coverage: Remove pending_dups from the span refiner

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -61,7 +61,7 @@ pub(super) fn generate_coverage_spans(
             hir_info,
             basic_coverage_blocks,
         );
-        let coverage_spans = SpansRefiner::refine_sorted_spans(basic_coverage_blocks, sorted_spans);
+        let coverage_spans = SpansRefiner::refine_sorted_spans(sorted_spans);
         mappings.extend(coverage_spans.into_iter().map(|RefinedCovspan { bcb, span, .. }| {
             // Each span produced by the generator represents an ordinary code region.
             BcbMapping { kind: BcbMappingKind::Code(bcb), span }
@@ -88,8 +88,6 @@ pub(super) fn generate_coverage_spans(
 
 #[derive(Debug)]
 struct CurrCovspan {
-    /// This is used as the basis for [`PrevCovspan::original_span`], so it must
-    /// not be modified.
     span: Span,
     bcb: BasicCoverageBlock,
     is_closure: bool,
@@ -102,7 +100,7 @@ impl CurrCovspan {
 
     fn into_prev(self) -> PrevCovspan {
         let Self { span, bcb, is_closure } = self;
-        PrevCovspan { original_span: span, span, bcb, merged_spans: vec![span], is_closure }
+        PrevCovspan { span, bcb, merged_spans: vec![span], is_closure }
     }
 
     fn into_refined(self) -> RefinedCovspan {
@@ -115,7 +113,6 @@ impl CurrCovspan {
 
 #[derive(Debug)]
 struct PrevCovspan {
-    original_span: Span,
     span: Span,
     bcb: BasicCoverageBlock,
     /// List of all the original spans from MIR that have been merged into this
@@ -142,35 +139,8 @@ impl PrevCovspan {
         }
     }
 
-    fn into_dup(self) -> DuplicateCovspan {
-        let Self { original_span, span, bcb, merged_spans: _, is_closure } = self;
-        // Only unmodified spans end up in `pending_dups`.
-        debug_assert_eq!(original_span, span);
-        DuplicateCovspan { span, bcb, is_closure }
-    }
-
-    fn refined_copy(&self) -> RefinedCovspan {
-        let &Self { original_span: _, span, bcb, merged_spans: _, is_closure } = self;
-        RefinedCovspan { span, bcb, is_closure }
-    }
-
-    fn into_refined(self) -> RefinedCovspan {
-        self.refined_copy()
-    }
-}
-
-#[derive(Debug)]
-struct DuplicateCovspan {
-    span: Span,
-    bcb: BasicCoverageBlock,
-    is_closure: bool,
-}
-
-impl DuplicateCovspan {
-    /// Returns a copy of this covspan, as a [`RefinedCovspan`].
-    /// Should only be called in places that would otherwise clone this covspan.
     fn refined_copy(&self) -> RefinedCovspan {
-        let &Self { span, bcb, is_closure } = self;
+        let &Self { span, bcb, merged_spans: _, is_closure } = self;
         RefinedCovspan { span, bcb, is_closure }
     }
 
@@ -205,10 +175,7 @@ impl RefinedCovspan {
 ///  * Merge spans that represent continuous (both in source code and control flow), non-branching
 ///    execution
 ///  * Carve out (leave uncovered) any span that will be counted by another MIR (notably, closures)
-struct SpansRefiner<'a> {
-    /// The BasicCoverageBlock Control Flow Graph (BCB CFG).
-    basic_coverage_blocks: &'a CoverageGraph,
-
+struct SpansRefiner {
     /// The initial set of coverage spans, sorted by `Span` (`lo` and `hi`) and by relative
     /// dominance between the `BasicCoverageBlock`s of equal `Span`s.
     sorted_spans_iter: std::vec::IntoIter<SpanFromMir>,
@@ -223,36 +190,22 @@ struct SpansRefiner<'a> {
     /// If that `curr` was discarded, `prev` retains its value from the previous iteration.
     some_prev: Option<PrevCovspan>,
 
-    /// One or more coverage spans with the same `Span` but different `BasicCoverageBlock`s, and
-    /// no `BasicCoverageBlock` in this list dominates another `BasicCoverageBlock` in the list.
-    /// If a new `curr` span also fits this criteria (compared to an existing list of
-    /// `pending_dups`), that `curr` moves to `prev` before possibly being added to
-    /// the `pending_dups` list, on the next iteration. As a result, if `prev` and `pending_dups`
-    /// have the same `Span`, the criteria for `pending_dups` holds for `prev` as well: a `prev`
-    /// with a matching `Span` does not dominate any `pending_dup` and no `pending_dup` dominates a
-    /// `prev` with a matching `Span`)
-    pending_dups: Vec<DuplicateCovspan>,
-
     /// The final coverage spans to add to the coverage map. A `Counter` or `Expression`
     /// will also be injected into the MIR for each BCB that has associated spans.
     refined_spans: Vec<RefinedCovspan>,
 }
 
