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Diffstat (limited to 'compiler/rustc_mir_transform/src/coverage/graph.rs')
| -rw-r--r-- | compiler/rustc_mir_transform/src/coverage/graph.rs | 753 |
1 files changed, 753 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/coverage/graph.rs b/compiler/rustc_mir_transform/src/coverage/graph.rs new file mode 100644 index 000000000..759ea7cd3 --- /dev/null +++ b/compiler/rustc_mir_transform/src/coverage/graph.rs @@ -0,0 +1,753 @@ +use super::Error; + +use itertools::Itertools; +use rustc_data_structures::fx::FxHashMap; +use rustc_data_structures::graph::dominators::{self, Dominators}; +use rustc_data_structures::graph::{self, GraphSuccessors, WithNumNodes, WithStartNode}; +use rustc_index::bit_set::BitSet; +use rustc_index::vec::IndexVec; +use rustc_middle::mir::coverage::*; +use rustc_middle::mir::{self, BasicBlock, BasicBlockData, Terminator, TerminatorKind}; + +use std::ops::{Index, IndexMut}; + +const ID_SEPARATOR: &str = ","; + +/// A coverage-specific simplification of the MIR control flow graph (CFG). The `CoverageGraph`s +/// nodes are `BasicCoverageBlock`s, which encompass one or more MIR `BasicBlock`s, plus a +/// `CoverageKind` counter (to be added by `CoverageCounters::make_bcb_counters`), and an optional +/// set of additional counters--if needed--to count incoming edges, if there are more than one. +/// (These "edge counters" are eventually converted into new MIR `BasicBlock`s.) +#[derive(Debug)] +pub(super) struct CoverageGraph { + bcbs: IndexVec<BasicCoverageBlock, BasicCoverageBlockData>, + bb_to_bcb: IndexVec<BasicBlock, Option<BasicCoverageBlock>>, + pub successors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>, + pub predecessors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>, + dominators: Option<Dominators<BasicCoverageBlock>>, +} + +impl CoverageGraph { + pub fn from_mir(mir_body: &mir::Body<'_>) -> Self { + let (bcbs, bb_to_bcb) = Self::compute_basic_coverage_blocks(mir_body); + + // Pre-transform MIR `BasicBlock` successors and predecessors into the BasicCoverageBlock + // equivalents. Note that since the BasicCoverageBlock graph has been fully simplified, the + // each predecessor of a BCB leader_bb should be in a unique BCB. It is possible for a + // `SwitchInt` to have multiple targets to the same destination `BasicBlock`, so + // de-duplication is required. This is done without reordering the successors. + + let bcbs_len = bcbs.len(); + let mut seen = IndexVec::from_elem_n(false, bcbs_len); + let successors = IndexVec::from_fn_n( + |bcb| { + for b in seen.iter_mut() { + *b = false; + } + let bcb_data = &bcbs[bcb]; + let mut bcb_successors = Vec::new(); + for successor in + bcb_filtered_successors(&mir_body, &bcb_data.terminator(mir_body).kind) + .filter_map(|successor_bb| bb_to_bcb[successor_bb]) + { + if !seen[successor] { + seen[successor] = true; + bcb_successors.push(successor); + } + } + bcb_successors + }, + bcbs.len(), + ); + + let mut predecessors = IndexVec::from_elem_n(Vec::new(), bcbs.len()); + for (bcb, bcb_successors) in successors.iter_enumerated() { + for &successor in bcb_successors { + predecessors[successor].push(bcb); + } + } + + let mut basic_coverage_blocks = + Self { bcbs, bb_to_bcb, successors, predecessors, dominators: None }; + let dominators = dominators::dominators(&basic_coverage_blocks); + basic_coverage_blocks.dominators = Some(dominators); + basic_coverage_blocks + } + + fn compute_basic_coverage_blocks( + mir_body: &mir::Body<'_>, + ) -> ( + IndexVec<BasicCoverageBlock, BasicCoverageBlockData>, + IndexVec<BasicBlock, Option<BasicCoverageBlock>>, + ) { + let num_basic_blocks = mir_body.basic_blocks.len(); + let mut bcbs = IndexVec::with_capacity(num_basic_blocks); + let mut bb_to_bcb = IndexVec::from_elem_n(None, num_basic_blocks); + + // Walk the MIR CFG using a Preorder traversal, which