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coverage: Remove the custom graph traversal from counter creation
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The logic behind this traversal order sounds compelling, but in practice it
doesn't seem to produce enough benefit to justify its complexity.
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@Zalathar
Zalathar committed Oct 8, 2024
1 parent ae442f3 commit 9b89793
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Showing 21 changed files with 1,120 additions and 1,306 deletions.
23 changes: 4 additions & 19 deletions compiler/rustc_mir_transform/src/coverage/counters.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,9 +7,9 @@ use rustc_index::IndexVec;
use rustc_index::bit_set::BitSet;
use rustc_middle::bug;
use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};
use tracing::{debug, debug_span, instrument};
use tracing::{debug, instrument};

use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, TraverseCoverageGraphWithLoops};
use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph};

/// The coverage counter or counter expression associated with a particular
/// BCB node or BCB edge.
Expand Down Expand Up @@ -290,24 +290,9 @@ impl<'a> MakeBcbCounters<'a> {

// Traverse the coverage graph, ensuring that every node that needs a
// coverage counter has one.
//
// The traversal tries to ensure that, when a loop is encountered, all
// nodes within the loop are visited before visiting any nodes outside
// the loop. It also keeps track of which loop(s) the traversal is
// currently inside.
let mut traversal = TraverseCoverageGraphWithLoops::new(self.basic_coverage_blocks);
while let Some(bcb) = traversal.next() {
let _span = debug_span!("traversal", ?bcb).entered();
if self.bcb_needs_counter.contains(bcb) {
self.make_node_counter_and_out_edge_counters(bcb);
}
for bcb in self.bcb_needs_counter.iter() {
self.make_node_counter_and_out_edge_counters(bcb);
}

assert!(
traversal.is_complete(),
"`TraverseCoverageGraphWithLoops` missed some `BasicCoverageBlock`s: {:?}",
traversal.unvisited(),
);
}

/// Make sure the given node has a node counter, and then make sure each of
Expand Down
167 changes: 1 addition & 166 deletions compiler/rustc_mir_transform/src/coverage/graph.rs
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Original file line number Diff line number Diff line change
@@ -1,14 +1,12 @@
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;
use rustc_data_structures::graph::dominators::{self, Dominators};
use rustc_data_structures::graph::{self, DirectedGraph, StartNode};
use rustc_index::IndexVec;
use rustc_index::bit_set::BitSet;
use rustc_middle::bug;
use rustc_middle::mir::{self, BasicBlock, Terminator, TerminatorKind};
use tracing::debug;

Expand Down Expand Up @@ -151,11 +149,6 @@ impl CoverageGraph {
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,
Expand Down Expand Up @@ -400,164 +393,6 @@ fn bcb_filtered_successors<'a, 'tcx>(terminator: &'a Terminator<'tcx>) -> Covera
}
}

/// 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 }
}

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,
Expand Down
16 changes: 8 additions & 8 deletions tests/coverage/async.cov-map
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Original file line number Diff line number Diff line change
Expand Up @@ -155,25 +155,25 @@ Number of file 0 mappings: 1
Max counter seen: c0

Function name: async::i::{closure#0}
Raw bytes (63): 0x[01, 01, 02, 07, 19, 11, 15, 0b, 01, 2d, 13, 04, 0c, 09, 05, 09, 00, 0a, 01, 00, 0e, 00, 18, 05, 00, 1c, 00, 21, 09, 00, 27, 00, 30, 15, 01, 09, 00, 0a, 0d, 00, 0e, 00, 17, 1d, 00, 1b, 00, 20, 15, 00, 24, 00, 26, 19, 01, 0e, 00, 10, 03, 02, 01, 00, 02]
Raw bytes (63): 0x[01, 01, 02, 07, 15, 0d, 11, 0b, 01, 2d, 13, 04, 0c, 09, 05, 09, 00, 0a, 01, 00, 0e, 00, 18, 05, 00, 1c, 00, 21, 09, 00, 27, 00, 30, 11, 01, 09, 00, 0a, 19, 00, 0e, 00, 17, 1d, 00, 1b, 00, 20, 11, 00, 24, 00, 26, 15, 01, 0e, 00, 10, 03, 02, 01, 00, 02]
Number of files: 1
- file 0 => global file 1
Number of expressions: 2
- expression 0 operands: lhs = Expression(1, Add), rhs = Counter(6)
- expression 1 operands: lhs = Counter(4), rhs = Counter(5)
- expression 0 operands: lhs = Expression(1, Add), rhs = Counter(5)
- expression 1 operands: lhs = Counter(3), rhs = Counter(4)
Number of file 0 mappings: 11
- Code(Counter(0)) at (prev + 45, 19) to (start + 4, 12)
- Code(Counter(2)) at (prev + 5, 9) to (start + 0, 10)
- Code(Counter(0)) at (prev + 0, 14) to (start + 0, 24)
- Code(Counter(1)) at (prev + 0, 28) to (start + 0, 33)
- Code(Counter(2)) at (prev + 0, 39) to (start + 0, 48)
- Code(Counter(5)) at (prev + 1, 9) to (start + 0, 10)
- Code(Counter(3)) at (prev + 0, 14) to (start + 0, 23)
- Code(Counter(4)) at (prev + 1, 9) to (start + 0, 10)
- Code(Counter(6)) at (prev + 0, 14) to (start + 0, 23)
- Code(Counter(7)) at (prev + 0, 27) to (start + 0, 32)
- Code(Counter(5)) at (prev + 0, 36) to (start + 0, 38)
- Code(Counter(6)) at (prev + 1, 14) to (start + 0, 16)
- Code(Counter(4)) at (prev + 0, 36) to (start + 0, 38)
- Code(Counter(5)) at (prev + 1, 14) to (start + 0, 16)
- Code(Expression(0, Add)) at (prev + 2, 1) to (start + 0, 2)
= ((c4 + c5) + c6)
= ((c3 + c4) + c5)
Max counter seen: c7

