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use std::fmt::{self, Debug};
use rustc_data_structures::captures::Captures;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::graph::DirectedGraph;
use rustc_index::IndexVec;
use rustc_middle::bug;
use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};
use tracing::{debug, debug_span, instrument};
use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, TraverseCoverageGraphWithLoops};
/// The coverage counter or counter expression associated with a particular
/// BCB node or BCB edge.
#[derive(Clone, Copy, PartialEq, Eq, Hash)]
pub(super) enum BcbCounter {
Counter { id: CounterId },
Expression { id: ExpressionId },
}
impl BcbCounter {
pub(super) fn as_term(&self) -> CovTerm {
match *self {
BcbCounter::Counter { id, .. } => CovTerm::Counter(id),
BcbCounter::Expression { id, .. } => CovTerm::Expression(id),
}
}
}
impl Debug for BcbCounter {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
Self::Counter { id, .. } => write!(fmt, "Counter({:?})", id.index()),
Self::Expression { id } => write!(fmt, "Expression({:?})", id.index()),
}
}
}
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
struct BcbExpression {
lhs: BcbCounter,
op: Op,
rhs: BcbCounter,
}
#[derive(Debug)]
pub(super) enum CounterIncrementSite {
Node { bcb: BasicCoverageBlock },
Edge { from_bcb: BasicCoverageBlock, to_bcb: BasicCoverageBlock },
}
/// Generates and stores coverage counter and coverage expression information
/// associated with nodes/edges in the BCB graph.
pub(super) struct CoverageCounters {
/// List of places where a counter-increment statement should be injected
/// into MIR, each with its corresponding counter ID.
counter_increment_sites: IndexVec<CounterId, CounterIncrementSite>,
/// Coverage counters/expressions that are associated with individual BCBs.
bcb_counters: IndexVec<BasicCoverageBlock, Option<BcbCounter>>,
/// Coverage counters/expressions that are associated with the control-flow
/// edge between two BCBs.
///
/// We currently don't iterate over this map, but if we do in the future,
/// switch it back to `FxIndexMap` to avoid query stability hazards.
bcb_edge_counters: FxHashMap<(BasicCoverageBlock, BasicCoverageBlock), BcbCounter>,
/// Table of expression data, associating each expression ID with its
/// corresponding operator (+ or -) and its LHS/RHS operands.
expressions: IndexVec<ExpressionId, BcbExpression>,
/// Remember expressions that have already been created (or simplified),
/// so that we don't create unnecessary duplicates.
expressions_memo: FxHashMap<BcbExpression, BcbCounter>,
}
impl CoverageCounters {
/// Makes [`BcbCounter`] `Counter`s and `Expressions` for the `BasicCoverageBlock`s directly or
/// indirectly associated with coverage spans, and accumulates additional `Expression`s
/// representing intermediate values.
pub(super) fn make_bcb_counters(
basic_coverage_blocks: &CoverageGraph,
bcb_has_coverage_spans: impl Fn(BasicCoverageBlock) -> bool,
) -> Self {
let num_bcbs = basic_coverage_blocks.num_nodes();
let mut this = Self {
counter_increment_sites: IndexVec::new(),
bcb_counters: IndexVec::from_elem_n(None, num_bcbs),
bcb_edge_counters: FxHashMap::default(),
expressions: IndexVec::new(),
expressions_memo: FxHashMap::default(),
};
MakeBcbCounters::new(&mut this, basic_coverage_blocks)
.make_bcb_counters(bcb_has_coverage_spans);
this
}
fn make_counter(&mut self, site: CounterIncrementSite) -> BcbCounter {
let id = self.counter_increment_sites.push(site);
BcbCounter::Counter { id }
}
fn make_expression(&mut self, lhs: BcbCounter, op: Op, rhs: BcbCounter) -> BcbCounter {
let new_expr = BcbExpression { lhs, op, rhs };
*self
.expressions_memo
.entry(new_expr)
.or_insert_with(|| Self::make_expression_inner(&mut self.expressions, new_expr))
}
/// This is an associated function so that we can call it while borrowing
/// `&mut self.expressions_memo`.
fn make_expression_inner(
expressions: &mut IndexVec<ExpressionId, BcbExpression>,
new_expr: BcbExpression,
) -> BcbCounter {
// Simplify expressions using basic algebra.
