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coverage: Avoid referring to out-edges as "branches"

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This makes the graph terminology a bit more consistent, and avoids potential
confusion with branch coverage.
This commit is contained in:
Zalathar 2024-09-10 20:44:15 +10:00
parent 96d545a33b
commit 8be70c7b2c
1 changed files with 80 additions and 97 deletions
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177
compiler/rustc_mir_transform/src/coverage/counters.rs
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@ -253,12 +253,6 @@ fn new(
Self { coverage_counters, basic_coverage_blocks } Self { coverage_counters, basic_coverage_blocks }
} }
/// If two `BasicCoverageBlock`s branch from another `BasicCoverageBlock`, one of the branches
/// can be counted by `Expression` by subtracting the other branch from the branching
/// block. Otherwise, the `BasicCoverageBlock` executed the least should have the `Counter`.
/// One way to predict which branch executes the least is by considering loops. A loop is exited
/// at a branch, so the branch that jumps to a `BasicCoverageBlock` outside the loop is almost
/// always executed less than the branch that does not exit the loop.
fn make_bcb_counters(&mut self, bcb_needs_counter: impl Fn(BasicCoverageBlock) -> bool) { fn make_bcb_counters(&mut self, bcb_needs_counter: impl Fn(BasicCoverageBlock) -> bool) {
debug!("make_bcb_counters(): adding a counter or expression to each BasicCoverageBlock"); debug!("make_bcb_counters(): adding a counter or expression to each BasicCoverageBlock");
@ -273,7 +267,7 @@ fn make_bcb_counters(&mut self, bcb_needs_counter: impl Fn(BasicCoverageBlock) -
while let Some(bcb) = traversal.next() { while let Some(bcb) = traversal.next() {
let _span = debug_span!("traversal", ?bcb).entered(); let _span = debug_span!("traversal", ?bcb).entered();
if bcb_needs_counter(bcb) { if bcb_needs_counter(bcb) {
self.make_node_and_branch_counters(&traversal, bcb); self.make_node_counter_and_out_edge_counters(&traversal, bcb);
} }
} }
@ -284,74 +278,66 @@ fn make_bcb_counters(&mut self, bcb_needs_counter: impl Fn(BasicCoverageBlock) -
); );
} }
/// Make sure the given node has a node counter, and then make sure each of
/// its out-edges has an edge counter (if appropriate).
#[instrument(level = "debug", skip(self, traversal))] #[instrument(level = "debug", skip(self, traversal))]
fn make_node_and_branch_counters( fn make_node_counter_and_out_edge_counters(
&mut self, &mut self,
traversal: &TraverseCoverageGraphWithLoops<'_>, traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock, from_bcb: BasicCoverageBlock,
) { ) {
// First, ensure that this node has a counter of some kind. // First, ensure that this node has a counter of some kind.
// We might also use its term later to compute one of the branch counters. // We might also use that counter to compute one of the out-edge counters.
let from_bcb_operand = self.get_or_make_counter_operand(from_bcb); let from_bcb_operand = self.get_or_make_counter_operand(from_bcb);
let branch_target_bcbs = self.basic_coverage_blocks.successors[from_bcb].as_slice(); let successors = self.basic_coverage_blocks.successors[from_bcb].as_slice();
// If this node doesn't have multiple out-edges, or all of its out-edges // 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. // already have counters, then we don't need to create edge counters.
let needs_branch_counters = branch_target_bcbs.len() > 1 let needs_out_edge_counters = successors.len() > 1
&& branch_target_bcbs && successors.iter().any(|&to_bcb| self.edge_has_no_counter(from_bcb, to_bcb));
.iter() if !needs_out_edge_counters {
.any(|&to_bcb| self.branch_has_no_counter(from_bcb, to_bcb));
if !needs_branch_counters {
return; return;
} }
debug!( if tracing::enabled!(tracing::Level::DEBUG) {
"{from_bcb:?} has some branch(es) without counters:\n {}", let _span =
branch_target_bcbs debug_span!("node has some out-edges without counters", ?from_bcb).entered();
.iter() for &to_bcb in successors {
.map(|&to_bcb| { debug!(?to_bcb, counter=?self.edge_counter(from_bcb, to_bcb));
format!("{from_bcb:?}->{to_bcb:?}: {:?}", self.branch_counter(from_bcb, to_bcb)) }
}) }
.collect::<Vec<_>>()
.join("\n "),
);
// Of the branch edges that don't have counters yet, one can be given an expression // Of the out-edges that don't have counters yet, one can be given an expression
// (computed from the other edges) instead of a dedicated counter. // (computed from the other out-edges) instead of a dedicated counter.
