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Rollup merge of #132022 - Zalathar:dominator-order, r=tmiasko

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Move `cmp_in_dominator_order` out of graph dominator computation

Dominator-order information is only needed for coverage graphs, and is easy enough to collect by just traversing the graph again.

This avoids wasted work when computing graph dominators for any other purpose.
This commit is contained in:
Matthias Krüger 2024-10-22 15:28:50 +02:00 • committed by GitHub
commit f5aa456646
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2 changed files with 24 additions and 24 deletions
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25
compiler/rustc_mir_transform/src/coverage/graph.rs
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@ -21,6 +21,10 @@ pub(crate) struct CoverageGraph {
pub(crate) successors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
pub(crate) predecessors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
dominators: Option<Dominators<BasicCoverageBlock>>,
/// Allows nodes to be compared in some total order such that _if_
/// `a` dominates `b`, then `a < b`. If neither node dominates the other,
/// their relative order is consistent but arbitrary.
dominator_order_rank: IndexVec<BasicCoverageBlock, u32>,
}
impl CoverageGraph {
@ -54,10 +58,27 @@ impl CoverageGraph {
}
}
let mut this = Self { bcbs, bb_to_bcb, successors, predecessors, dominators: None };
let num_nodes = bcbs.len();
let mut this = Self {
bcbs,
bb_to_bcb,
successors,
predecessors,
dominators: None,
dominator_order_rank: IndexVec::from_elem_n(0, num_nodes),
};
assert_eq!(num_nodes, this.num_nodes());
this.dominators = Some(dominators::dominators(&this));
// The dominator rank of each node is just its index in a reverse-postorder traversal.
let reverse_post_order = graph::iterate::reverse_post_order(&this, this.start_node());
// The coverage graph is created by traversal, so all nodes are reachable.
assert_eq!(reverse_post_order.len(), this.num_nodes());
for (rank, bcb) in (0u32..).zip(reverse_post_order) {
this.dominator_order_rank[bcb] = rank;
}
// The coverage graph's entry-point node (bcb0) always starts with bb0,
// which never has predecessors. Any other blocks merged into bcb0 can't
// have multiple (coverage-relevant) predecessors, so bcb0 always has
@ -162,7 +183,7 @@ impl CoverageGraph {
a: BasicCoverageBlock,
b: BasicCoverageBlock,
) -> Ordering {
self.dominators.as_ref().unwrap().cmp_in_dominator_order(a, b)
self.dominator_order_rank[a].cmp(&self.dominator_order_rank[b])
}
/// Returns the source of this node's sole in-edge, if it has exactly one.