//
// Some of these cases might not actually occur in practice, depending
// on the details of how the instrumentor builds expressions.
let BcbExpression { lhs, op, rhs } = new_expr;
if let BcbCounter::Expression { id } = lhs {
let lhs_expr = &expressions[id];
// Simplify `(a - b) + b` to `a`.
if lhs_expr.op == Op::Subtract && op == Op::Add && lhs_expr.rhs == rhs {
return lhs_expr.lhs;
}
// Simplify `(a + b) - b` to `a`.
if lhs_expr.op == Op::Add && op == Op::Subtract && lhs_expr.rhs == rhs {
return lhs_expr.lhs;
}
// Simplify `(a + b) - a` to `b`.
if lhs_expr.op == Op::Add && op == Op::Subtract && lhs_expr.lhs == rhs {
return lhs_expr.rhs;
}
}
if let BcbCounter::Expression { id } = rhs {
let rhs_expr = &expressions[id];
// Simplify `a + (b - a)` to `b`.
if op == Op::Add && rhs_expr.op == Op::Subtract && lhs == rhs_expr.rhs {
return rhs_expr.lhs;
}
// Simplify `a - (a - b)` to `b`.
if op == Op::Subtract && rhs_expr.op == Op::Subtract && lhs == rhs_expr.lhs {
return rhs_expr.rhs;
}
}
// Simplification failed, so actually create the new expression.
let id = expressions.push(new_expr);
BcbCounter::Expression { id }
}
/// Variant of `make_expression` that makes `lhs` optional and assumes [`Op::Add`].
///
/// This is useful when using [`Iterator::fold`] to build an arbitrary-length sum.
fn make_sum_expression(&mut self, lhs: Option<BcbCounter>, rhs: BcbCounter) -> BcbCounter {
let Some(lhs) = lhs else { return rhs };
self.make_expression(lhs, Op::Add, rhs)
}
pub(super) fn num_counters(&self) -> usize {
self.counter_increment_sites.len()
}
fn set_bcb_counter(&mut self, bcb: BasicCoverageBlock, counter_kind: BcbCounter) -> BcbCounter {
if let Some(replaced) = self.bcb_counters[bcb].replace(counter_kind) {
bug!(
"attempt to set a BasicCoverageBlock coverage counter more than once; \
{bcb:?} already had counter {replaced:?}",
);
} else {
counter_kind
}
}
fn set_bcb_edge_counter(
&mut self,
from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock,
counter_kind: BcbCounter,
) -> BcbCounter {
if let Some(replaced) = self.bcb_edge_counters.insert((from_bcb, to_bcb), counter_kind) {
bug!(
"attempt to set an edge counter more than once; from_bcb: \
{from_bcb:?} already had counter {replaced:?}",
);
} else {
counter_kind
}
}
pub(super) fn bcb_counter(&self, bcb: BasicCoverageBlock) -> Option<BcbCounter> {
self.bcb_counters[bcb]
}
/// Returns an iterator over all the nodes/edges in the coverage graph that
/// should have a counter-increment statement injected into MIR, along with
/// each site's corresponding counter ID.
pub(super) fn counter_increment_sites(
&self,
) -> impl Iterator<Item = (CounterId, &CounterIncrementSite)> {
self.counter_increment_sites.iter_enumerated()
}
/// Returns an iterator over the subset of BCB nodes that have been associated
/// with a counter *expression*, along with the ID of that expression.
pub(super) fn bcb_nodes_with_coverage_expressions(
&self,
) -> impl Iterator<Item = (BasicCoverageBlock, ExpressionId)> + Captures<'_> {
self.bcb_counters.iter_enumerated().filter_map(|(bcb, &counter_kind)| match counter_kind {
// Yield the BCB along with its associated expression ID.
Some(BcbCounter::Expression { id }) => Some((bcb, id)),
// This BCB is associated with a counter or nothing, so skip it.