starts from `START_BLOCK` and follows + // each block terminator's `successors()`. Coverage spans must map to actual source code, + // so compiler generated blocks and paths can be ignored. To that end, the CFG traversal + // intentionally omits unwind paths. + // FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and + // `catch_unwind()` handlers. + let mir_cfg_without_unwind = ShortCircuitPreorder::new(&mir_body, bcb_filtered_successors); + + let mut basic_blocks = Vec::new(); + for (bb, data) in mir_cfg_without_unwind { + if let Some(last) = basic_blocks.last() { + let predecessors = &mir_body.basic_blocks.predecessors()[bb]; + if predecessors.len() > 1 || !predecessors.contains(last) { + // The `bb` has more than one _incoming_ edge, and should start its own + // `BasicCoverageBlockData`. (Note, the `basic_blocks` vector does not yet + // include `bb`; it contains a sequence of one or more sequential basic_blocks + // with no intermediate branches in or out. Save these as a new + // `BasicCoverageBlockData` before starting the new one.) + Self::add_basic_coverage_block( + &mut bcbs, + &mut bb_to_bcb, + basic_blocks.split_off(0), + ); + debug!( + " because {}", + if predecessors.len() > 1 { + "predecessors.len() > 1".to_owned() + } else { + format!("bb {} is not in precessors: {:?}", bb.index(), predecessors) + } + ); + } + } + basic_blocks.push(bb); + + let term = data.terminator(); + + match term.kind { + TerminatorKind::Return { .. } + | TerminatorKind::Abort + | TerminatorKind::Yield { .. } + | TerminatorKind::SwitchInt { .. } => { + // The `bb` has more than one _outgoing_ edge, or exits the function. Save the + // current sequence of `basic_blocks` gathered to this point, as a new + // `BasicCoverageBlockData`. + Self::add_basic_coverage_block( + &mut bcbs, + &mut bb_to_bcb, + basic_blocks.split_off(0), + ); + debug!(" because term.kind = {:?}", term.kind); + // Note that this condition is based on `TerminatorKind`, even though it + // theoretically boils down to `successors().len() != 1`; that is, either zero + // (e.g., `Return`, `Abort`) or multiple successors (e.g., `SwitchInt`), but + // since the BCB CFG ignores things like unwind branches (which exist in the + // `Terminator`s `successors()` list) checking the number of successors won't + // work. + } + + // The following `TerminatorKind`s are either not expected outside an unwind branch, + // or they should not (under normal circumstances) branch. Coverage graphs are + // simplified by assuring coverage results are accurate for program executions that + // don't panic. + // + // Programs that panic and unwind may record slightly inaccurate coverage results + // for a coverage region containing the `Terminator` that began the panic. This + // is as intended. (See Issue #78544 for a possible future option to support + // coverage in test programs that panic.) + TerminatorKind::Goto { .. } + | TerminatorKind::Resume + | TerminatorKind::Unreachable + | TerminatorKind::Drop { .. } + | TerminatorKind::DropAndReplace { .. } + | TerminatorKind::Call { .. } + | TerminatorKind::GeneratorDrop + | TerminatorKind::Assert { .. } + | TerminatorKind::FalseEdge { .. } + | TerminatorKind::FalseUnwind { .. } + | TerminatorKind::InlineAsm { .. } => {} + } + } + + if !basic_blocks.is_empty() { + // process any remaining basic_blocks into a final `BasicCoverageBlockData` + Self::add_basic_coverage_block(&mut bcbs, &mut bb_to_bcb, basic_blocks.split_off(0)); + debug!(" because the end of the MIR CFG was reached while traversing"); + } + + (bcbs, bb_to_bcb) + } + + fn add_basic_coverage_block( + bcbs: &mut IndexVec<BasicCoverageBlock, BasicCoverageBlockData>, + bb_to_bcb: &mut IndexVec<BasicBlock, Option<BasicCoverageBlock>>, + basic_blocks: Vec<BasicBlock>, + ) { + let bcb = BasicCoverageBlock::from_usize(bcbs.len()); + for &bb in basic_blocks.iter() { + bb_to_bcb[bb] = Some(bcb); + } + let bcb_data = BasicCoverageBlockData::from(basic_blocks); + debug!