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use std::cmp::Ordering;
use std::collections::VecDeque;
use std::ops::{Index, IndexMut};
use rustc_data_structures::captures::Captures;
use rustc_data_structures::fx::FxHashSet;
use rustc_data_structures::graph::dominators::{self, Dominators};
use rustc_data_structures::graph::{self, DirectedGraph, StartNode};
use rustc_index::bit_set::BitSet;
use rustc_index::IndexVec;
use rustc_middle::bug;
use rustc_middle::mir::{self, BasicBlock, Terminator, TerminatorKind};
use tracing::debug;
/// 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.
#[derive(Debug)]
pub(crate) struct CoverageGraph {
bcbs: IndexVec<BasicCoverageBlock, BasicCoverageBlockData>,
bb_to_bcb: IndexVec<BasicBlock, Option<BasicCoverageBlock>>,
pub(crate) successors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
pub(crate) predecessors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
dominators: Option<Dominators<BasicCoverageBlock>>,
}
impl CoverageGraph {
pub(crate) 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 successors = IndexVec::from_fn_n(
|bcb| {
let mut seen_bcbs = FxHashSet::default();
let terminator = mir_body[bcbs[bcb].last_bb()].terminator();
bcb_filtered_successors(terminator)
.into_iter()
.filter_map(|successor_bb| bb_to_bcb[successor_bb])
// Remove duplicate successor BCBs, keeping only the first.
.filter(|&successor_bcb| seen_bcbs.insert(successor_bcb))
.collect::<Vec<_>>()
},
bcbs.len(),
);
let mut predecessors = IndexVec::from_elem(Vec::new(), &bcbs);
for (bcb, bcb_successors) in successors.iter_enumerated() {
for &successor in bcb_successors {
predecessors[successor].push(bcb);
}
}
let mut this = Self { bcbs, bb_to_bcb, successors, predecessors, dominators: None };
this.dominators = Some(dominators::dominators(&this));
// The coverage graph's entry-point node (bcb0) always starts with bb0,
// which never has predecessors. Any other blocks merged into bcb0 can't
// have multiple (coverage-relevant) predecessors, so bcb0 always has
// zero in-edges.
assert!(this[START_BCB].leader_bb() == mir::START_BLOCK);
assert!(this.predecessors[START_BCB].is_empty());
this
}
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::<BasicCoverageBlock, _>::with_capacity(num_basic_blocks);
let mut bb_to_bcb = IndexVec::from_elem_n(None, num_basic_blocks);
let mut add_basic_coverage_block = |basic_blocks: &mut Vec<BasicBlock>| {
// Take the accumulated list of blocks, leaving the vector empty
// to be used by subsequent BCBs.
let basic_blocks = std::mem::take(basic_blocks);
let bcb = bcbs.next_index();
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);
};
// 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.
// Accumulates a chain of blocks that will be combined into one BCB.
let mut basic_blocks = Vec::new();
let filtered_successors = |bb| bcb_filtered_successors(mir_body[bb].terminator());
for bb in short_circuit_preorder(mir_body, filtered_successors)
.filter(|&bb| mir_body[bb].terminator().kind != TerminatorKind::Unreachable)
{
// If the previous block can't be chained into `bb`, flush the accumulated
// blocks into a new BCB, then start building the next chain.
if let Some(&prev) = basic_blocks.last()
&& (!filtered_successors(prev).is_chainable() || {
// If `bb` has multiple predecessor blocks, or `prev` isn't
// one of its predecessors, we can't chain and must flush.
let predecessors = &mir_body.basic_blocks.predecessors()[bb];
predecessors.len() > 1 || !predecessors.contains(&prev)
})
{
debug!(
terminator_kind = ?mir_body[prev].terminator().kind,
predecessors = ?&mir_body.basic_blocks.predecessors()[bb],
"can't chain from {prev:?} to {bb:?}"
);
add_basic_coverage_block(&mut basic_blocks);
}
basic_blocks.push(bb);
}
if !basic_blocks.is_empty() {
debug!("flushing accumulated blocks into one last BCB");
add_basic_coverage_block(&mut basic_blocks);
}
(bcbs, bb_to_bcb)
}
#[inline(always)]
pub(crate) fn iter_enumerated(
&self,
) -> impl Iterator<Item = (BasicCoverageBlock, &BasicCoverageBlockData)> {
self.bcbs.iter_enumerated()
}
#[inline(always)]
pub(crate) 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(crate) fn dominates(&self, dom: BasicCoverageBlock, node: BasicCoverageBlock) -> bool {
self.dominators.as_ref().unwrap().dominates(dom, node)
}
#[inline(always)]
pub(crate) fn cmp_in_dominator_order(
&self,
a: BasicCoverageBlock,
b: BasicCoverageBlock,
) -> Ordering {
self.dominators.as_ref().unwrap().cmp_in_dominator_order(a, b)
}
/// Returns true if the given node has 2 or more in-edges, i.e. 2 or more
/// predecessors.