Some(BcbCounter::Counter { .. }) | None => None,
})
}
pub(super) fn into_expressions(self) -> IndexVec<ExpressionId, Expression> {
let old_len = self.expressions.len();
let expressions = self
.expressions
.into_iter()
.map(|BcbExpression { lhs, op, rhs }| Expression {
lhs: lhs.as_term(),
op,
rhs: rhs.as_term(),
})
.collect::<IndexVec<ExpressionId, _>>();
// Expression IDs are indexes into this vector, so make sure we didn't
// accidentally invalidate them by changing its length.
assert_eq!(old_len, expressions.len());
expressions
}
}
/// Traverse the `CoverageGraph` and add either a `Counter` or `Expression` to every BCB, to be
/// injected with coverage spans. `Expressions` have no runtime overhead, so if a viable expression
/// (adding or subtracting two other counters or expressions) can compute the same result as an
/// embedded counter, an `Expression` should be used.
struct MakeBcbCounters<'a> {
coverage_counters: &'a mut CoverageCounters,
basic_coverage_blocks: &'a CoverageGraph,
}
impl<'a> MakeBcbCounters<'a> {
fn new(
coverage_counters: &'a mut CoverageCounters,
basic_coverage_blocks: &'a CoverageGraph,
) -> Self {
Self { coverage_counters, basic_coverage_blocks }
}
/// If two `BasicCoverageBlock`s branch from another `BasicCoverageBlock`, one of the branches
/// can be counted by `Expression` by subtracting the other branch from the branching
/// block. Otherwise, the `BasicCoverageBlock` executed the least should have the `Counter`.
/// One way to predict which branch executes the least is by considering loops. A loop is exited
/// at a branch, so the branch that jumps to a `BasicCoverageBlock` outside the loop is almost
/// always executed less than the branch that does not exit the loop.
fn make_bcb_counters(&mut self, bcb_has_coverage_spans: impl Fn(BasicCoverageBlock) -> bool) {
debug!("make_bcb_counters(): adding a counter or expression to each BasicCoverageBlock");
// Walk the `CoverageGraph`. For each `BasicCoverageBlock` node with an associated
// coverage span, add a counter. If the `BasicCoverageBlock` branches, add a counter or
// expression to each branch `BasicCoverageBlock` (if the branch BCB has only one incoming
// edge) or edge from the branching BCB to the branch BCB (if the branch BCB has multiple
// incoming edges).
//
// The `TraverseCoverageGraphWithLoops` traversal ensures that, when a loop is encountered,
// all `BasicCoverageBlock` nodes in the loop are visited before visiting any node outside
// the loop. The `traversal` state includes a `context_stack`, providing a way to know if
// the current BCB is in one or more nested loops or not.
let mut traversal = TraverseCoverageGraphWithLoops::new(self.basic_coverage_blocks);
while let Some(bcb) = traversal.next() {
if bcb_has_coverage_spans(bcb) {
debug!("{:?} has at least one coverage span. Get or make its counter", bcb);
self.make_node_and_branch_counters(&traversal, bcb);
} else {
debug!(
"{:?} does not have any coverage spans. A counter will only be added if \
and when a covered BCB has an expression dependency.",
bcb,
);
}
}
assert!(
traversal.is_complete(),
"`TraverseCoverageGraphWithLoops` missed some `BasicCoverageBlock`s: {:?}",
traversal.unvisited(),
);
}
fn make_node_and_branch_counters(
&mut self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock,
) {
// First, ensure that this node has a counter of some kind.
// We might also use its term later to compute one of the branch counters.
let from_bcb_operand = self.get_or_make_counter_operand(from_bcb);
let branch_target_bcbs = self.basic_coverage_blocks.successors[from_bcb].as_slice();
// If this node doesn't have multiple out-edges, or all of its out-edges
// already have counters, then we don't need to create edge counters.
let needs_branch_counters = branch_target_bcbs.len() > 1
&& branch_target_bcbs
.iter()
.any(|&to_bcb| self.branch_has_no_counter(from_bcb, to_bcb));
if !needs_branch_counters {
return;
}
debug!(
"{from_bcb:?} has some branch(es) without counters:\n {}",
branch_target_bcbs
.iter()
.map(|&to_bcb| {
format!("{from_bcb:?}->{to_bcb:?}: {:?}", self.branch_counter(from_bcb, to_bcb))
})
.collect::<Vec<_>>()
.join("\n "),
);
// Of the branch edges that don't have counters yet, one can be given an expression
// (computed from the other edges) instead of a dedicated counter.
let expression_to_bcb = self.choose_preferred_expression_branch(traversal, from_bcb);
// For each branch arm other than the one that was chosen to get an expression,
// ensure that it has a counter (existing counter/expression or a new counter),
// and accumulate the corresponding terms into a single sum term.
let sum_of_all_other_branches: BcbCounter = {
let _span = debug_span!("sum_of_all_other_branches", ?expression_to_bcb).entered();
branch_target_bcbs
.iter()
.copied()
// Skip the chosen branch, since we'll calculate it from the other branches.
