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coverage: Simplify choosing an out-edge to be given a counter expression

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By building the list of candidate edges up-front, and making the
candidate-selection method fallible, we can remove a few pieces of awkward
code.
This commit is contained in:
Zalathar 2024-09-13 16:34:49 +10:00
parent 771659d264
commit e24310b07c
1 changed files with 49 additions and 70 deletions
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119
compiler/rustc_mir_transform/src/coverage/counters.rs
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@ -321,25 +321,23 @@ fn make_node_counter_and_out_edge_counters(
if !self.basic_coverage_blocks[from_bcb].is_out_summable {
return;
}
// If this node doesn't have multiple out-edges, or all of its out-edges
// already have counters, then we don't need to create edge counters.
let needs_out_edge_counters = successors.len() > 1
&& successors.iter().any(|&to_bcb| self.edge_has_no_counter(from_bcb, to_bcb));
if !needs_out_edge_counters {
// Determine the set of out-edges that don't yet have edge counters.
let candidate_successors = self
.bcb_successors(from_bcb)
.iter()
.copied()
.filter(|&to_bcb| self.edge_has_no_counter(from_bcb, to_bcb))
.collect::<Vec<_>>();
debug!(?candidate_successors);
// If there are out-edges without counters, choose one to be given an expression
// (computed from this node and the other out-edges) instead of a physical counter.
let Some(expression_to_bcb) =
self.choose_out_edge_for_expression(traversal, &candidate_successors)
else {
return;
}
if tracing::enabled!(tracing::Level::DEBUG) {
let _span =
debug_span!("node has some out-edges without counters", ?from_bcb).entered();
for &to_bcb in successors {
debug!(?to_bcb, counter=?self.edge_counter(from_bcb, to_bcb));
}
}
// Of the out-edges that don't have counters yet, one can be given an expression
// (computed from the other out-edges) instead of a dedicated counter.
let expression_to_bcb = self.choose_out_edge_for_expression(traversal, from_bcb);
};
// For each out-edge other than the one that was chosen to get an expression,
// ensure that it has a counter (existing counter/expression or a new counter),
@ -351,10 +349,11 @@ fn make_node_counter_and_out_edge_counters(
.filter(|&to_bcb| to_bcb != expression_to_bcb)
.map(|to_bcb| self.get_or_make_edge_counter(from_bcb, to_bcb))
.collect::<Vec<_>>();
let sum_of_all_other_out_edges: BcbCounter = self
.coverage_counters
.make_sum(&other_out_edge_counters)
.expect("there must be at least one other out-edge");
let Some(sum_of_all_other_out_edges) =
self.coverage_counters.make_sum(&other_out_edge_counters)
else {
return;
};
// Now create an expression for the chosen edge, by taking the counter
// for its source node and subtracting the sum of its sibling out-edges.
@ -440,79 +439,59 @@ fn get_or_make_edge_counter(
self.coverage_counters.make_phys_edge_counter(from_bcb, to_bcb)
}
/// Choose one of the out-edges of `from_bcb` to receive an expression
/// instead of a physical counter, and returns that edge's target node.
///
/// - Precondition: The node must have at least one out-edge without a counter.
/// - Postcondition: The selected edge does not have an edge counter.
/// Given a set of candidate out-edges (represented by their successor node),
/// choose one to be given a counter expression instead of a physical counter.
fn choose_out_edge_for_expression(
&self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock,
) -> BasicCoverageBlock {
if let Some(reloop_target) = self.find_good_reloop_edge(traversal, from_bcb) {
assert!(self.edge_has_no_counter(from_bcb, reloop_target));
candidate_successors: &[BasicCoverageBlock],
) -> Option<BasicCoverageBlock> {
// Try to find a candidate that leads back to the top of a loop,
// because reloop edges tend to be executed more times than loop-exit edges.
if let Some(reloop_target) = self.find_good_reloop_edge(traversal, &candidate_successors) {
debug!("Selecting reloop target {reloop_target:?} to get an expression");
return reloop_target;
return Some(reloop_target);
}
// We couldn't identify a "good" edge, so just choose any edge that
// doesn't already have a counter.
let arbitrary_target = self
.bcb_successors(from_bcb)
.iter()
.copied()
.find(|&to_bcb| self.edge_has_no_counter(from_bcb, to_bcb))
.expect("precondition: at least one out-edge without a counter");
// We couldn't identify a "good" edge, so just choose an arbitrary one.
let arbitrary_target = candidate_successors.first().copied()?;
debug!(?arbitrary_target, "selecting arbitrary out-edge to get an expression");
arbitrary_target
Some(arbitrary_target)
}
/// Tries to find an edge that leads back to the top of a loop, and that
/// doesn't already have a counter. Such edges are good candidates to
/// be given an expression (instead of a physical counter), because they
/// will tend to be executed more times than a loop-exit edge.
/// Given a set of candidate out-edges (represented by their successor node),
/// tries to find one that leads back to the top of a loop.
///
/// Reloop edges are good candidates for counter expressions, because they
/// will tend to be executed more times than a loop-exit edge, so it's nice
/// for them to be able to avoid a physical counter increment.
fn find_good_reloop_edge(
&self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock,
candidate_successors: &[BasicCoverageBlock],
) -> Option<BasicCoverageBlock> {
let successors = self.bcb_successors(from_bcb);
// If there are no candidates, avoid iterating over the loop stack.
if candidate_successors.is_empty() {
return None;
}
// Consider each loop on the current traversal context stack, top-down.
for reloop_bcbs in traversal.reloop_bcbs_per_loop() {
let mut all_edges_exit_this_loop = true;
// Try to find an out-edge that doesn't exit this loop and doesn't
// already have a counter.
for &target_bcb in successors {
// Try to find a candidate edge that doesn't exit this loop.
for &target_bcb in candidate_successors {
// An edge is a reloop edge if its target dominates any BCB that has
// an edge back to the loop header. (Otherwise it's an exit edge.)
let is_reloop_edge = reloop_bcbs.iter().any(|&reloop_bcb| {
self.basic_coverage_blocks.dominates(target_bcb, reloop_bcb)
});
if is_reloop_edge {
all_edges_exit_this_loop = false;
if self.edge_has_no_counter(from_bcb, target_bcb) {
// We found a good out-edge to be given an expression.
return Some(target_bcb);
}
// Keep looking for another reloop edge without a counter.
} else {
// This edge exits the loop.
// We found a good out-edge to be given an expression.
return Some(target_bcb);
}
}
if !all_edges_exit_this_loop {
// We found one or more reloop edges, but all of them already
// have counters. Let the caller choose one of the other edges.
debug!("All reloop edges had counters; skipping the other loops");
return None;
}
// All of the out-edges exit this loop, so keep looking for a good
// reloop edge for one of the outer loops.
// All of the candidate edges exit this loop, so keep looking
// for a good reloop edge for one of the outer loops.
}
None