let expression_to_bcb = self.choose_preferred_expression_branch(traversal, from_bcb); let expression_to_bcb = self.choose_out_edge_for_expression(traversal, from_bcb);
// For each branch arm other than the one that was chosen to get an expression, // 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), // ensure that it has a counter (existing counter/expression or a new counter),
// and accumulate the corresponding terms into a single sum term. // and accumulate the corresponding counters into a single sum expression.
let sum_of_all_other_branches: BcbCounter = { let sum_of_all_other_out_edges: BcbCounter = {
let _span = debug_span!("sum_of_all_other_branches", ?expression_to_bcb).entered(); let _span = debug_span!("sum_of_all_other_out_edges", ?expression_to_bcb).entered();
branch_target_bcbs successors
.iter() .iter()
.copied() .copied()
// Skip the chosen branch, since we'll calculate it from the other branches. // Skip the chosen edge, since we'll calculate its count from this sum.
.filter(|&to_bcb| to_bcb != expression_to_bcb) .filter(|&to_bcb| to_bcb != expression_to_bcb)
.fold(None, |accum, to_bcb| { .fold(None, |accum, to_bcb| {
let _span = debug_span!("to_bcb", ?accum, ?to_bcb).entered(); let _span = debug_span!("to_bcb", ?accum, ?to_bcb).entered();
let branch_counter = self.get_or_make_edge_counter_operand(from_bcb, to_bcb); let edge_counter = self.get_or_make_edge_counter_operand(from_bcb, to_bcb);
Some(self.coverage_counters.make_sum_expression(accum, branch_counter)) Some(self.coverage_counters.make_sum_expression(accum, edge_counter))
}) })
.expect("there must be at least one other branch") .expect("there must be at least one other out-edge")
}; };
// For the branch that was chosen to get an expression, create that expression // Now create an expression for the chosen edge, by taking the counter
// by taking the count of the node we're branching from, and subtracting the // for its source node and subtracting the sum of its sibling out-edges.
// sum of all the other branches.
debug!(
"Making an expression for the selected expression_branch: \
{expression_to_bcb:?} (expression_branch predecessors: {:?})",
self.bcb_predecessors(expression_to_bcb),
);
let expression = self.coverage_counters.make_expression( let expression = self.coverage_counters.make_expression(
from_bcb_operand, from_bcb_operand,
Op::Subtract, Op::Subtract,
sum_of_all_other_branches, sum_of_all_other_out_edges,
); );
debug!("{expression_to_bcb:?} gets an expression: {expression:?}"); debug!("{expression_to_bcb:?} gets an expression: {expression:?}");
if self.basic_coverage_blocks.bcb_has_multiple_in_edges(expression_to_bcb) { if self.basic_coverage_blocks.bcb_has_multiple_in_edges(expression_to_bcb) {
self.coverage_counters.set_bcb_edge_counter(from_bcb, expression_to_bcb, expression); self.coverage_counters.set_bcb_edge_counter(from_bcb, expression_to_bcb, expression);
@ -442,82 +428,79 @@ fn get_or_make_edge_counter_operand(
self.coverage_counters.set_bcb_edge_counter(from_bcb, to_bcb, counter_kind) self.coverage_counters.set_bcb_edge_counter(from_bcb, to_bcb, counter_kind)
} }
/// Select a branch for the expression, either the recommended `reloop_branch`, or if none was /// Choose one of the out-edges of `from_bcb` to receive an expression
/// found, select any branch. /// instead of a physical counter, and returns that edge's target node.
fn choose_preferred_expression_branch( ///
/// - Precondition: The node must have at least one out-edge without a counter.
/// - Postcondition: The selected edge does not have an edge counter.
