, branches, and function calls/returns.
If the language being compiled is type safe, the compiler's type checker is correct, and the language lacks the ability to create pointers referencing local variables, (such as ML, Haskell, or Java) then some useful optimizations can be made.[1] There are many cases where we know that two memory locations must be in different alias classes:
When performing alias analysis for code, every load and store to memory needs to be labeled with its class. We then have the useful property, given memory locations
Ai
Bj
i,j
i=j
Ai
Bj
i ≠ j
Analysis based on flow, can be applied to programs in a language with references or type-casting. Flow based analysis can be used in lieu of or to supplement type based analysis. In flow based analysis, new alias classes are created for each memory allocation, and for every global and local variable whose address has been used. References may point to more than one value over time and thus may be in more than one alias class. This means that each memory location has a set of alias classes instead of a single alias class.