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Rollup merge of #117651 - Zalathar:fold-sums, r=cjgillot

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coverage: Simplify building coverage expressions based on sums

This is a combination of some interlinked changes to the code that creates coverage counters/expressions for nodes and edges in the coverage graph:

- Some preparatory cleanups in `MakeBcbCounters::make_branch_counters`
- Use `BcbCounter` (instead of `CovTerm`) when building coverage expressions
  - This makes it easier to introduce a fold for building sums
- Simplify the creation of coverage expressions based on sums, by having `Iterator::fold` do much of the work
- Get rid of the awkward `BcbBranch` enum, and replace it with graph edges represented as `(from_bcb, to_bcb)`
  - This further simplifies the body of the fold
This commit is contained in:
Michael Goulet 2023-11-25 17:23:32 -05:00 • committed by GitHub
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307
compiler/rustc_mir_transform/src/coverage/counters.rs
View file

@ -1,18 +1,16 @@
use super::graph;
use graph::{BasicCoverageBlock, BcbBranch, CoverageGraph, TraverseCoverageGraphWithLoops};
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::graph::WithNumNodes;
use rustc_index::bit_set::BitSet;
use rustc_index::IndexVec;
use rustc_middle::mir::coverage::*;
use super::graph::{BasicCoverageBlock, CoverageGraph, TraverseCoverageGraphWithLoops};
use std::fmt::{self, Debug};
/// The coverage counter or counter expression associated with a particular
/// BCB node or BCB edge.
#[derive(Clone)]
#[derive(Clone, Copy)]
pub(super) enum BcbCounter {
Counter { id: CounterId },
Expression { id: ExpressionId },
@ -88,11 +86,20 @@ impl CoverageCounters {
BcbCounter::Counter { id }
}
fn make_expression(&mut self, lhs: CovTerm, op: Op, rhs: CovTerm) -> BcbCounter {
let id = self.expressions.push(Expression { lhs, op, rhs });
fn make_expression(&mut self, lhs: BcbCounter, op: Op, rhs: BcbCounter) -> BcbCounter {
let expression = Expression { lhs: lhs.as_term(), op, rhs: rhs.as_term() };
let id = self.expressions.push(expression);
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)
}
/// Counter IDs start from one and go up.
fn next_counter(&mut self) -> CounterId {
let next = self.next_counter_id;
@ -109,7 +116,7 @@ impl CoverageCounters {
self.expressions.len()
}
fn set_bcb_counter(&mut self, bcb: BasicCoverageBlock, counter_kind: BcbCounter) -> CovTerm {
fn set_bcb_counter(&mut self, bcb: BasicCoverageBlock, counter_kind: BcbCounter) -> BcbCounter {
assert!(
// If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also
// have an expression (to be injected into an existing `BasicBlock` represented by this
@ -118,14 +125,13 @@ impl CoverageCounters {
"attempt to add a `Counter` to a BCB target with existing incoming edge counters"
);
let term = counter_kind.as_term();
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 {
term
counter_kind
}
}
@ -134,7 +140,7 @@ impl CoverageCounters {
from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock,
counter_kind: BcbCounter,
) -> CovTerm {
) -> BcbCounter {
// If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also
// have an expression (to be injected into an existing `BasicBlock` represented by this
// `BasicCoverageBlock`).
@ -148,19 +154,18 @@ impl CoverageCounters {
}
self.bcb_has_incoming_edge_counters.insert(to_bcb);
let term = counter_kind.as_term();
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 {
term
counter_kind
}
}
pub(super) fn bcb_counter(&self, bcb: BasicCoverageBlock) -> Option<&BcbCounter> {
self.bcb_counters[bcb].as_ref()
pub(super) fn bcb_counter(&self, bcb: BasicCoverageBlock) -> Option<BcbCounter> {
self.bcb_counters[bcb]
}
pub(super) fn bcb_node_counters(
@ -226,11 +231,7 @@ impl<'a> MakeBcbCounters<'a> {
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);
let branching_counter_operand = self.get_or_make_counter_operand(bcb);
if self.bcb_needs_branch_counters(bcb) {
self.make_branch_counters(&traversal, bcb, branching_counter_operand);
}
self.make_node_and_branch_counters(&traversal, bcb);
} else {
debug!(
"{:?} does not have any coverage spans. A counter will only be added if \
@ -247,100 +248,93 @@ impl<'a> MakeBcbCounters<'a> {
);
}
fn make_branch_counters(
fn make_node_and_branch_counters(
&mut self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
branching_bcb: BasicCoverageBlock,
branching_counter_operand: CovTerm,
from_bcb: BasicCoverageBlock,
) {
let branches = self.bcb_branches(branching_bcb);
debug!(
