6d1c396399
Filtering out unreachable successors is only needed by the main graph traversal loop, so we can move the filtering step into that loop instead, eliminating the need to pass the MIR body into `bcb_filtered_successors`.
564 lines
23 KiB
Rust
564 lines
23 KiB
Rust
use rustc_data_structures::captures::Captures;
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use rustc_data_structures::graph::dominators::{self, Dominators};
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use rustc_data_structures::graph::{self, GraphSuccessors, WithNumNodes, WithStartNode};
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use rustc_index::bit_set::BitSet;
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use rustc_index::{IndexSlice, IndexVec};
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use rustc_middle::mir::{self, BasicBlock, Terminator, TerminatorKind};
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use std::cmp::Ordering;
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use std::collections::VecDeque;
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use std::ops::{Index, IndexMut};
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/// A coverage-specific simplification of the MIR control flow graph (CFG). The `CoverageGraph`s
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/// nodes are `BasicCoverageBlock`s, which encompass one or more MIR `BasicBlock`s.
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#[derive(Debug)]
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pub(super) struct CoverageGraph {
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bcbs: IndexVec<BasicCoverageBlock, BasicCoverageBlockData>,
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bb_to_bcb: IndexVec<BasicBlock, Option<BasicCoverageBlock>>,
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pub successors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
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pub predecessors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
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dominators: Option<Dominators<BasicCoverageBlock>>,
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}
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impl CoverageGraph {
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pub fn from_mir(mir_body: &mir::Body<'_>) -> Self {
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let (bcbs, bb_to_bcb) = Self::compute_basic_coverage_blocks(mir_body);
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// Pre-transform MIR `BasicBlock` successors and predecessors into the BasicCoverageBlock
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// equivalents. Note that since the BasicCoverageBlock graph has been fully simplified, the
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// each predecessor of a BCB leader_bb should be in a unique BCB. It is possible for a
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// `SwitchInt` to have multiple targets to the same destination `BasicBlock`, so
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// de-duplication is required. This is done without reordering the successors.
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let mut seen = IndexVec::from_elem(false, &bcbs);
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let successors = IndexVec::from_fn_n(
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|bcb| {
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for b in seen.iter_mut() {
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*b = false;
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}
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let bcb_data = &bcbs[bcb];
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let mut bcb_successors = Vec::new();
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for successor in bcb_filtered_successors(mir_body[bcb_data.last_bb()].terminator())
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.filter_map(|successor_bb| bb_to_bcb[successor_bb])
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{
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if !seen[successor] {
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seen[successor] = true;
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bcb_successors.push(successor);
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}
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}
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bcb_successors
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},
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bcbs.len(),
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);
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let mut predecessors = IndexVec::from_elem(Vec::new(), &bcbs);
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for (bcb, bcb_successors) in successors.iter_enumerated() {
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for &successor in bcb_successors {
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predecessors[successor].push(bcb);
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}
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}
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let mut basic_coverage_blocks =
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Self { bcbs, bb_to_bcb, successors, predecessors, dominators: None };
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let dominators = dominators::dominators(&basic_coverage_blocks);
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basic_coverage_blocks.dominators = Some(dominators);
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// The coverage graph's entry-point node (bcb0) always starts with bb0,
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// which never has predecessors. Any other blocks merged into bcb0 can't
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// have multiple (coverage-relevant) predecessors, so bcb0 always has
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// zero in-edges.
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assert!(basic_coverage_blocks[START_BCB].leader_bb() == mir::START_BLOCK);
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assert!(basic_coverage_blocks.predecessors[START_BCB].is_empty());
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basic_coverage_blocks
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}
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fn compute_basic_coverage_blocks(
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mir_body: &mir::Body<'_>,
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) -> (
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IndexVec<BasicCoverageBlock, BasicCoverageBlockData>,
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IndexVec<BasicBlock, Option<BasicCoverageBlock>>,
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) {
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let num_basic_blocks = mir_body.basic_blocks.len();
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let mut bcbs = IndexVec::with_capacity(num_basic_blocks);
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let mut bb_to_bcb = IndexVec::from_elem_n(None, num_basic_blocks);
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// Walk the MIR CFG using a Preorder traversal, which starts from `START_BLOCK` and follows
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// each block terminator's `successors()`. Coverage spans must map to actual source code,
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// so compiler generated blocks and paths can be ignored. To that end, the CFG traversal
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// intentionally omits unwind paths.
