Computing the DAG of a program given source code or AST
I assume you're looking for ways to compute the Control Flow Graph of a given program.

This graph depends (obviously) on the actual operational semantics of the language you are interested in, and so cannot be computed from the AST alone, in the sense that some knowledge of the language semantics themselves are needed.

For a given language it is possible to build the CFG given the AST of the parsed program, and indeed this is usually a very important step in compilers to perform flow analysis. This can be found in chapter 17 of Appel or in chapter 7 of Muchnick.