Today we continue to discuss the WRL research report on the Swift Java compiler. We were reviewing the class and method analysis section of the paper. We saw how the Swift compiler applies class-hierarchy analysis to resolve virtual calls. Swift also maintains a variety of information about methods in a hash table. It can be used to resolve method calls. Type propagation is useful for resolving some virtual method calls, The use of SSA form makes the makes the type propagation flow sensitive.
Exact types are determined in several ways.
First, exact types are known when an object or array is directly allocated.
Second, Swift can compute on demand whether a method returns an object with an exact type.
Third, Swift can do field analyses to determine if a load from a field  of an object always returns an object with an exact type.
Next we review Field Analysis. This is an expensive inter-procedural analysis. It involves reading and storing field attributes such as access modifiers so that they can be honored during the program execution.
As an example, if there is a field called points and it is marked private, the compiler only needs to scan the instance methods in class Plane to determine its properties.If  points is non-null, it must reference an array with base type Point and a fixed size of three. Swift uses exact type information from field analyses to help resolve method calls.Null checks can be eliminated for fields that are known to be non-null.Swift uses page protection to implement null-checks. While Swift uses page protection to implement null-checks without any extra code, eliminating null checks is still useful because it gives the compiler more flexibility in code motion. Lastly, a property of a field is computed on-demand only if required for a potential optimization.