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// FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and
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// `catch_unwind()` handlers.
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let mut basic_blocks = Vec::new();
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let filtered_successors = |bb| bcb_filtered_successors(mir_body[bb].terminator());
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for bb in short_circuit_preorder(mir_body, filtered_successors)
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.filter(|&bb| mir_body[bb].terminator().kind != TerminatorKind::Unreachable)
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{
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if let Some(last) = basic_blocks.last() {
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let predecessors = &mir_body.basic_blocks.predecessors()[bb];
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if predecessors.len() > 1 || !predecessors.contains(last) {
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// The `bb` has more than one _incoming_ edge, and should start its own
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// `BasicCoverageBlockData`. (Note, the `basic_blocks` vector does not yet
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// include `bb`; it contains a sequence of one or more sequential basic_blocks
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// with no intermediate branches in or out. Save these as a new
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// `BasicCoverageBlockData` before starting the new one.)
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Self::add_basic_coverage_block(
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&mut bcbs,
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&mut bb_to_bcb,
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basic_blocks.split_off(0),
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);
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debug!(
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" because {}",
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if predecessors.len() > 1 {
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"predecessors.len() > 1".to_owned()
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} else {
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format!("bb {} is not in predecessors: {:?}", bb.index(), predecessors)
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}
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);
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}
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}
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basic_blocks.push(bb);
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let term = mir_body[bb].terminator();
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match term.kind {
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TerminatorKind::Return { .. }
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| TerminatorKind::UnwindTerminate(_)
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| TerminatorKind::Yield { .. }
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| TerminatorKind::SwitchInt { .. } => {
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// The `bb` has more than one _outgoing_ edge, or exits the function. Save the
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// current sequence of `basic_blocks` gathered to this point, as a new
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// `BasicCoverageBlockData`.
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Self::add_basic_coverage_block(
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&mut bcbs,
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&mut bb_to_bcb,
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basic_blocks.split_off(0),
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);
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debug!(" because term.kind = {:?}", term.kind);
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// Note that this condition is based on `TerminatorKind`, even though it
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// theoretically boils down to `successors().len() != 1`; that is, either zero
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// (e.g., `Return`, `Terminate`) or multiple successors (e.g., `SwitchInt`), but
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// since the BCB CFG ignores things like unwind branches (which exist in the
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// `Terminator`s `successors()` list) checking the number of successors won't
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// work.
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}
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// The following `TerminatorKind`s are either not expected outside an unwind branch,
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// or they should not (under normal circumstances) branch. Coverage graphs are
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// simplified by assuring coverage results are accurate for program executions that
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// don't panic.
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//
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// Programs that panic and unwind may record slightly inaccurate coverage results
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// for a coverage region containing the `Terminator` that began the panic. This
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// is as intended. (See Issue #78544 for a possible future option to support
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// coverage in test programs that panic.)
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TerminatorKind::Goto { .. }
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| TerminatorKind::UnwindResume
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| TerminatorKind::Unreachable
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| TerminatorKind::Drop { .. }
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| TerminatorKind::Call { .. }
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| TerminatorKind::CoroutineDrop
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| TerminatorKind::Assert { .. }
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| TerminatorKind::FalseEdge { .. }
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| TerminatorKind::FalseUnwind { .. }
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| TerminatorKind::InlineAsm { .. } => {}
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}
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}
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if !basic_blocks.is_empty() {
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// process any remaining basic_blocks into a final `BasicCoverageBlockData`
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Self::add_basic_coverage_block(&mut bcbs, &mut bb_to_bcb, basic_blocks.split_off(0));
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debug!(" because the end of the MIR CFG was reached while traversing");
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}
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(bcbs, bb_to_bcb)
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}
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fn add_basic_coverage_block(
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bcbs: &mut IndexVec<BasicCoverageBlock, BasicCoverageBlockData>,
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bb_to_bcb: &mut IndexSlice<BasicBlock, Option<BasicCoverageBlock>>,
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basic_blocks: Vec<BasicBlock>,
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) {
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let bcb = bcbs.next_index();
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for &bb in basic_blocks.iter() {
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bb_to_bcb[bb] = Some(bcb);
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}
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let bcb_data = BasicCoverageBlockData::from(basic_blocks);
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debug!("adding bcb{}: {:?}", bcb.index(), bcb_data);
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bcbs.push(bcb_data);
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}
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#[inline(always)]
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pub fn iter_enumerated(
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&self,
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) -> impl Iterator<Item = (BasicCoverageBlock, &BasicCoverageBlockData)> {
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self.bcbs.iter_enumerated()
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}
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#[inline(always)]
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pub fn bcb_from_bb(&self, bb: BasicBlock) -> Option<BasicCoverageBlock> {
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if bb.index() < self.bb_to_bcb.len() { self.bb_to_bcb[bb] } else { None }
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}
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#[inline(always)]
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pub fn dominates(&self, dom: BasicCoverageBlock, node: BasicCoverageBlock) -> bool {
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self.dominators.as_ref().unwrap().dominates(dom, node)
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}
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#[inline(always)]
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pub fn cmp_in_dominator_order(&self, a: BasicCoverageBlock, b: BasicCoverageBlock) -> Ordering {
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self.dominators.as_ref().unwrap().cmp_in_dominator_order(a, b)
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}
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/// Returns true if the given node has 2 or more in-edges, i.e. 2 or more
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/// predecessors.
