GHC.Data.Graph.Collapse

A "supernode" stands for a collection of one or more nodes (basic blocks) that have been coalesced by the Hecht-Ullman algorithm. A collection in a supernode constitutes a reducible subgraph of a control-flow graph. (When an entire control-flow graph is collapsed to a single supernode, the flow graph is reducible.)

The idea of node splitting is to collapse a control-flow graph down to a single supernode, then materialize (`inflate') the reducible equivalent graph from that supernode. The Supernode class defines only the methods needed to collapse; rematerialization is the responsibility of the client.

During the Hecht-Ullman algorithm, every supernode has a unique entry point, which is given by superLabel. But this invariant is not guaranteed by the class methods and is not a law of the class. The mapLabels function rewrites all labels that appear in a supernode (both definitions and uses). The freshen function replaces every appearance of a defined label with a fresh label. (Appearances include both definitions and uses.)

Laws: superLabel (n <> n') == superLabel n blocks (n <> n') == blocks n union blocks n' mapLabels f (n <> n') = mapLabels f n <> mapLabels f n' mapLabels id == id mapLabels (f . g) == mapLabels f . mapLabels g

(We expect freshen to distribute over <>, but because of the fresh names involved, formulating a precise law is a bit challenging.)

Using the algorithm of Hecht and Ullman (1972), collapse a graph into a single node, splitting nodes as needed. Record visualization events in monad m.

The identity monad as a VizCollapseMonad. Use this monad when you want efficiency in graph collapse.

Module : GHC.Data.Graph.Collapse Description : Implement the "collapsing" algorithm Hecht and Ullman

A control-flow graph is reducible if and only if it is collapsible according to the definition of Hecht and Ullman (1972). This module implements the collapsing algorithm of Hecht and Ullman, and if it encounters a graph that is not collapsible, it splits nodes until the graph is fully collapsed. It then reports what nodes (if any) had to be split in order to collapse the graph. The information is used upstream to node-split Cmm graphs.

The module uses the inductive graph representation cloned from the Functional Graph Library (Hackage package fgl, modules GHC.Data.Graph.Inductive.*.)

If you want to visualize the graph-collapsing algorithm, create an instance of monad VizCollapseMonad. Each step in the algorithm is announced to the monad as a side effect. If you don't care about visualization, you would use the NullCollapseViz monad, in which these operations are no-ops.