"{:?} has some branch(es) without counters:\n {}",
branching_bcb,
branches
// 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()
.map(|branch| { format!("{:?}: {:?}", branch, self.branch_counter(branch)) })
.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 "),
);
// Use the `traversal` state to decide if a subset of the branches exit a loop, making it
// likely that branch is executed less than branches that do not exit the same loop. In this
// case, any branch that does not exit the loop (and has not already been assigned a
// counter) should be counted by expression, if possible. (If a preferred expression branch
// is not selected based on the loop context, select any branch without an existing
// counter.)
let expression_branch = self.choose_preferred_expression_branch(traversal, &branches);
// 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);
// Assign a Counter or Expression to each branch, plus additional `Expression`s, as needed,
// to sum up intermediate results.
let mut some_sumup_counter_operand = None;
for branch in branches {
// Skip the selected `expression_branch`, if any. It's expression will be assigned after
// all others.
if branch != expression_branch {
let branch_counter_operand = if branch.is_only_path_to_target() {
debug!(
" {:?} has only one incoming edge (from {:?}), so adding a \
counter",
branch, branching_bcb
);
self.get_or_make_counter_operand(branch.target_bcb)
} else {
debug!(" {:?} has multiple incoming edges, so adding an edge counter", branch);
self.get_or_make_edge_counter_operand(branching_bcb, branch.target_bcb)
};
if let Some(sumup_counter_operand) =
some_sumup_counter_operand.replace(branch_counter_operand)
{
let intermediate_expression = self.coverage_counters.make_expression(
branch_counter_operand,
Op::Add,
sumup_counter_operand,
);
debug!(" [new intermediate expression: {:?}]", intermediate_expression);
let intermediate_expression_operand = intermediate_expression.as_term();
some_sumup_counter_operand.replace(intermediate_expression_operand);
}
}
}
// 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")
};
// Assign the final expression to the `expression_branch` by subtracting the total of all
// other branches from the counter of the branching BCB.
let sumup_counter_operand =
some_sumup_counter_operand.expect("sumup_counter_operand should have a value");
// 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_branch predecessors: {:?})",
expression_branch,
self.bcb_predecessors(expression_branch.target_bcb),
"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(
branching_counter_operand,
from_bcb_operand,
Op::Subtract,
sumup_counter_operand,
sum_of_all_other_branches,
);
debug!("{:?} gets an expression: {:?}", expression_branch, expression);
let bcb = expression_branch.target_bcb;
if expression_branch.is_only_path_to_target() {
self.coverage_counters.set_bcb_counter(bcb, expression);
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_edge_counter(branching_bcb, bcb, expression);
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) -> CovTerm {
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] {
if let Some(counter_kind) = self.coverage_counters.bcb_counters[bcb] {
debug!("{bcb:?} already has a counter: {counter_kind:?}");
return counter_kind.as_term();
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.bcb_has_one_path_to_target(bcb);
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();
if one_path_to_target {
@ -355,40 +349,25 @@ impl<'a> MakeBcbCounters<'a> {
return self.coverage_counters.set_bcb_counter(bcb, counter_kind);
}
// A BCB with multiple incoming edges can compute its count by `Expression`, summing up the
// counters and/or expressions of its incoming edges. This will recursively get or create
// counters for those incoming edges first, then call `make_expression()` to sum them up,
// with additional intermediate expressions as needed.
let _sumup_debug_span = debug_span!("(preparing sum-up expression)").entered();
// 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")
};
let mut predecessors = self.bcb_predecessors(bcb).to_owned().into_iter();
let first_edge_counter_operand =
self.get_or_make_edge_counter_operand(predecessors.next().unwrap(), bcb);
let mut some_sumup_edge_counter_operand = None;
for predecessor in predecessors {
let edge_counter_operand = self.get_or_make_edge_counter_operand(predecessor, bcb);
if let Some(sumup_edge_counter_operand) =
some_sumup_edge_counter_operand.replace(edge_counter_operand)
{
let intermediate_expression = self.coverage_counters.make_expression(
sumup_edge_counter_operand,
Op::Add,
