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Add comment on cause of panic in dominators algorithm

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This commit is contained in:
Bryan Garza 2023-01-06 16:26:56 +00:00
parent a8c9528e06
commit f9982ea24a
1 changed files with 41 additions and 1 deletions
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42
compiler/rustc_data_structures/src/graph/dominators/mod.rs
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@ -135,7 +135,47 @@ pub fn dominators<G: ControlFlowGraph>(graph: G) -> Dominators<G::Node> {
// This loop computes the semi[w] for w.
semi[w] = w;
for v in graph.predecessors(pre_order_to_real[w]) {
// Reachable vertices may have unreachable predecessors, so ignore any of them
// TL;DR: Reachable vertices may have unreachable predecessors, so ignore any of them.
//
// Ignore blocks which are not connected to the entry block.
//
// The algorithm that was used to traverse the graph and build the
// `pre_order_to_real` and `real_to_pre_order` vectors does so by
// starting from the entry block and following the successors.
// Therefore, any blocks not reachable from the entry block will be
// set to `None` in the `pre_order_to_real` vector.
//
// For example, in this graph, A and B should be skipped:
//
// ┌─────┐
// │ │
// └──┬──┘
// │
// ┌──▼──┐ ┌─────┐
// │ │ │ A │
// └──┬──┘ └──┬──┘
// │ │
// ┌───────┴───────┐ │
// │ │ │
// ┌──▼──┐ ┌──▼──┐ ┌──▼──┐
// │ │ │ │ │ B │
// └──┬──┘ └──┬──┘ └──┬──┘
// │ └──────┬─────┘
// ┌──▼──┐ │
// │ │ │
// └──┬──┘ ┌──▼──┐
// │ │ │
// │ └─────┘
// ┌──▼──┐
// │ │
// └──┬──┘
// │
// ┌──▼──┐
// │ │
// └─────┘
//
// ...this may be the case if a MirPass modifies the CFG to remove
// or rearrange certain blocks/edges.
let Some(v) = real_to_pre_order[v] else {
continue
};