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7 Find components

This is a post-pass to ICR conversion that massages the flow graph into the shape subsequent phases expect. Things done: Compute the depth-first ordering for the flow graph. Find the components (disconnected parts) of the flow graph.

This pass need only be redone when newly converted code has been added to the flow graph. The reanalyze flag in the component structure should be set by people who mess things up.

We create the initial DFO using a variant of the basic algorithm. The initial DFO computation breaks the ICR up into components, which are parts that can be compiled independently. This is done to increase the efficiency of large block compilations. In addition to improving locality of reference and reducing the size of flow analysis problems, this allows back-end data structures to be reclaimed after the compilation of each component.

ICR optimization can change the connectivity of the flow graph by discovering new calls or eliminating dead code. Initial DFO determination splits up the flow graph into separate components, but does so conservatively, ensuring that parts that might become joined (due to local call conversion) are joined from the start. Initial DFO computation also guarantees that all code which shares a lexical environment is in the same component so that environment analysis needs to operate only on a single component at a time.

[This can get a bit hairy, since code seemingly reachable from the environment entry may be reachable from a NLX into that environment. Also, function references must be considered as links joining components even though the flow graph doesn’t represent these.]

After initial DFO determination, components are neither split nor joined. The standard DFO computation doesn’t attempt to split components that have been disconnected.