From 96d545a33bb6dc4ed3c1c80b997cce2035c571a6 Mon Sep 17 00:00:00 2001 From: Zalathar Date: Tue, 10 Sep 2024 20:16:43 +1000 Subject: [PATCH 1/3] coverage: Avoid referring to "coverage spans" in counter creation The counter-creation code needs to know which BCB nodes require counters, but isn't interested in why, so treat that as an opaque detail. --- .../src/coverage/counters.rs | 43 +++++++------------ 1 file changed, 15 insertions(+), 28 deletions(-) diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs index ea1c0d2df4517..c9eceff9b507b 100644 --- a/compiler/rustc_mir_transform/src/coverage/counters.rs +++ b/compiler/rustc_mir_transform/src/coverage/counters.rs @@ -74,12 +74,11 @@ pub(super) struct CoverageCounters { } impl CoverageCounters { - /// Makes [`BcbCounter`] `Counter`s and `Expressions` for the `BasicCoverageBlock`s directly or - /// indirectly associated with coverage spans, and accumulates additional `Expression`s - /// representing intermediate values. + /// Ensures that each BCB node needing a counter has one, by creating physical + /// counters or counter expressions for nodes and edges as required. pub(super) fn make_bcb_counters( basic_coverage_blocks: &CoverageGraph, - bcb_has_coverage_spans: impl Fn(BasicCoverageBlock) -> bool, + bcb_needs_counter: impl Fn(BasicCoverageBlock) -> bool, ) -> Self { let num_bcbs = basic_coverage_blocks.num_nodes(); @@ -91,8 +90,7 @@ impl CoverageCounters { expressions_memo: FxHashMap::default(), }; - MakeBcbCounters::new(&mut this, basic_coverage_blocks) - .make_bcb_counters(bcb_has_coverage_spans); + MakeBcbCounters::new(&mut this, basic_coverage_blocks).make_bcb_counters(bcb_needs_counter); this } @@ -241,10 +239,7 @@ impl CoverageCounters { } } -/// Traverse the `CoverageGraph` and add either a `Counter` or `Expression` to every BCB, to be -/// injected with coverage spans. `Expressions` have no runtime overhead, so if a viable expression -/// (adding or subtracting two other counters or expressions) can compute the same result as an -/// embedded counter, an `Expression` should be used. +/// Helper struct that allows counter creation to inspect the BCB graph. struct MakeBcbCounters<'a> { coverage_counters: &'a mut CoverageCounters, basic_coverage_blocks: &'a CoverageGraph, @@ -264,30 +259,21 @@ impl<'a> MakeBcbCounters<'a> { /// 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_has_coverage_spans: 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"); - // Walk the `CoverageGraph`. For each `BasicCoverageBlock` node with an associated - // coverage span, add a counter. If the `BasicCoverageBlock` branches, add a counter or - // expression to each branch `BasicCoverageBlock` (if the branch BCB has only one incoming - // edge) or edge from the branching BCB to the branch BCB (if the branch BCB has multiple - // incoming edges). + // Traverse the coverage graph, ensuring that every node that needs a + // coverage counter has one. // - // The `TraverseCoverageGraphWithLoops` traversal ensures that, when a loop is encountered, - // all `BasicCoverageBlock` nodes in the loop are visited before visiting any node outside - // the loop. The `traversal` state includes a `context_stack`, providing a way to know if - // the current BCB is in one or more nested loops or not. + // The traversal tries to ensure that, when a loop is encountered, all + // nodes within the loop are visited before visiting any nodes outside + // the loop. It also keeps track of which loop(s) the traversal is + // currently inside. let mut traversal = TraverseCoverageGraphWithLoops::new(self.basic_coverage_blocks); while let Some(bcb) = traversal.next() { - if bcb_has_coverage_spans(bcb) { - debug!("{:?} has at least one coverage span. Get or make its counter", bcb); + let _span = debug_span!("traversal", ?bcb).entered(); + if bcb_needs_counter(bcb) { self.make_node_and_branch_counters(&traversal, bcb); - } else { - debug!( - "{:?} does not have any coverage spans. A counter will only be added if \ - and when a covered BCB has an expression dependency.", - bcb, - ); } } @@ -298,6 +284,7 @@ impl<'a> MakeBcbCounters<'a> { ); } + #[instrument(level = "debug", skip(self, traversal))] fn make_node_and_branch_counters( &mut self, traversal: &TraverseCoverageGraphWithLoops<'_>, From 8be70c7b2c88be04e07adcf0c2dafbd0e8a6d70f Mon Sep 17 00:00:00 2001 From: Zalathar Date: Tue, 10 Sep 2024 20:44:15 +1000 Subject: [PATCH 2/3] coverage: Avoid referring to out-edges as "branches" This makes the graph terminology a bit more consistent, and avoids potential confusion with branch coverage. --- .../src/coverage/counters.rs | 177 ++++++++---------- 1 file changed, 80 insertions(+), 97 deletions(-) diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs index c9eceff9b507b..f55d6cf8efc71 100644 --- a/compiler/rustc_mir_transform/src/coverage/counters.rs +++ b/compiler/rustc_mir_transform/src/coverage/counters.rs @@ -253,12 +253,6 @@ impl<'a> MakeBcbCounters<'a> { 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) { debug!