("adding bcb{}: {:?}", bcb.index(), bcb_data); + bcbs.push(bcb_data); + } + + #[inline(always)] + pub fn iter_enumerated( + &self, + ) -> impl Iterator<Item = (BasicCoverageBlock, &BasicCoverageBlockData)> { + self.bcbs.iter_enumerated() + } + + #[inline(always)] + pub fn iter_enumerated_mut( + &mut self, + ) -> impl Iterator<Item = (BasicCoverageBlock, &mut BasicCoverageBlockData)> { + self.bcbs.iter_enumerated_mut() + } + + #[inline(always)] + pub fn bcb_from_bb(&self, bb: BasicBlock) -> Option<BasicCoverageBlock> { + if bb.index() < self.bb_to_bcb.len() { self.bb_to_bcb[bb] } else { None } + } + + #[inline(always)] + pub fn is_dominated_by(&self, node: BasicCoverageBlock, dom: BasicCoverageBlock) -> bool { + self.dominators.as_ref().unwrap().is_dominated_by(node, dom) + } + + #[inline(always)] + pub fn dominators(&self) -> &Dominators<BasicCoverageBlock> { + self.dominators.as_ref().unwrap() + } +} + +impl Index<BasicCoverageBlock> for CoverageGraph { + type Output = BasicCoverageBlockData; + + #[inline] + fn index(&self, index: BasicCoverageBlock) -> &BasicCoverageBlockData { + &self.bcbs[index] + } +} + +impl IndexMut<BasicCoverageBlock> for CoverageGraph { + #[inline] + fn index_mut(&mut self, index: BasicCoverageBlock) -> &mut BasicCoverageBlockData { + &mut self.bcbs[index] + } +} + +impl graph::DirectedGraph for CoverageGraph { + type Node = BasicCoverageBlock; +} + +impl graph::WithNumNodes for CoverageGraph { + #[inline] + fn num_nodes(&self) -> usize { + self.bcbs.len() + } +} + +impl graph::WithStartNode for CoverageGraph { + #[inline] + fn start_node(&self) -> Self::Node { + self.bcb_from_bb(mir::START_BLOCK) + .expect("mir::START_BLOCK should be in a BasicCoverageBlock") + } +} + +type BcbSuccessors<'graph> = std::slice::Iter<'graph, BasicCoverageBlock>; + +impl<'graph> graph::GraphSuccessors<'graph> for CoverageGraph { + type Item = BasicCoverageBlock; + type Iter = std::iter::Cloned<BcbSuccessors<'graph>>; +} + +impl graph::WithSuccessors for CoverageGraph { + #[inline] + fn successors(&self, node: Self::Node) -> <Self as GraphSuccessors<'_>>::Iter { + self.successors[node].iter().cloned() + } +} + +impl<'graph> graph::GraphPredecessors<'graph> for CoverageGraph { + type Item = BasicCoverageBlock; + type Iter = std::iter::Copied<std::slice::Iter<'graph, BasicCoverageBlock>>; +} + +impl graph::WithPredecessors for CoverageGraph { + #[inline] + fn predecessors(&self, node: Self::Node) -> <Self as graph::GraphPredecessors<'_>>::Iter { + self.predecessors[node].iter().copied() + } +} + +rustc_index::newtype_index! { + /// A node in the control-flow graph of CoverageGraph. + pub(super) struct BasicCoverageBlock { + DEBUG_FORMAT = "bcb{}", + const START_BCB = 0, + } +} + +/// `BasicCoverageBlockData` holds the data indexed by a `BasicCoverageBlock`. +/// +/// A `BasicCoverageBlock` (BCB) represents the maximal-length sequence of MIR `BasicBlock`s without +/// conditional branches, and form a new, simplified, coverage-specific Control Flow Graph, without +/// altering the original MIR CFG. +/// +/// Note that running the MIR `SimplifyCfg` transform is not sufficient (and therefore not +/// necessary). The BCB-based CFG is a more aggressive simplification. For example: +/// +/// * The BCB CFG ignores (trims) branches not relevant to coverage, such as unwind-related code, +/// that is injected by the Rust compiler but has no physical source code to count. This also +/// means a BasicBlock with a `Call` terminator can be merged into its primary successor target +/// block, in the same BCB. (But, note: Issue #78544: "MIR InstrumentCoverage: Improve coverage +/// of `#[should_panic]` tests and `catch_unwind()` handlers") +/// * Some BasicBlock terminators support Rust-specific concerns--like