edge_counter_operand,
);
debug!("new intermediate expression: {intermediate_expression:?}");
let intermediate_expression_operand = intermediate_expression.as_term();
some_sumup_edge_counter_operand.replace(intermediate_expression_operand);
}
}
let counter_kind = self.coverage_counters.make_expression(
first_edge_counter_operand,
Op::Add,
some_sumup_edge_counter_operand.unwrap(),
);
drop(_sumup_debug_span);
debug!("{bcb:?} gets a new counter (sum of predecessor counters): {counter_kind:?}");
self.coverage_counters.set_bcb_counter(bcb, counter_kind)
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))]
@ -396,20 +375,26 @@ impl<'a> MakeBcbCounters<'a> {
&mut self,
from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock,
) -> CovTerm {
) -> 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.
let successors = self.bcb_successors(from_bcb).iter();
if successors.len() == 1 {
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) =
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.as_term();
return counter_kind;
}
// Make a new counter to count this edge.
@ -423,16 +408,19 @@ impl<'a> MakeBcbCounters<'a> {
fn choose_preferred_expression_branch(
&self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
branches: &[BcbBranch],
) -> BcbBranch {
let good_reloop_branch = self.find_good_reloop_branch(traversal, branches);
if let Some(reloop_branch) = good_reloop_branch {
assert!(self.branch_has_no_counter(&reloop_branch));
debug!("Selecting reloop branch {reloop_branch:?} to get an expression");
reloop_branch
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 =
branches.iter().find(|&branch| self.branch_has_no_counter(branch)).expect(
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",
);
@ -453,26 +441,28 @@ impl<'a> MakeBcbCounters<'a> {
fn find_good_reloop_branch(
&self,
traversal: &TraverseCoverageGraphWithLoops<'_>,
branches: &[BcbBranch],
) -> Option<BcbBranch> {
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 in branches {
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)
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(&branch) {
if self.branch_has_no_counter(from_bcb, branch_target_bcb) {
// We found a good branch to be given an expression.
return Some(branch);
return Some(branch_target_bcb);
}
// Keep looking for another reloop branch without a counter.
} else {
@ -505,36 +495,23 @@ impl<'a> MakeBcbCounters<'a> {
}
#[inline]
fn bcb_branches(&self, from_bcb: BasicCoverageBlock) -> Vec<BcbBranch> {
self.bcb_successors(from_bcb)
.iter()
.map(|&to_bcb| BcbBranch::from_to(from_bcb, to_bcb, self.basic_coverage_blocks))
.collect::<Vec<_>>()
fn branch_has_no_counter(
&self,
from_bcb: BasicCoverageBlock,
to_bcb: BasicCoverageBlock,
) -> bool {
self.branch_counter(from_bcb, to_bcb).is_none()
}
fn bcb_needs_branch_counters(&self, bcb: BasicCoverageBlock) -> bool {
let branch_needs_a_counter = |branch: &BcbBranch| self.branch_has_no_counter(branch);
let branches = self.bcb_branches(bcb);
branches.len() > 1 && branches.iter().any(branch_needs_a_counter)
}
fn branch_has_no_counter(&self, branch: &BcbBranch) -> bool {
self.branch_counter(branch).is_none()
}
fn branch_counter(&self, branch: &BcbBranch) -> Option<&BcbCounter> {
let to_bcb = branch.target_bcb;
if let Some(from_bcb) = branch.edge_from_bcb {
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()
}
}
/// Returns true if the BasicCoverageBlock has zero or one incoming edge. (If zero, it should be
/// the entry point for the function.)
#[inline]
fn bcb_has_one_path_to_target(&self, bcb: BasicCoverageBlock) -> bool {
self.bcb_predecessors(bcb).len() <= 1
}
}

66
compiler/rustc_mir_transform/src/coverage/graph.rs
View file

@ -62,6 +62,14 @@ impl CoverageGraph {
Self { bcbs, bb_to_bcb, successors, predecessors, dominators: None };
let dominators = dominators::dominators(&basic_coverage_blocks);
basic_coverage_blocks.dominators = Some(dominators);
// 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!(basic_coverage_blocks[START_BCB].leader_bb() == mir::START_BLOCK);
assert!(basic_coverage_blocks.predecessors[START_BCB].is_empty());
basic_coverage_blocks
}
@ -199,6 +207,25 @@ impl CoverageGraph {
pub 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(super) 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 {
@ -319,45 +346,6 @@ impl BasicCoverageBlockData {
}
}
/// 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 subset of a block's successors that are relevant to the coverage
// graph, i.e. those that do not represent unwinds or unreachable branches.
// FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and