("make_bcb_counters(): adding a counter or expression to each BasicCoverageBlock"); @@ -273,7 +267,7 @@ impl<'a> MakeBcbCounters<'a> { while let Some(bcb) = traversal.next() { let _span = debug_span!("traversal", ?bcb).entered(); 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 @@ impl<'a> MakeBcbCounters<'a> { ); } + /// 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))] - fn make_node_and_branch_counters( + fn make_node_counter_and_out_edge_counters( &mut self, traversal: &TraverseCoverageGraphWithLoops<'_>, from_bcb: BasicCoverageBlock, ) { // 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 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 // already have counters, then we don't need to create edge counters. - let needs_branch_counters = branch_target_bcbs.len() > 1 - && branch_target_bcbs - .iter() - .any(|&to_bcb| self.branch_has_no_counter(from_bcb, to_bcb)); - if !needs_branch_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 { return; } - debug!( - "{from_bcb:?} has some branch(es) without counters:\n {}", - branch_target_bcbs - .iter() - .map(|&to_bcb| { - format!("{from_bcb:?}->{to_bcb:?}: {:?}", self.branch_counter(from_bcb, to_bcb)) - }) - .collect::>() - .join("\n "), - ); + 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 branch edges that don't have counters yet, one can be given an expression - // (computed from the other edges) instead of a dedicated counter. - let expression_to_bcb = self.choose_preferred_expression_branch(traversal, from_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 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), - // and accumulate the corresponding terms into a single sum term. - let sum_of_all_other_branches: BcbCounter = { - let _span = debug_span!("sum_of_all_other_branches", ?expression_to_bcb).entered(); - branch_target_bcbs + // and accumulate the corresponding counters into a single sum expression. + let sum_of_all_other_out_edges: BcbCounter = { + let _span = debug_span!("sum_of_all_other_out_edges", ?expression_to_bcb).entered(); + successors .iter() .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) .fold(None, |accum, to_bcb| { let _span = debug_span!("to_bcb", ?accum, ?to_bcb).entered(); - let branch_counter = self.get_or_make_edge_counter_operand(from_bcb, to_bcb); - Some(self.coverage_counters.make_sum_expression(accum, branch_counter)) + let edge_counter = self.get_or_make_edge_counter_operand(from_bcb, to_bcb); + 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 - // by taking the count of the node we're branching from, and subtracting the - // 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), - ); + // 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. let expression = self.coverage_counters.make_expression( from_bcb_operand, Op::Subtract, - sum_of_all_other_branches, + sum_of_all_other_out_edges, ); + debug!("{expression_to_bcb:?} gets an expression: {expression:?}"); 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); @@ -442,82 +428,79 @@ impl<'a> MakeBcbCounters<'a> { 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 - /// found, select any branch. - fn choose_preferred_expression_branch( + /// 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. + fn choose_out_edge_for_expression( &self, traversal: &TraverseCoverageGraphWithLoops<'_>, from_bcb: BasicCoverageBlock, ) -> BasicCoverageBlock { - let good_reloop_branch = self.find_good_reloop_branch(traversal, from_bcb); - if let Some(reloop_target) = good_reloop_branch { - assert!(self.branch_has_no_counter(from_bcb, reloop_target)); + if let Some(reloop_target) = self.find_good_reloop_edge(traversal, from_bcb) { + assert!(self.edge_has_no_counter(from_bcb, reloop_target)); debug!("Selecting reloop target {reloop_target:?} to get an expression"); - 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 + return 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"); + 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 - /// doesn't already have a counter. Such branches are good candidates to + /// 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 branch. - fn find_good_reloop_branch( + /// will tend to be executed more times than a loop-exit edge. + fn find_good_reloop_edge( &self, traversal: &TraverseCoverageGraphWithLoops<'_>, from_bcb: BasicCoverageBlock, ) -> Option { - 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. 