borrow-checking--that are +/// not relevant to coverage analysis. `FalseUnwind`, for example, can be treated the same as +/// a `Goto`, and merged with its successor into the same BCB. +/// +/// Each BCB with at least one computed `CoverageSpan` will have no more than one `Counter`. +/// In some cases, a BCB's execution count can be computed by `Expression`. Additional +/// disjoint `CoverageSpan`s in a BCB can also be counted by `Expression` (by adding `ZERO` +/// to the BCB's primary counter or expression). +/// +/// The BCB CFG is critical to simplifying the coverage analysis by ensuring graph path-based +/// queries (`is_dominated_by()`, `predecessors`, `successors`, etc.) have branch (control flow) +/// significance. +#[derive(Debug, Clone)] +pub(super) struct BasicCoverageBlockData { + pub basic_blocks: Vec<BasicBlock>, + pub counter_kind: Option<CoverageKind>, + edge_from_bcbs: Option<FxHashMap<BasicCoverageBlock, CoverageKind>>, +} + +impl BasicCoverageBlockData { + pub fn from(basic_blocks: Vec<BasicBlock>) -> Self { + assert!(basic_blocks.len() > 0); + Self { basic_blocks, counter_kind: None, edge_from_bcbs: None } + } + + #[inline(always)] + pub fn leader_bb(&self) -> BasicBlock { + self.basic_blocks[0] + } + + #[inline(always)] + pub fn last_bb(&self) -> BasicBlock { + *self.basic_blocks.last().unwrap() + } + + #[inline(always)] + pub fn terminator<'a, 'tcx>(&self, mir_body: &'a mir::Body<'tcx>) -> &'a Terminator<'tcx> { + &mir_body[self.last_bb()].terminator() + } + + pub fn set_counter( + &mut self, + counter_kind: CoverageKind, + ) -> Result<ExpressionOperandId, Error> { + debug_assert!( + // If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also + // have an expression (to be injected into an existing `BasicBlock` represented by this + // `BasicCoverageBlock`). + self.edge_from_bcbs.is_none() || counter_kind.is_expression(), + "attempt to add a `Counter` to a BCB target with existing incoming edge counters" + ); + let operand = counter_kind.as_operand_id(); + if let Some(replaced) = self.counter_kind.replace(counter_kind) { + Error::from_string(format!( + "attempt to set a BasicCoverageBlock coverage counter more than once; \ + {:?} already had counter {:?}", + self, replaced, + )) + } else { + Ok(operand) + } + } + + #[inline(always)] + pub fn counter(&self) -> Option<&CoverageKind> { + self.counter_kind.as_ref() + } + + #[inline(always)] + pub fn take_counter(&mut self) -> Option<CoverageKind> { + self.counter_kind.take() + } + + pub fn set_edge_counter_from( + &mut self, + from_bcb: BasicCoverageBlock, + counter_kind: CoverageKind, + ) -> Result<ExpressionOperandId, Error> { + if level_enabled!(tracing::Level::DEBUG) { + // If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also + // have an expression (to be injected into an existing `BasicBlock` represented by this + // `BasicCoverageBlock`). + if !self.counter_kind.as_ref().map_or(true, |c| c.is_expression()) { + return Error::from_string(format!( + "attempt to add an incoming edge counter from {:?} when the target BCB already \ + has a `Counter`", + from_bcb + )); + } + } + let operand = counter_kind.as_operand_id(); + if let Some(replaced) = + self.edge_from_bcbs.get_or_insert_default().insert(from_bcb, counter_kind) + { + Error::from_string(format!( + "attempt to set an edge counter more than once; from_bcb: \ + {:?} already had counter {:?}", + from_bcb, replaced, + )) + } else { + Ok(operand) + } + } + + #[inline] + pub fn edge_counter_from(&self, from_bcb: BasicCoverageBlock) -> Option<&CoverageKind> { + if let Some(edge_from_bcbs) = &self.edge_from_bcbs { + edge_from_bcbs.get(&from_bcb) + } else { + None + } + } + + #[inline] + pub fn take_edge_counters( + &mut self, + ) -> Option<impl Iterator<Item = (BasicCoverageBlock, CoverageKind)>> { + self.edge_from_bcbs.take().map(|m| m.into_iter()) + } + + pub fn id(&self) -> String { + format!