Data flow analysis for Nim. We transform the AST into a linear list of instructions first to make this easier to handle: There are only 2 different branching instructions: 'goto X' is an unconditional goto, 'fork X' is a conditional goto (either the next instruction or 'X' can be taken). Exhaustive case statements are translated so that the last branch is transformed into an 'else' branch. return and break are all covered by 'goto'.

Control flow through exception handling: Contrary to popular belief, exception handling doesn't cause many problems for this DFA representation, raise is a statement that goes to the outer finally or except if there is one, otherwise it is the same as return. Every call is treated as a call that can potentially raise. However, without a surrounding try we don't emit these fork ReturnLabel instructions in order to speed up the dataflow analysis passes.

The data structures and algorithms used here are inspired by "A Graph–Free Approach to Data–Flow Analysis" by Markus Mohnen. https://link.springer.com/content/pdf/10.1007/3-540-45937-5_6.pdf