Function name: async::j
Expand Down
31 changes: 17 additions & 14 deletions tests/coverage/branch/if.cov-map
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Original file line number Diff line number Diff line change
@@ -1,28 +1,31 @@
Function name: if::branch_and
Raw bytes (56): 0x[01, 01, 04, 05, 09, 0d, 02, 11, 0f, 0d, 02, 08, 01, 2b, 01, 01, 10, 05, 03, 08, 00, 09, 20, 09, 02, 00, 08, 00, 09, 09, 00, 0d, 00, 0e, 20, 11, 0d, 00, 0d, 00, 0e, 11, 00, 0f, 02, 06, 0f, 02, 0c, 02, 06, 0b, 03, 01, 00, 02]
Raw bytes (62): 0x[01, 01, 07, 05, 09, 09, 0d, 1a, 02, 09, 0d, 0d, 17, 1a, 02, 09, 0d, 08, 01, 2b, 01, 01, 10, 05, 03, 08, 00, 09, 20, 09, 02, 00, 08, 00, 09, 09, 00, 0d, 00, 0e, 20, 0d, 1a, 00, 0d, 00, 0e, 0d, 00, 0f, 02, 06, 17, 02, 0c, 02, 06, 13, 03, 01, 00, 02]
Number of files: 1
- file 0 => global file 1
Number of expressions: 4
Number of expressions: 7
- expression 0 operands: lhs = Counter(1), rhs = Counter(2)
- expression 1 operands: lhs = Counter(3), rhs = Expression(0, Sub)
- expression 2 operands: lhs = Counter(4), rhs = Expression(3, Add)
- expression 3 operands: lhs = Counter(3), rhs = Expression(0, Sub)
- expression 1 operands: lhs = Counter(2), rhs = Counter(3)
- expression 2 operands: lhs = Expression(6, Sub), rhs = Expression(0, Sub)
- expression 3 operands: lhs = Counter(2), rhs = Counter(3)
- expression 4 operands: lhs = Counter(3), rhs = Expression(5, Add)
- expression 5 operands: lhs = Expression(6, Sub), rhs = Expression(0, Sub)
- expression 6 operands: lhs = Counter(2), rhs = Counter(3)
Number of file 0 mappings: 8
- Code(Counter(0)) at (prev + 43, 1) to (start + 1, 16)
- Code(Counter(1)) at (prev + 3, 8) to (start + 0, 9)
- Branch { true: Counter(2), false: Expression(0, Sub) } at (prev + 0, 8) to (start + 0, 9)
true = c2
false = (c1 - c2)
- Code(Counter(2)) at (prev + 0, 13) to (start + 0, 14)
- Branch { true: Counter(4), false: Counter(3) } at (prev + 0, 13) to (start + 0, 14)
true = c4
false = c3
- Code(Counter(4)) at (prev + 0, 15) to (start + 2, 6)
- Code(Expression(3, Add)) at (prev + 2, 12) to (start + 2, 6)
= (c3 + (c1 - c2))
- Code(Expression(2, Add)) at (prev + 3, 1) to (start + 0, 2)
= (c4 + (c3 + (c1 - c2)))
Max counter seen: c4
- Branch { true: Counter(3), false: Expression(6, Sub) } at (prev + 0, 13) to (start + 0, 14)
true = c3
false = (c2 - c3)
- Code(Counter(3)) at (prev + 0, 15) to (start + 2, 6)
- Code(Expression(5, Add)) at (prev + 2, 12) to (start + 2, 6)
= ((c2 - c3) + (c1 - c2))
- Code(Expression(4, Add)) at (prev + 3, 1) to (start + 0, 2)
= (c3 + ((c2 - c3) + (c1 - c2)))
Max counter seen: c3

Function name: if::branch_not
Raw bytes (116): 0x[01, 01, 07, 05, 09, 05, 0d, 05, 0d, 05, 11, 05, 11, 05, 15, 05, 15, 12, 01, 0c, 01, 01, 10, 05, 03, 08, 00, 09, 20, 09, 02, 00, 08, 00, 09, 09, 01, 09, 00, 11, 02, 01, 06, 00, 07, 05, 01, 08, 00, 0a, 20, 0a, 0d, 00, 08, 00, 0a, 0a, 00, 0b, 02, 06, 0d, 02, 06, 00, 07, 05, 01, 08, 00, 0b, 20, 11, 12, 00, 08, 00, 0b, 11, 00, 0c, 02, 06, 12, 02, 06, 00, 07, 05, 01, 08, 00, 0c, 20, 1a, 15, 00, 08, 00, 0c, 1a, 00, 0d, 02, 06, 15, 02, 06, 00, 07, 05, 01, 01, 00, 02]
Expand Down
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