("@{}", self.basic_blocks.iter().map(|bb| bb.index().to_string()).join(ID_SEPARATOR)) + } +} + +/// Represents a successor from a branching BasicCoverageBlock (such as the arms of a `SwitchInt`) +/// as either the successor BCB itself, if it has only one incoming edge, or the successor _plus_ +/// the specific branching BCB, representing the edge between the two. The latter case +/// distinguishes this incoming edge from other incoming edges to the same `target_bcb`. +#[derive(Clone, Copy, PartialEq, Eq)] +pub(super) struct BcbBranch { + pub edge_from_bcb: Option<BasicCoverageBlock>, + pub target_bcb: BasicCoverageBlock, +} + +impl BcbBranch { + pub fn from_to( + from_bcb: BasicCoverageBlock, + to_bcb: BasicCoverageBlock, + basic_coverage_blocks: &CoverageGraph, + ) -> Self { + let edge_from_bcb = if basic_coverage_blocks.predecessors[to_bcb].len() > 1 { + Some(from_bcb) + } else { + None + }; + Self { edge_from_bcb, target_bcb: to_bcb } + } + + pub fn counter<'a>( + &self, + basic_coverage_blocks: &'a CoverageGraph, + ) -> Option<&'a CoverageKind> { + if let Some(from_bcb) = self.edge_from_bcb { + basic_coverage_blocks[self.target_bcb].edge_counter_from(from_bcb) + } else { + basic_coverage_blocks[self.target_bcb].counter() + } + } + + pub fn is_only_path_to_target(&self) -> bool { + self.edge_from_bcb.is_none() + } +} + +impl std::fmt::Debug for BcbBranch { + fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { + if let Some(from_bcb) = self.edge_from_bcb { + write!(fmt, "{:?}->{:?}", from_bcb, self.target_bcb) + } else { + write!(fmt, "{:?}", self.target_bcb) + } + } +} + +// Returns the `Terminator`s non-unwind successors. +// FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and +// `catch_unwind()` handlers. +fn bcb_filtered_successors<'a, 'tcx>( + body: &'a mir::Body<'tcx>, + term_kind: &'a TerminatorKind<'tcx>, +) -> Box<dyn Iterator<Item = BasicBlock> + 'a> { + Box::new( + match &term_kind { + // SwitchInt successors are never unwind, and all of them should be traversed. + TerminatorKind::SwitchInt { ref targets, .. } => { + None.into_iter().chain(targets.all_targets().into_iter().copied()) + } + // For all other kinds, return only the first successor, if any, and ignore unwinds. + // NOTE: `chain(&[])` is required to coerce the `option::iter` (from + // `next().into_iter()`) into the `mir::Successors` aliased type. + _ => term_kind.successors().next().into_iter().chain((&[]).into_iter().copied()), + } + .filter(move |&successor| body[successor].terminator().kind != TerminatorKind::Unreachable), + ) +} + +/// Maintains separate worklists for each loop in the BasicCoverageBlock CFG, plus one for the +/// CoverageGraph outside all loops. This supports traversing the BCB CFG in a way that +/// ensures a loop is completely traversed before processing Blocks after the end of the loop. +#[derive(Debug)] +pub(super) struct TraversalContext { + /// From one or more backedges returning to a loop header. + pub loop_backedges: Option<(Vec<BasicCoverageBlock>, BasicCoverageBlock)>, + + /// worklist, to be traversed, of CoverageGraph in the loop with the given loop + /// backedges, such that the loop is the inner inner-most loop containing these + /// CoverageGraph + pub worklist: Vec<BasicCoverageBlock>, +} + +pub(super) struct TraverseCoverageGraphWithLoops { + pub backedges: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>, + pub context_stack: Vec<TraversalContext>, + visited: BitSet<BasicCoverageBlock>, +} + +impl TraverseCoverageGraphWithLoops { + pub fn new(basic_coverage_blocks: &CoverageGraph) -> Self { + let start_bcb = basic_coverage_blocks.start_node(); + let backedges = find_loop_backedges(basic_coverage_blocks); + let context_stack = + vec![TraversalContext { loop_backedges: None, worklist: vec![start_bcb] }]; + // `context_stack` starts with a `TraversalContext` for the main function context (beginning + // with the `start` BasicCoverageBlock of the function). New worklists are pushed to the top + // of the stack as loops are entered, and popped off of the stack when a loop's worklist is + // exhausted. + let visited = BitSet::new_empty(basic_coverage_blocks.num_nodes()); + Self { backedges, context_stack, visited } + } + + pub fn next(&mut self, basic_coverage_blocks: &CoverageGraph) -> Option<BasicCoverageBlock> { + debug!