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///
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/// This property is interesting to code that assigns counters to nodes and
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/// edges, because if a node _doesn't_ have multiple in-edges, then there's
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/// no benefit in having a separate counter for its in-edge, because it
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/// would have the same value as the node's own counter.
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///
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/// FIXME: That assumption might not be true for [`TerminatorKind::Yield`]?
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#[inline(always)]
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pub(super) fn bcb_has_multiple_in_edges(&self, bcb: BasicCoverageBlock) -> bool {
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// Even though bcb0 conceptually has an extra virtual in-edge due to
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// being the entry point, we've already asserted that it has no _other_
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// in-edges, so there's no possibility of it having _multiple_ in-edges.
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// (And since its virtual in-edge doesn't exist in the graph, that edge
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// can't have a separate counter anyway.)
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self.predecessors[bcb].len() > 1
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}
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}
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impl Index<BasicCoverageBlock> for CoverageGraph {
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type Output = BasicCoverageBlockData;
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#[inline]
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fn index(&self, index: BasicCoverageBlock) -> &BasicCoverageBlockData {
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&self.bcbs[index]
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}
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}
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impl IndexMut<BasicCoverageBlock> for CoverageGraph {
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#[inline]
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fn index_mut(&mut self, index: BasicCoverageBlock) -> &mut BasicCoverageBlockData {
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&mut self.bcbs[index]
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}
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}
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impl graph::DirectedGraph for CoverageGraph {
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type Node = BasicCoverageBlock;
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}
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impl graph::WithNumNodes for CoverageGraph {
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#[inline]
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fn num_nodes(&self) -> usize {
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self.bcbs.len()
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}
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}
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impl graph::WithStartNode for CoverageGraph {
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#[inline]
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fn start_node(&self) -> Self::Node {
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self.bcb_from_bb(mir::START_BLOCK)
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.expect("mir::START_BLOCK should be in a BasicCoverageBlock")
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}
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}
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type BcbSuccessors<'graph> = std::slice::Iter<'graph, BasicCoverageBlock>;
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impl<'graph> graph::GraphSuccessors<'graph> for CoverageGraph {
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type Item = BasicCoverageBlock;
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type Iter = std::iter::Cloned<BcbSuccessors<'graph>>;
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}
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impl graph::WithSuccessors for CoverageGraph {
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#[inline]
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fn successors(&self, node: Self::Node) -> <Self as GraphSuccessors<'_>>::Iter {
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self.successors[node].iter().cloned()
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}
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}
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impl<'graph> graph::GraphPredecessors<'graph> for CoverageGraph {
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type Item = BasicCoverageBlock;
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type Iter = std::iter::Copied<std::slice::Iter<'graph, BasicCoverageBlock>>;
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}
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impl graph::WithPredecessors for CoverageGraph {
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#[inline]
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fn predecessors(&self, node: Self::Node) -> <Self as graph::GraphPredecessors<'_>>::Iter {
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self.predecessors[node].iter().copied()
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}
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}
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rustc_index::newtype_index! {
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/// A node in the control-flow graph of CoverageGraph.
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#[orderable]
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#[debug_format = "bcb{}"]
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pub(super) struct BasicCoverageBlock {
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const START_BCB = 0;
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}
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}
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/// `BasicCoverageBlockData` holds the data indexed by a `BasicCoverageBlock`.