fn choose_out_edge_for_expression(
&self, &self,
traversal: &TraverseCoverageGraphWithLoops<'_>, traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock, from_bcb: BasicCoverageBlock,
) -> BasicCoverageBlock { ) -> BasicCoverageBlock {
let good_reloop_branch = self.find_good_reloop_branch(traversal, from_bcb); if let Some(reloop_target) = self.find_good_reloop_edge(traversal, from_bcb) {
if let Some(reloop_target) = good_reloop_branch { assert!(self.edge_has_no_counter(from_bcb, reloop_target));
assert!(self.branch_has_no_counter(from_bcb, reloop_target));
debug!("Selecting reloop target {reloop_target:?} to get an expression"); debug!("Selecting reloop target {reloop_target:?} to get an expression");
reloop_target return reloop_target;
} else {
let &branch_without_counter = self
.bcb_successors(from_bcb)
.iter()
.find(|&&to_bcb| self.branch_has_no_counter(from_bcb, to_bcb))
.expect(
"needs_branch_counters was `true` so there should be at least one \
branch",
);
debug!(
"Selecting any branch={:?} that still needs a counter, to get the \
`Expression` because there was no `reloop_branch`, or it already had a \
counter",
branch_without_counter
);
branch_without_counter
} }
// 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");
debug!(?arbitrary_target, "selecting arbitrary out-edge to get an expression");
arbitrary_target
} }
/// Tries to find a branch that leads back to the top of a loop, and that /// Tries to find an edge that leads back to the top of a loop, and that
/// doesn't already have a counter. Such branches are good candidates to /// doesn't already have a counter. Such edges are good candidates to
/// be given an expression (instead of a physical counter), because they /// be given an expression (instead of a physical counter), because they
/// will tend to be executed more times than a loop-exit branch. /// will tend to be executed more times than a loop-exit edge.
fn find_good_reloop_branch( fn find_good_reloop_edge(
&self, &self,
traversal: &TraverseCoverageGraphWithLoops<'_>, traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock, from_bcb: BasicCoverageBlock,
) -> Option<BasicCoverageBlock> { ) -> Option<BasicCoverageBlock> {
let branch_target_bcbs = self.bcb_successors(from_bcb); let successors = self.bcb_successors(from_bcb);
// Consider each loop on the current traversal context stack, top-down. // Consider each loop on the current traversal context stack, top-down.
for reloop_bcbs in traversal.reloop_bcbs_per_loop() { for reloop_bcbs in traversal.reloop_bcbs_per_loop() {
let mut all_branches_exit_this_loop = true; let mut all_edges_exit_this_loop = true;
// Try to find a branch that doesn't exit this loop and doesn't // Try to find an out-edge that doesn't exit this loop and doesn't
// already have a counter. // already have a counter.
for &branch_target_bcb in branch_target_bcbs { for &target_bcb in successors {
// A branch is a reloop branch if it dominates any BCB that has // An edge is a reloop edge if its target dominates any BCB that has
// an edge back to the loop header. (Other branches are exits.) // an edge back to the loop header. (Otherwise it's an exit edge.)
let is_reloop_branch = reloop_bcbs.iter().any(|&reloop_bcb| { let is_reloop_edge = reloop_bcbs.iter().any(|&reloop_bcb| {
self.basic_coverage_blocks.dominates(branch_target_bcb, reloop_bcb) self.basic_coverage_blocks.dominates(target_bcb, reloop_bcb)
}); });
if is_reloop_branch { if is_reloop_edge {
all_branches_exit_this_loop = false; all_edges_exit_this_loop = false;
if self.branch_has_no_counter(from_bcb, branch_target_bcb) { if self.edge_has_no_counter(from_bcb, target_bcb) {
// We found a good branch to be given an expression. // We found a good out-edge to be given an expression.
return Some(branch_target_bcb); return Some(target_bcb);
} }
// Keep looking for another reloop branch without a counter. // Keep looking for another reloop edge without a counter.
} else { } else {
// This branch exits the loop. // This edge exits the loop.
} }
} }
if !all_branches_exit_this_loop { if !all_edges_exit_this_loop {
// We found one or more reloop branches, but all of them already // We found one or more reloop edges, but all of them already
// have counters. Let the caller choose one of the exit branches. // have counters. Let the caller choose one of the other edges.
debug!("All reloop branches had counters; skip checking the other loops"); debug!("All reloop edges had counters; skipping the other loops");
return None; return None;
} }
// All of the branches exit this loop, so keep looking for a good // All of the out-edges exit this loop, so keep looking for a good
// reloop branch for one of the outer loops. // reloop edge for one of the outer loops.
} }
None None
@ -534,15 +517,15 @@ fn bcb_successors(&self, bcb: BasicCoverageBlock) -> &[BasicCoverageBlock] {
} }
#[inline] #[inline]
fn branch_has_no_counter( fn edge_has_no_counter(
&self, &self,
from_bcb: BasicCoverageBlock, from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock, to_bcb: BasicCoverageBlock,
) -> bool { ) -> bool {
self.branch_counter(from_bcb, to_bcb).is_none() self.edge_counter(from_bcb, to_bcb).is_none()
} }
fn branch_counter( fn edge_counter(
&self, &self,
from_bcb: BasicCoverageBlock, from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock, to_bcb: BasicCoverageBlock,