( + "TraverseCoverageGraphWithLoops::next - context_stack: {:?}", + self.context_stack.iter().rev().collect::<Vec<_>>() + ); + while let Some(next_bcb) = { + // Strip contexts with empty worklists from the top of the stack + while self.context_stack.last().map_or(false, |context| context.worklist.is_empty()) { + self.context_stack.pop(); + } + // Pop the next bcb off of the current context_stack. If none, all BCBs were visited. + self.context_stack.last_mut().map_or(None, |context| context.worklist.pop()) + } { + if !self.visited.insert(next_bcb) { + debug!("Already visited: {:?}", next_bcb); + continue; + } + debug!("Visiting {:?}", next_bcb); + if self.backedges[next_bcb].len() > 0 { + debug!("{:?} is a loop header! Start a new TraversalContext...", next_bcb); + self.context_stack.push(TraversalContext { + loop_backedges: Some((self.backedges[next_bcb].clone(), next_bcb)), + worklist: Vec::new(), + }); + } + self.extend_worklist(basic_coverage_blocks, next_bcb); + return Some(next_bcb); + } + None + } + + pub fn extend_worklist( + &mut self, + basic_coverage_blocks: &CoverageGraph, + bcb: BasicCoverageBlock, + ) { + let successors = &basic_coverage_blocks.successors[bcb]; + debug!("{:?} has {} successors:", bcb, successors.len()); + for &successor in successors { + if successor == bcb { + debug!( + "{:?} has itself as its own successor. (Note, the compiled code will \ + generate an infinite loop.)", + bcb + ); + // Don't re-add this successor to the worklist. We are already processing it. + break; + } + for context in self.context_stack.iter_mut().rev() { + // Add successors of the current BCB to the appropriate context. Successors that + // stay within a loop are added to the BCBs context worklist. Successors that + // exit the loop (they are not dominated by the loop header) must be reachable + // from other BCBs outside the loop, and they will be added to a different + // worklist. + // + // Branching blocks (with more than one successor) must be processed before + // blocks with only one successor, to prevent unnecessarily complicating + // `Expression`s by creating a Counter in a `BasicCoverageBlock` that the + // branching block would have given an `Expression` (or vice versa). + let (some_successor_to_add, some_loop_header) = + if let Some((_, loop_header)) = context.loop_backedges { + if basic_coverage_blocks.is_dominated_by(successor, loop_header) { + (Some(successor), Some(loop_header)) + } else { + (None, None) + } + } else { + (Some(successor), None) + }; + if let Some(successor_to_add) = some_successor_to_add { + if basic_coverage_blocks.successors[successor_to_add].len() > 1 { + debug!( + "{:?} successor is branching. Prioritize it at the beginning of \ + the {}", + successor_to_add, + if let Some(loop_header) = some_loop_header { + format!("worklist for the loop headed by {:?}", loop_header) + } else { + String::from("non-loop worklist") + }, + ); + context.worklist.insert(0, successor_to_add); + } else { + debug!( + "{:?} successor is non-branching. Defer it to the end of the {}", + successor_to_add, + if let Some(loop_header) = some_loop_header { + format!("worklist for the loop headed by {:?