People, who know intermediate representations from text books, are often confused because Firm shows edges in the wrong direction. Instead of data flow, Firm provides data dependencies, which is usually more convenient in practice. So, "forward" means from the End to the Start node.
These are the most common edges. An operation is connected with a data dependency to its operands.
Reversed, these edges form (sort of) a control flow graph. However, these edges go to the operation (not the block), which is used to jump to the source block.
These edges are not control dependencies!
A reversed control flow edge, where the block of the target node dominates (or is) the source block.
Do not confuse back edges with reversed edges!
Memory dependencies represent a special form of data dependency, where the value is the memory. In contrast to other SSA values, memory cannot be duplicated. Intuitively, a store operation consumes the whole memory state and produces a new state.
If code is dead, but reachable, e.g. infinite loops, the keep edges are Firm’s hack to "keep" the code, because they provide a path from the End node.
Additional dependencies with no special semantics.
Sometimes it is necessary to walk a Firm graph in data flow order. So libFirm provides the following mechanisms.
Outs are the reversed form of the dependencies above. However, any change to the graph makes this edges inconsistent, but the corresponding flag is not set implicitly.
These are like outs, but are updated automatically. These automatic updates can be switched on and off (edges_activate, edges_deactivate). However, keep in mind that this means that dead code does not disappear implicitly, because it is still referenced by out edges.