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. - for &branch_target_bcb in branch_target_bcbs { - // A branch is a reloop branch if it dominates any BCB that has - // an edge back to the loop header. (Other branches are exits.) - let is_reloop_branch = reloop_bcbs.iter().any(|&reloop_bcb| { - self.basic_coverage_blocks.dominates(branch_target_bcb, reloop_bcb) + for &target_bcb in 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_branch { - all_branches_exit_this_loop = false; - if self.branch_has_no_counter(from_bcb, branch_target_bcb) { - // We found a good branch to be given an expression. - return Some(branch_target_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 branch without a counter. + // Keep looking for another reloop edge without a counter. } else { - // This branch exits the loop. + // This edge exits the loop. } } - if !all_branches_exit_this_loop { - // We found one or more reloop branches, but all of them already - // have counters. Let the caller choose one of the exit branches. - debug!("All reloop branches had counters; skip checking the other loops"); + 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 branches exit this loop, so keep looking for a good - // reloop branch for one of the outer loops. + // All of the out-edges exit this loop, so keep looking for a good + // reloop edge for one of the outer loops. } None @@ -534,15 +517,15 @@ impl<'a> MakeBcbCounters<'a> { } #[inline] - fn branch_has_no_counter( + fn edge_has_no_counter( &self, from_bcb: BasicCoverageBlock, to_bcb: BasicCoverageBlock, ) -> 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, from_bcb: BasicCoverageBlock, to_bcb: BasicCoverageBlock, From 10cd5e8386a1ed20dc7fe07b481d3965e51e592f Mon Sep 17 00:00:00 2001 From: Zalathar Date: Tue, 10 Sep 2024 20:48:28 +1000 Subject: [PATCH 3/3] coverage: Avoid referring to "operands" in counter creation --- .../rustc_mir_transform/src/coverage/counters.rs | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs index f55d6cf8efc71..1c240366afa2e 100644 --- a/compiler/rustc_mir_transform/src/coverage/counters.rs +++ b/compiler/rustc_mir_transform/src/coverage/counters.rs @@ -288,7 +288,7 @@ impl<'a> MakeBcbCounters<'a> { ) { // First, ensure that this node has a counter of some kind. // 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 node_counter = self.get_or_make_node_counter(from_bcb); let successors = self.basic_coverage_blocks.successors[from_bcb].as_slice(); @@ -324,7 +324,7 @@ impl<'a> MakeBcbCounters<'a> { .filter(|&to_bcb| to_bcb != expression_to_bcb) .fold(None, |accum, to_bcb| { let _span = debug_span!("to_bcb", ?accum, ?to_bcb).entered(); - let edge_counter = self.get_or_make_edge_counter_operand(from_bcb, to_bcb); + let edge_counter = self.get_or_make_edge_counter(from_bcb, to_bcb); Some(self.coverage_counters.make_sum_expression(accum, edge_counter)) }) .expect("there must be at least one other out-edge") @@ -333,7 +333,7 @@ impl<'a> MakeBcbCounters<'a> { // 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. let expression = self.coverage_counters.make_expression( - from_bcb_operand, + node_counter, Op::Subtract, sum_of_all_other_out_edges, ); @@ -347,7 +347,7 @@ impl<'a> MakeBcbCounters<'a> { } #[instrument(level = "debug", skip(self))] - fn get_or_make_counter_operand(&mut self, bcb: BasicCoverageBlock) -> BcbCounter { + fn get_or_make_node_counter(&mut self, bcb: BasicCoverageBlock) -> BcbCounter { // If the BCB already has a counter, return it. if let Some(counter_kind) = self.coverage_counters.bcb_counters[bcb] { debug!("{bcb:?} already has a counter: {counter_kind:?}"); @@ -384,7 +384,7 @@ impl<'a> MakeBcbCounters<'a> { .copied() .fold(None, |accum, from_bcb| { let _span = debug_span!("from_bcb", ?accum, ?from_bcb).entered(); - let edge_counter = self.get_or_make_edge_counter_operand(from_bcb, bcb); + let edge_counter = self.get_or_make_edge_counter(from_bcb, bcb); Some(self.coverage_counters.make_sum_expression(accum, edge_counter)) }) .expect("there must be at least one in-edge") @@ -395,7 +395,7 @@ impl<'a> MakeBcbCounters<'a> { } #[instrument(level = "debug", skip(self))] - fn get_or_make_edge_counter_operand( + fn get_or_make_edge_counter( &mut self, from_bcb: BasicCoverageBlock, to_bcb: BasicCoverageBlock, @@ -404,13 +404,13 @@ impl<'a> MakeBcbCounters<'a> { // a node counter instead, since it will have the same value. if !self.basic_coverage_blocks.bcb_has_multiple_in_edges(to_bcb) { assert_eq!([from_bcb].as_slice(), self.basic_coverage_blocks.predecessors[to_bcb]); - return self.get_or_make_counter_operand(to_bcb); + return self.get_or_make_node_counter(to_bcb); } // If the source BCB has only one successor (assumed to be the given target), an edge // counter is unnecessary. Just get or make a counter for the source BCB. if self.bcb_successors(from_bcb).len() == 1 { - return self.get_or_make_counter_operand(from_bcb); + return self.get_or_make_node_counter(from_bcb); } // If the edge already has a counter, return it.