}", loop_header) + } else { + String::from("non-loop worklist") + }, + ); + context.worklist.push(successor_to_add); + } + break; + } + } + } + } + + pub fn is_complete(&self) -> bool { + self.visited.count() == self.visited.domain_size() + } + + pub fn unvisited(&self) -> Vec<BasicCoverageBlock> { + let mut unvisited_set: BitSet<BasicCoverageBlock> = + BitSet::new_filled(self.visited.domain_size()); + unvisited_set.subtract(&self.visited); + unvisited_set.iter().collect::<Vec<_>>() + } +} + +pub(super) fn find_loop_backedges( + basic_coverage_blocks: &CoverageGraph, +) -> IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>> { + let num_bcbs = basic_coverage_blocks.num_nodes(); + let mut backedges = IndexVec::from_elem_n(Vec::<BasicCoverageBlock>::new(), num_bcbs); + + // Identify loops by their backedges. + // + // The computational complexity is bounded by: n(s) x d where `n` is the number of + // `BasicCoverageBlock` nodes (the simplified/reduced representation of the CFG derived from the + // MIR); `s` is the average number of successors per node (which is most likely less than 2, and + // independent of the size of the function, so it can be treated as a constant); + // and `d` is the average number of dominators per node. + // + // The average number of dominators depends on the size and complexity of the function, and + // nodes near the start of the function's control flow graph typically have less dominators + // than nodes near the end of the CFG. Without doing a detailed mathematical analysis, I + // think the resulting complexity has the characteristics of O(n log n). + // + // The overall complexity appears to be comparable to many other MIR transform algorithms, and I + // don't expect that this function is creating a performance hot spot, but if this becomes an + // issue, there may be ways to optimize the `is_dominated_by` algorithm (as indicated by an + // existing `FIXME` comment in that code), or possibly ways to optimize it's usage here, perhaps + // by keeping track of results for visited `BasicCoverageBlock`s if they can be used to short + // circuit downstream `is_dominated_by` checks. + // + // For now, that kind of optimization seems unnecessarily complicated. + for (bcb, _) in basic_coverage_blocks.iter_enumerated() { + for &successor in &basic_coverage_blocks.successors[bcb] { + if basic_coverage_blocks.is_dominated_by(bcb, successor) { + let loop_header = successor; + let backedge_from_bcb = bcb; + debug!( + "Found BCB backedge: {:?} -> loop_header: {:?}", + backedge_from_bcb, loop_header + ); + backedges[loop_header].push(backedge_from_bcb); + } + } + } + backedges +} + +pub struct ShortCircuitPreorder< + 'a, + 'tcx, + F: Fn(&'a mir::Body<'tcx>, &'a TerminatorKind<'tcx>) -> Box<dyn Iterator<Item = BasicBlock> + 'a>, +> { + body: &'a mir::Body<'tcx>, + visited: BitSet<BasicBlock>, + worklist: Vec<BasicBlock>, + filtered_successors: F, +} + +impl< + 'a, + 'tcx, + F: Fn(&'a mir::Body<'tcx>, &'a TerminatorKind<'tcx>) -> Box<dyn Iterator<Item = BasicBlock> + 'a>, +> ShortCircuitPreorder<'a, 'tcx, F> +{ + pub fn new( + body: &'a mir::Body<'tcx>, + filtered_successors: F, + ) -> ShortCircuitPreorder<'a, 'tcx, F> { + let worklist = vec![mir::START_BLOCK]; + + ShortCircuitPreorder { + body, + visited: BitSet::new_empty(body.basic_blocks().len()), + worklist, + filtered_successors, + } + } +} + +impl< + 'a, + 'tcx, + F: Fn(&'a mir::Body<'tcx>, &'a TerminatorKind<'tcx>) -> Box<dyn Iterator<Item = BasicBlock> + 'a>, +> Iterator for ShortCircuitPreorder<'a, 'tcx, F> +{ + type Item = (BasicBlock, &'a BasicBlockData<'tcx>); + + fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> { + while let Some(idx) = self.worklist.pop() { + if !self.visited.insert(idx) { + continue; + } + + let data = &self.body[idx]; + + if let Some(ref term) = data.terminator { + self.worklist.extend((self.filtered_successors)(&self.body, &term.kind)); + } + + return Some((idx, data)); + } + + None + } + + fn size_hint(&self) -> (usize, Option<usize>) { + let size = self.body.basic_blocks().len() - self.visited.count(); + (size, Some(size)) + } +} |