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///
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/// A `BasicCoverageBlock` (BCB) represents the maximal-length sequence of MIR `BasicBlock`s without
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/// conditional branches, and form a new, simplified, coverage-specific Control Flow Graph, without
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/// altering the original MIR CFG.
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///
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/// Note that running the MIR `SimplifyCfg` transform is not sufficient (and therefore not
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/// necessary). The BCB-based CFG is a more aggressive simplification. For example:
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///
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/// * The BCB CFG ignores (trims) branches not relevant to coverage, such as unwind-related code,
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/// that is injected by the Rust compiler but has no physical source code to count. This also
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/// means a BasicBlock with a `Call` terminator can be merged into its primary successor target
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/// block, in the same BCB. (But, note: Issue #78544: "MIR InstrumentCoverage: Improve coverage
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/// of `#[should_panic]` tests and `catch_unwind()` handlers")
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/// * Some BasicBlock terminators support Rust-specific concerns--like borrow-checking--that are
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/// not relevant to coverage analysis. `FalseUnwind`, for example, can be treated the same as
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/// a `Goto`, and merged with its successor into the same BCB.
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///
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/// Each BCB with at least one computed coverage span will have no more than one `Counter`.
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/// In some cases, a BCB's execution count can be computed by `Expression`. Additional
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/// disjoint coverage spans in a BCB can also be counted by `Expression` (by adding `ZERO`
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/// to the BCB's primary counter or expression).
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///
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/// The BCB CFG is critical to simplifying the coverage analysis by ensuring graph path-based
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/// queries (`dominates()`, `predecessors`, `successors`, etc.) have branch (control flow)
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/// significance.
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#[derive(Debug, Clone)]
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pub(super) struct BasicCoverageBlockData {
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pub basic_blocks: Vec<BasicBlock>,
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}
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impl BasicCoverageBlockData {
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pub fn from(basic_blocks: Vec<BasicBlock>) -> Self {
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assert!(basic_blocks.len() > 0);
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Self { basic_blocks }
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}
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#[inline(always)]
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pub fn leader_bb(&self) -> BasicBlock {
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self.basic_blocks[0]
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}
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#[inline(always)]
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pub fn last_bb(&self) -> BasicBlock {
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*self.basic_blocks.last().unwrap()
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}
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}
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// Returns the subset of a block's successors that are relevant to the coverage
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// graph, i.e. those that do not represent unwinds or false edges.
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// FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and
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// `catch_unwind()` handlers.
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fn bcb_filtered_successors<'a, 'tcx>(
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terminator: &'a Terminator<'tcx>,
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) -> impl Iterator<Item = BasicBlock> + Captures<'a> + Captures<'tcx> {
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let take_n_successors = match terminator.kind {
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// SwitchInt successors are never unwinds, so all of them should be traversed.
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TerminatorKind::SwitchInt { .. } => usize::MAX,
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// For all other kinds, return only the first successor (if any), ignoring any
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// unwind successors.
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_ => 1,
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};
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terminator.successors().take(take_n_successors)
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}
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/// Maintains separate worklists for each loop in the BasicCoverageBlock CFG, plus one for the
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/// CoverageGraph outside all loops. This supports traversing the BCB CFG in a way that
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/// ensures a loop is completely traversed before processing Blocks after the end of the loop.
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#[derive(Debug)]
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pub(super) struct TraversalContext {
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/// BCB with one or more incoming loop backedges, indicating which loop
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/// this context is for.
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///
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/// If `None`, this is the non-loop context for the function as a whole.
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loop_header: Option<BasicCoverageBlock>,
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/// Worklist of BCBs to be processed in this context.
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worklist: VecDeque<BasicCoverageBlock>,
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}
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pub(super) struct TraverseCoverageGraphWithLoops<'a> {
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basic_coverage_blocks: &'a CoverageGraph,
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backedges: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
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context_stack: Vec<TraversalContext>,
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visited: BitSet<BasicCoverageBlock>,
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}
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impl<'a> TraverseCoverageGraphWithLoops<'a> {
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pub(super) fn new(basic_coverage_blocks: &'a CoverageGraph) -> Self {
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let backedges = find_loop_backedges(basic_coverage_blocks);
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let worklist = VecDeque::from([basic_coverage_blocks.start_node()]);
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let context_stack = vec![TraversalContext { loop_header: None, worklist }];
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// `context_stack` starts with a `TraversalContext` for the main function context (beginning
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// with the `start` BasicCoverageBlock of the function). New worklists are pushed to the top
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// of the stack as loops are entered, and popped off of the stack when a loop's worklist is
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// exhausted.