.filter(|&to_bcb| to_bcb != expression_to_bcb)
.fold(None, |accum, to_bcb| {
let _span = debug_span!("to_bcb", ?accum, ?to_bcb).entered();
let branch_counter = self.get_or_make_edge_counter_operand(from_bcb, to_bcb);
Some(self.coverage_counters.make_sum_expression(accum, branch_counter))
})
.expect("there must be at least one other branch")
};
// For the branch that was chosen to get an expression, create that expression
// by taking the count of the node we're branching from, and subtracting the
// sum of all the other branches.
debug!(
"Making an expression for the selected expression_branch: \
{expression_to_bcb:?} (expression_branch predecessors: {:?})",
self.bcb_predecessors(expression_to_bcb),
);
let expression = self.coverage_counters.make_expression(
from_bcb_operand,
Op::Subtract,
sum_of_all_other_branches,
);
debug!("{expression_to_bcb:?} gets an expression: {expression:?}");
if self.basic_coverage_blocks.bcb_has_multiple_in_edges(expression_to_bcb) {
self.coverage_counters.set_bcb_edge_counter(from_bcb, expression_to_bcb, expression);
} else {
self.coverage_counters.set_bcb_counter(expression_to_bcb, expression);
}
}
#[instrument(level = "debug", skip(self))]
fn get_or_make_counter_operand(&mut self, bcb: BasicCoverageBlock) -> BcbCounter {
// If the BCB already has a counter, return it.
if let Some(counter_kind) = self.coverage_counters.bcb_counters[bcb] {
debug!("{bcb:?} already has a counter: {counter_kind:?}");
return counter_kind;
}
// A BCB with only one incoming edge gets a simple `Counter` (via `make_counter()`).
// Also, a BCB that loops back to itself gets a simple `Counter`. This may indicate the
// program results in a tight infinite loop, but it should still compile.
let one_path_to_target = !self.basic_coverage_blocks.bcb_has_multiple_in_edges(bcb);
if one_path_to_target || self.bcb_predecessors(bcb).contains(&bcb) {
let counter_kind =
self.coverage_counters.make_counter(CounterIncrementSite::Node { bcb });
if one_path_to_target {
debug!("{bcb:?} gets a new counter: {counter_kind:?}");
} else {
debug!(
"{bcb:?} has itself as its own predecessor. It can't be part of its own \
Expression sum, so it will get its own new counter: {counter_kind:?}. \
(Note, the compiled code will generate an infinite loop.)",
);
}
return self.coverage_counters.set_bcb_counter(bcb, counter_kind);
}
// A BCB with multiple incoming edges can compute its count by ensuring that counters
// exist for each of those edges, and then adding them up to get a total count.
let sum_of_in_edges: BcbCounter = {
let _span = debug_span!("sum_of_in_edges", ?bcb).entered();
// We avoid calling `self.bcb_predecessors` here so that we can
// call methods on `&mut self` inside the fold.
self.basic_coverage_blocks.predecessors[bcb]
.iter()
.copied()
.fold(None, |accum, from_bcb| {
let _span = debug_span!("from_bcb", ?accum, ?from_bcb).entered();
let edge_counter = self.get_or_make_edge_counter_operand(from_bcb, bcb);
Some(self.coverage_counters.make_sum_expression(accum, edge_counter))
})
.expect("there must be at least one in-edge")
};
debug!("{bcb:?} gets a new counter (sum of predecessor counters): {sum_of_in_edges:?}");
self.coverage_counters.set_bcb_counter(bcb, sum_of_in_edges)
}
#[instrument(level = "debug", skip(self))]
fn get_or_make_edge_counter_operand(
&mut self,
from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock,
) -> BcbCounter {
// If the target BCB has only one in-edge (i.e. this one), then create
// a node counter instead, since it will have the same value.
if !self.basic_coverage_blocks.bcb_has_multiple_in_edges(to_bcb) {
assert_eq!([from_bcb].as_slice(), self.basic_coverage_blocks.predecessors[to_bcb]);
return self.get_or_make_counter_operand(to_bcb);
}
// If the source BCB has only one successor (assumed to be the given target), an edge
// counter is unnecessary. Just get or make a counter for the source BCB.
