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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::{IndexSlice, IndexVec};
use rustc_middle::mir::{self, BasicBlock, BasicBlockData, Terminator, TerminatorKind};
use std::cmp::Ordering;
use std::ops::{Index, IndexMut};
/// 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(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 mut seen = IndexVec::from_elem(false, &bcbs);
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(Vec::new(), &bcbs);
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 predecessors: {:?}", bb.index(), predecessors)
}
);
}
}
basic_blocks.push(bb);
let term = data.terminator();
match term.kind {
TerminatorKind::Return { .. }
| TerminatorKind::UnwindTerminate(_)
| 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`, `Terminate`) 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::UnwindResume
| TerminatorKind::Unreachable
| TerminatorKind::Drop { .. }
| 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 IndexSlice<BasicBlock, Option<BasicCoverageBlock>>,
basic_blocks: Vec<BasicBlock>,
) {
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);
}
#[inline(always)]
pub fn iter_enumerated(
&self,
) -> impl Iterator<Item = (BasicCoverageBlock, &BasicCoverageBlockData)> {
self.bcbs.iter_enumerated()
}
#[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 dominates(&self, dom: BasicCoverageBlock, node: BasicCoverageBlock) -> bool {
self.dominators.as_ref().unwrap().dominates(dom, node)
}
#[inline(always)]
pub fn cmp_in_dominator_order(&self, a: BasicCoverageBlock, b: BasicCoverageBlock) -> Ordering {
self.dominators.as_ref().unwrap().cmp_in_dominator_order(a, b)
}
}
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.
#[debug_format = "bcb{}"]
pub(super) 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(super) struct BasicCoverageBlockData {
pub basic_blocks: Vec<BasicBlock>,
}
impl BasicCoverageBlockData {
pub fn from(basic_blocks: Vec<BasicBlock>) -> Self {
assert!(basic_blocks.len() > 0);
Self { basic_blocks }
}
#[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()
}
}
/// 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 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(context) = self.context_stack.last_mut() {
if let Some(next_bcb) = 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);
} else {
// Strip contexts with empty worklists from the top of the stack
self.context_stack.pop();
}
}
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.dominates(loop_header, successor) {
(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.
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
}
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))
}
}