Injecting expressions in place of counters where helpful

Implementing the Graph traits for the BasicCoverageBlock
graph.

optimized replacement of counters with expressions plus new BCB graphviz

* Avoid adding coverage to unreachable blocks.
* Special case for Goto at the end of the body. Make it non-reportable.

Improved debugging and formatting options (from env)

Don't automatically add counters to BCBs without CoverageSpans. They may
still get counters but only if there are dependencies from
other BCBs that have spans, I think.

Make CodeRegions optional for Counters too. It is
possible to inject counters (`llvm.instrprof.increment` intrinsic calls
without corresponding code regions in the coverage map. An expression
can still uses these counter values.

Refactored instrument_coverage.rs -> instrument_coverage/mod.rs, and
then broke up the mod into multiple files.

Compiling with coverage, with the expression optimization, works on
the json5format crate and its dependencies.

Refactored debug features from mod.rs to debug.rs

1 files changed, 319 insertions, 2 deletions

diff --git a/compiler/rustc_mir/src/transform/coverage/graph.rs b/compiler/rustc_mir/src/transform/coverage/graph.rs
index 3d3e76f907b..84062541701 100644
--- a/compiler/rustc_mir/src/transform/coverage/graph.rs
+++ b/compiler/rustc_mir/src/transform/coverage/graph.rs
@@ -1,7 +1,11 @@
+use super::Error;
+
+use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::graph::dominators::{self, Dominators};
-use rustc_data_structures::graph::{self, GraphSuccessors, WithNumNodes};
+use rustc_data_structures::graph::{self, GraphSuccessors, WithNumNodes, WithStartNode};
 use rustc_index::bit_set::BitSet;
 use rustc_index::vec::IndexVec;
+use rustc_middle::mir::coverage::*;
 use rustc_middle::mir::{self, BasicBlock, BasicBlockData, Terminator, TerminatorKind};
 
 use std::ops::{Index, IndexMut};
@@ -184,6 +188,13 @@ impl CoverageGraph {
     }
 
     #[inline(always)]
+    pub fn iter_enumerated_mut(
+        &mut self,
+    ) -> impl Iterator<Item = (BasicCoverageBlock, &mut BasicCoverageBlockData)> {
+        self.bcbs.iter_enumerated_mut()
+    }
+
+    #[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 }
     }
@@ -293,12 +304,14 @@ rustc_index::newtype_index! {
 #[derive(Debug, Clone)]
 pub(crate) struct BasicCoverageBlockData {
     pub basic_blocks: Vec<BasicBlock>,
+    pub counter_kind: Option<CoverageKind>,
+    edge_from_bcbs: Option<FxHashMap<BasicCoverageBlock, CoverageKind>>,
 }
 
 impl BasicCoverageBlockData {
     pub fn from(basic_blocks: Vec<BasicBlock>) -> Self {
         assert!(basic_blocks.len() > 0);
-        Self { basic_blocks }
+        Self { basic_blocks, counter_kind: None, edge_from_bcbs: None }
     }
 
     #[inline(always)]
@@ -316,6 +329,91 @@ impl BasicCoverageBlockData {
         &mir_body[self.last_bb()].terminator()
     }
 
+    #[inline(always)]
+    pub fn set_counter(
+        &mut self,
+        counter_kind: CoverageKind,
+    ) -> Result<ExpressionOperandId, Error> {
+        debug_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
+            // `BasicCoverageBlock`).
+            self.edge_from_bcbs.is_none() || counter_kind.is_expression(),
+            "attempt to add a `Counter` to a BCB target with existing incoming edge counters"
+        );
+        let operand = counter_kind.as_operand_id();
+        let expect_none = self.counter_kind.replace(counter_kind);
+        if expect_none.is_some() {
+            return Error::from_string(format!(
+                "attempt to set a BasicCoverageBlock coverage counter more than once; \
+                {:?} already had counter {:?}",
+                self,
+                expect_none.unwrap(),
+            ));
+        }
+        Ok(operand)
+    }
+
+    #[inline(always)]
+    pub fn counter(&self) -> Option<&CoverageKind> {
+        self.counter_kind.as_ref()
+    }
+
+    #[inline(always)]
+    pub fn take_counter(&mut self) -> Option<CoverageKind> {
+        self.counter_kind.take()
+    }
+
+    #[inline(always)]
+    pub fn set_edge_counter_from(
+        &mut self,
+        from_bcb: BasicCoverageBlock,
+        counter_kind: CoverageKind,
+    ) -> Result<ExpressionOperandId, Error> {
+        if level_enabled!(tracing::Level::DEBUG) {
+            // 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`).
+            if !self.counter_kind.as_ref().map_or(true, |c| c.is_expression()) {
+                return Error::from_string(format!(
+                    "attempt to add an incoming edge counter from {:?} when the target BCB already \
+                    has a `Counter`",
+                    from_bcb
+                ));
+            }
+        }
+        let operand = counter_kind.as_operand_id();
+        let expect_none = self
+            .edge_from_bcbs
+            .get_or_insert_with(|| FxHashMap::default())
+            .insert(from_bcb, counter_kind);
+        if expect_none.is_some() {
+            return Error::from_string(format!(
+                "attempt to set an edge counter more than once; from_bcb: \
+                {:?} already had counter {:?}",
+                from_bcb,
+                expect_none.unwrap(),
+            ));
+        }
+        Ok(operand)
+    }
+
+    #[inline(always)]
+    pub fn edge_counter_from(&self, from_bcb: BasicCoverageBlock) -> Option<&CoverageKind> {
+        if let Some(edge_from_bcbs) = &self.edge_from_bcbs {
+            edge_from_bcbs.get(&from_bcb)
+        } else {
+            None
+        }
+    }
+
+    #[inline(always)]
+    pub fn take_edge_counters(
+        &mut self,
+    ) -> Option<impl Iterator<Item = (BasicCoverageBlock, CoverageKind)>> {
+        self.edge_from_bcbs.take().map_or(None, |m| Some(m.into_iter()))
+    }
+
     pub fn id(&self) -> String {
         format!(
             "@{}",
@@ -328,6 +426,56 @@ 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(crate) 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 counter<'a>(
+        &self,
+        basic_coverage_blocks: &'a CoverageGraph,
+    ) -> Option<&'a CoverageKind> {
+        if let Some(from_bcb) = self.edge_from_bcb {
+            basic_coverage_blocks[self.target_bcb].edge_counter_from(from_bcb)
+        } else {
+            basic_coverage_blocks[self.target_bcb].counter()
+        }
+    }
+
+    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)
+        }
+    }
+}
+
 fn bcb_filtered_successors<'a, 'tcx>(
     body: &'tcx &'a mir::Body<'tcx>,
     term_kind: &'tcx TerminatorKind<'tcx>,
@@ -344,6 +492,175 @@ fn bcb_filtered_successors<'a, 'tcx>(
     .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(crate) 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(crate) 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 mut context_stack = Vec::new();
+        context_stack.push(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(next_bcb) = {
+            // Strip contexts with empty worklists from the top of the stack
+            while self.context_stack.last().map_or(false, |context| context.worklist.is_empty()) {
+                self.context_stack.pop();
+            }
+            // Pop the next bcb off of the current context_stack. If none, all BCBs were visited.
+            self.context_stack.last_mut().map_or(None, |context| 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);
+        }
+        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.is_dominated_by(successor, loop_header) {
+                            (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<_>>()
+    }
+}
+
+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.is_dominated_by(bcb, successor) {
+                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,