if self.bcb_successors(from_bcb).len() == 1 {
return self.get_or_make_counter_operand(from_bcb);
}
// If the edge already has a counter, return it.
if let Some(&counter_kind) =
self.coverage_counters.bcb_edge_counters.get(&(from_bcb, to_bcb))
{
debug!("Edge {from_bcb:?}->{to_bcb:?} already has a counter: {counter_kind:?}");
return counter_kind;
}
// Make a new counter to count this edge.
let counter_kind =
self.coverage_counters.make_counter(CounterIncrementSite::Edge { from_bcb, to_bcb });
debug!("Edge {from_bcb:?}->{to_bcb:?} gets a new counter: {counter_kind:?}");
self.coverage_counters.set_bcb_edge_counter(from_bcb, to_bcb, counter_kind)
}
/// Select a branch for the expression, either the recommended `reloop_branch`, or if none was
/// found, select any branch.
fn choose_preferred_expression_branch(
&self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock,
) -> BasicCoverageBlock {
let good_reloop_branch = self.find_good_reloop_branch(traversal, from_bcb);
if let Some(reloop_target) = good_reloop_branch {
assert!(self.branch_has_no_counter(from_bcb, reloop_target));
debug!("Selecting reloop target {reloop_target:?} to get an expression");
reloop_target
} else {
let &branch_without_counter = self
.bcb_successors(from_bcb)
.iter()
.find(|&&to_bcb| self.branch_has_no_counter(from_bcb, to_bcb))
.expect(
"needs_branch_counters was `true` so there should be at least one \
branch",
);
debug!(
"Selecting any branch={:?} that still needs a counter, to get the \
`Expression` because there was no `reloop_branch`, or it already had a \
counter",
branch_without_counter
);
branch_without_counter
}
}
/// Tries to find a branch that leads back to the top of a loop, and that
/// doesn't already have a counter. Such branches are good candidates to
/// be given an expression (instead of a physical counter), because they
/// will tend to be executed more times than a loop-exit branch.
fn find_good_reloop_branch(
&self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
from_bcb: BasicCoverageBlock,
) -> Option<BasicCoverageBlock> {
let branch_target_bcbs = self.bcb_successors(from_bcb);
// Consider each loop on the current traversal context stack, top-down.
for reloop_bcbs in traversal.reloop_bcbs_per_loop() {
let mut all_branches_exit_this_loop = true;
// Try to find a branch that doesn't exit this loop and doesn't
// already have a counter.
for &branch_target_bcb in branch_target_bcbs {
// A branch is a reloop branch if it dominates any BCB that has
// an edge back to the loop header. (Other branches are exits.)
let is_reloop_branch = reloop_bcbs.iter().any(|&reloop_bcb| {
self.basic_coverage_blocks.dominates(branch_target_bcb, reloop_bcb)
});
if is_reloop_branch {
all_branches_exit_this_loop = false;
if self.branch_has_no_counter(from_bcb, branch_target_bcb) {
// We found a good branch to be given an expression.
return Some(branch_target_bcb);
}
// Keep looking for another reloop branch without a counter.
} else {
// This branch exits the loop.
}
}
if !all_branches_exit_this_loop {
// We found one or more reloop branches, but all of them already
// have counters. Let the caller choose one of the exit branches.
debug!("All reloop branches had counters; skip checking the other loops");
return None;
}
// All of the branches exit this loop, so keep looking for a good
// reloop branch for one of the outer loops.
}
None
}
#[inline]
fn bcb_predecessors(&self, bcb: BasicCoverageBlock) -> &[BasicCoverageBlock] {
&self.basic_coverage_blocks.predecessors[bcb]
}
#[inline]
fn bcb_successors(&self, bcb: BasicCoverageBlock) -> &[BasicCoverageBlock] {
&self.basic_coverage_blocks.successors[bcb]
}
#[inline]
fn branch_has_no_counter(
&self,
from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock,
) -> bool {
self.branch_counter(from_bcb, to_bcb).is_none()
}
fn branch_counter(
&self,
from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock,
) -> Option<&BcbCounter> {
if self.basic_coverage_blocks.bcb_has_multiple_in_edges(to_bcb) {
self.coverage_counters.bcb_edge_counters.get(&(from_bcb, to_bcb))
} else {
self.coverage_counters.bcb_counters[to_bcb].as_ref()
}
}
}