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let visited = BitSet::new_empty(basic_coverage_blocks.num_nodes());
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Self { basic_coverage_blocks, backedges, context_stack, visited }
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}
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/// For each loop on the loop context stack (top-down), yields a list of BCBs
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/// within that loop that have an outgoing edge back to the loop header.
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pub(super) fn reloop_bcbs_per_loop(&self) -> impl Iterator<Item = &[BasicCoverageBlock]> {
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self.context_stack
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.iter()
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.rev()
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.filter_map(|context| context.loop_header)
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.map(|header_bcb| self.backedges[header_bcb].as_slice())
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}
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pub(super) fn next(&mut self) -> Option<BasicCoverageBlock> {
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debug!(
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"TraverseCoverageGraphWithLoops::next - context_stack: {:?}",
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self.context_stack.iter().rev().collect::<Vec<_>>()
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);
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while let Some(context) = self.context_stack.last_mut() {
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if let Some(bcb) = context.worklist.pop_front() {
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if !self.visited.insert(bcb) {
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debug!("Already visited: {bcb:?}");
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continue;
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}
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debug!("Visiting {bcb:?}");
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if self.backedges[bcb].len() > 0 {
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debug!("{bcb:?} is a loop header! Start a new TraversalContext...");
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self.context_stack.push(TraversalContext {
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loop_header: Some(bcb),
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worklist: VecDeque::new(),
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});
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}
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self.add_successors_to_worklists(bcb);
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return Some(bcb);
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} else {
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// Strip contexts with empty worklists from the top of the stack
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self.context_stack.pop();
|
|
}
|
|
}
|
|
|
|
None
|
|
}
|
|
|
|
pub fn add_successors_to_worklists(&mut self, bcb: BasicCoverageBlock) {
|
|
let successors = &self.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.
|
|
// FIXME: This claims to skip just the self-successor, but it actually skips
|
|
// all other successors as well. Does that matter?
|
|
break;
|
|
}
|
|
|
|
// 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 context = self
|
|
.context_stack
|
|
.iter_mut()
|
|
.rev()
|
|
.find(|context| match context.loop_header {
|
|
Some(loop_header) => {
|
|
self.basic_coverage_blocks.dominates(loop_header, successor)
|
|
}
|
|
None => true,
|
|
})
|
|
.unwrap_or_else(|| bug!("should always fall back to the root non-loop context"));
|
|
debug!("adding to worklist for {:?}", context.loop_header);
|
|
|
|
// FIXME: The code below had debug messages claiming to add items to a
|
|
// particular end of the worklist, but was confused about which end was
|
|
// which. The existing behaviour has been preserved for now, but it's
|
|
// unclear what the intended behaviour was.
|
|
|
|
if self.basic_coverage_blocks.successors[successor].len() > 1 {
|
|
context.worklist.push_back(successor);
|
|
} else {
|
|
context.worklist.push_front(successor);
|
|
}
|
|
}
|
|
}
|
|
|
|
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.
|
|
for (bcb, _) in basic_coverage_blocks.iter_enumerated() {
|
|
for &successor in &basic_coverage_blocks.successors[bcb] {
|
|
if basic_coverage_blocks.dominates(successor, bcb) {
|
|
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
|
|
}
|
|
|
|
fn short_circuit_preorder<'a, 'tcx, F, Iter>(
|
|
body: &'a mir::Body<'tcx>,
|
|
filtered_successors: F,
|
|
) -> impl Iterator<Item = BasicBlock> + Captures<'a> + Captures<'tcx>
|
|
where
|
|
F: Fn(BasicBlock) -> Iter,
|
|
Iter: Iterator<Item = BasicBlock>,
|
|
{
|
|
let mut visited = BitSet::new_empty(body.basic_blocks.len());
|
|
let mut worklist = vec![mir::START_BLOCK];
|
|
|
|
std::iter::from_fn(move || {
|
|
while let Some(bb) = worklist.pop() {
|
|
if !visited.insert(bb) {
|
|
continue;
|
|
}
|
|
|
|
worklist.extend(filtered_successors(bb));
|
|
|
|
return Some(bb);
|
|
}
|
|
|
|
None
|
|
})
|
|
}
|