///
/// This property is interesting to code that assigns counters to nodes and
/// edges, because if a node _doesn't_ have multiple in-edges, then there's
/// no benefit in having a separate counter for its in-edge, because it
/// would have the same value as the node's own counter.
///
/// FIXME: That assumption might not be true for [`TerminatorKind::Yield`]?
#[inline(always)]
pub(crate) fn bcb_has_multiple_in_edges(&self, bcb: BasicCoverageBlock) -> bool {
// Even though bcb0 conceptually has an extra virtual in-edge due to
// being the entry point, we've already asserted that it has no _other_
// in-edges, so there's no possibility of it having _multiple_ in-edges.
// (And since its virtual in-edge doesn't exist in the graph, that edge
// can't have a separate counter anyway.)
self.predecessors[bcb].len() > 1
}
}
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;
#[inline]
fn num_nodes(&self) -> usize {
self.bcbs.len()
}
}
impl graph::StartNode 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")
}
}
impl graph::Successors for CoverageGraph {
#[inline]
fn successors(&self, node: Self::Node) -> impl Iterator<Item = Self::Node> {
self.successors[node].iter().copied()
}
}
impl graph::Predecessors for CoverageGraph {
#[inline]
fn predecessors(&self, node: Self::Node) -> impl Iterator<Item = Self::Node> {
self.predecessors[node].iter().copied()
}
}
rustc_index::newtype_index! {
/// A node in the control-flow graph of CoverageGraph.
#[orderable]
#[debug_format = "bcb{}"]
pub(crate) struct BasicCoverageBlock {
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 coverage span will have no more than one `Counter`.
/// In some cases, a BCB's execution count can be computed by `Expression`. Additional
/// disjoint coverage spans 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 (`dominates()`, `predecessors`, `successors`, etc.) have branch (control flow)
/// significance.
#[derive(Debug, Clone)]
pub(crate) struct BasicCoverageBlockData {
pub(crate) basic_blocks: Vec<BasicBlock>,
}
impl BasicCoverageBlockData {
fn from(basic_blocks: Vec<BasicBlock>) -> Self {
assert!(basic_blocks.len() > 0);
Self { basic_blocks }
}
#[inline(always)]
pub(crate) fn leader_bb(&self) -> BasicBlock {
self.basic_blocks[0]
}
#[inline(always)]
pub(crate) fn last_bb(&self) -> BasicBlock {
*self.basic_blocks.last().unwrap()
}
}
/// Holds the coverage-relevant successors of a basic block's terminator, and
/// indicates whether that block can potentially be combined into the same BCB
/// as its sole successor.
#[derive(Clone, Copy, Debug)]
enum CoverageSuccessors<'a> {
/// The terminator has exactly one straight-line successor, so its block can
/// potentially be combined into the same BCB as that successor.
Chainable(BasicBlock),
/// The block cannot be combined into the same BCB as its successor(s).
NotChainable(&'a [BasicBlock]),
}
impl CoverageSuccessors<'_> {
fn is_chainable(&self) -> bool {
match self {
Self::Chainable(_) => true,
Self::NotChainable(_) => false,
}
}
}
impl IntoIterator for CoverageSuccessors<'_> {
type Item = BasicBlock;
type IntoIter = impl DoubleEndedIterator<Item = Self::Item>;
fn into_iter(self) -> Self::IntoIter {
match self {
Self::Chainable(bb) => Some(bb).into_iter().chain((&[]).iter().copied()),
Self::NotChainable(bbs) => None.into_iter().chain(bbs.iter().copied()),
}
}
}
// Returns the subset of a block's successors that are relevant to the coverage
// graph, i.e. those that do not represent unwinds or false edges.
// FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and
// `catch_unwind()` handlers.
fn bcb_filtered_successors<'a, 'tcx>(terminator: &'a Terminator<'tcx>) -> CoverageSuccessors<'a> {
use TerminatorKind::*;
match terminator.kind {
// A switch terminator can have many coverage-relevant successors.
// (If there is exactly one successor, we still treat it as not chainable.)
SwitchInt { ref targets, .. } => CoverageSuccessors::NotChainable(targets.all_targets()),
// A yield terminator has exactly 1 successor, but should not be chained,
// because its resume edge has a different execution count.
Yield { ref resume, .. } => CoverageSuccessors::NotChainable(std::slice::from_ref(resume)),
// These terminators have exactly one coverage-relevant successor,
// and can be chained into it.
Assert { target, .. }
| Drop { target, .. }
| FalseEdge { real_target: target, .. }
| FalseUnwind { real_target: target, .. }
| Goto { target } => CoverageSuccessors::Chainable(target),
// A call terminator can normally be chained, except when they have no
// successor because they are known to diverge.
Call { target: maybe_target, .. } => match maybe_target {
Some(target) => CoverageSuccessors::Chainable(target),
None => CoverageSuccessors::NotChainable(&[]),
},
// An inline asm terminator can normally be chained, except when it diverges or uses asm
// goto.
InlineAsm { ref targets, .. } => {
if let [target] = targets[..] {
CoverageSuccessors::Chainable(target)
} else {
CoverageSuccessors::NotChainable(targets)
}
}
// These terminators have no coverage-relevant successors.
CoroutineDrop
| Return
| TailCall { .. }
| Unreachable
| UnwindResume
| UnwindTerminate(_) => CoverageSuccessors::NotChainable(&[]),
}
}
/// 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)]
struct TraversalContext {
/// BCB with one or more incoming loop backedges, indicating which loop
/// this context is for.
///
/// If `None`, this is the non-loop context for the function as a whole.
loop_header: Option<BasicCoverageBlock>,
/// Worklist of BCBs to be processed in this context.
worklist: VecDeque<BasicCoverageBlock>,
}
pub(crate) struct TraverseCoverageGraphWithLoops<'a> {
basic_coverage_blocks: &'a CoverageGraph,
backedges: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
context_stack: Vec<TraversalContext>,
visited: BitSet<BasicCoverageBlock>,
}
impl<'a> TraverseCoverageGraphWithLoops<'a> {
pub(crate) fn new(basic_coverage_blocks: &'a CoverageGraph) -> Self {
let backedges = find_loop_backedges(basic_coverage_blocks);
let worklist = VecDeque::from([basic_coverage_blocks.start_node()]);
let context_stack = vec![TraversalContext { loop_header: None, worklist }];
// `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 { basic_coverage_blocks, backedges, context_stack, visited }
}
/// For each loop on the loop context stack (top-down), yields a list of BCBs
/// within that loop that have an outgoing edge back to the loop header.
pub(crate) fn reloop_bcbs_per_loop(&self) -> impl Iterator<Item = &[BasicCoverageBlock]> {
self.context_stack
.iter()
.rev()
.filter_map(|context| context.loop_header)
.map(|header_bcb| self.backedges[header_bcb].as_slice())
}
pub(crate) fn next(&mut self) -> Option<BasicCoverageBlock> {
debug!(
"TraverseCoverageGraphWithLoops::next - context_stack: {:?}",
self.context_stack.iter().rev().collect::<Vec<_>>()
);
while let Some(context) = self.context_stack.last_mut() {
let Some(bcb) = context.worklist.pop_front() else {
// This stack level is exhausted; pop it and try the next one.
self.context_stack.pop();
continue;
};
if !self.visited.insert(bcb) {
debug!("Already visited: {bcb:?}");
continue;
}
debug!("Visiting {bcb:?}");
if self.backedges[bcb].len() > 0 {
debug!("{bcb:?} is a loop header! Start a new TraversalContext...");
self.context_stack
.push(TraversalContext { loop_header: Some(bcb), worklist: VecDeque::new() });
}
self.add_successors_to_worklists(bcb);
return Some(bcb);
}
None
}
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(crate) fn is_complete(&self) -> bool {
self.visited.count() == self.visited.domain_size()
}
pub(crate) 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<_>>()
}
}
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: IntoIterator<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
})
}