-impl<'a> SpansRefiner<'a> {
+impl SpansRefiner {
     /// Takes the initial list of (sorted) spans extracted from MIR, and "refines"
     /// them by merging compatible adjacent spans, removing redundant spans,
     /// and carving holes in spans when they overlap in unwanted ways.
-    fn refine_sorted_spans(
-        basic_coverage_blocks: &'a CoverageGraph,
-        sorted_spans: Vec<SpanFromMir>,
-    ) -> Vec<RefinedCovspan> {
+    fn refine_sorted_spans(sorted_spans: Vec<SpanFromMir>) -> Vec<RefinedCovspan> {
+        let sorted_spans_len = sorted_spans.len();
         let this = Self {
-            basic_coverage_blocks,
             sorted_spans_iter: sorted_spans.into_iter(),
             some_curr: None,
             some_prev: None,
-            pending_dups: Vec::new(),
-            refined_spans: Vec::with_capacity(basic_coverage_blocks.num_nodes() * 2),
+            refined_spans: Vec::with_capacity(sorted_spans_len),
         };
 
         this.to_refined_spans()
@@ -292,21 +245,11 @@ impl<'a> SpansRefiner<'a> {
                 self.take_curr(); // Discards curr.
             } else if curr.is_closure {
                 self.carve_out_span_for_closure();
-            } else if prev.original_span == prev.span && prev.span == curr.span {
-                // Prev and curr have the same span, and prev's span hasn't
-                // been modified by other spans.
-                self.update_pending_dups();
             } else {
                 self.cutoff_prev_at_overlapping_curr();
             }
         }
 
-        // Drain any remaining dups into the output.
-        for dup in self.pending_dups.drain(..) {
-            debug!("    ...adding at least one pending dup={:?}", dup);
-            self.refined_spans.push(dup.into_refined());
-        }
-
         // There is usually a final span remaining in `prev` after the loop ends,
         // so add it to the output as well.
         if let Some(prev) = self.some_prev.take() {
@@ -359,36 +302,6 @@ impl<'a> SpansRefiner<'a> {
         self.some_prev.take().unwrap_or_else(|| bug!("some_prev is None (take_prev)"))
     }
 
-    /// If there are `pending_dups` but `prev` is not a matching dup (`prev.span` doesn't match the
-    /// `pending_dups` spans), then one of the following two things happened during the previous
-    /// iteration:
-    ///   * the previous `curr` span (which is now `prev`) was not a duplicate of the pending_dups
-    ///     (in which case there should be at least two spans in `pending_dups`); or
-    ///   * the `span` of `prev` was modified by `curr_mut().merge_from(prev)` (in which case
-    ///     `pending_dups` could have as few as one span)
-    /// In either case, no more spans will match the span of `pending_dups`, so
-    /// add the `pending_dups` if they don't overlap `curr`, and clear the list.
-    fn maybe_flush_pending_dups(&mut self) {
-        let Some(last_dup) = self.pending_dups.last() else { return };
-        if last_dup.span == self.prev().span {
-            return;
-        }
-
-        debug!(
-            "    SAME spans, but pending_dups are NOT THE SAME, so BCBs matched on \
-            previous iteration, or prev started a new disjoint span"
-        );
-        if last_dup.span.hi() <= self.curr().span.lo() {
-            for dup in self.pending_dups.drain(..) {
-                debug!("    ...adding at least one pending={:?}", dup);
-                self.refined_spans.push(dup.into_refined());
-            }
-        } else {
-            self.pending_dups.clear();
-        }
-        assert!(self.pending_dups.is_empty());
-    }
-
     /// Advance `prev` to `curr` (if any), and `curr` to the next coverage span in sorted order.
     fn next_coverage_span(&mut self) -> bool {
         if let Some(curr) = self.some_curr.take() {
@@ -408,7 +321,6 @@ impl<'a> SpansRefiner<'a> {
                 );
             } else {
                 self.some_curr = Some(CurrCovspan::new(curr.span, curr.bcb, curr.is_closure));
-                self.maybe_flush_pending_dups();
                 return true;
             }
         }
@@ -433,13 +345,6 @@ impl<'a> SpansRefiner<'a> {
             let mut pre_closure = self.prev().refined_copy();
             pre_closure.span = pre_closure.span.with_hi(left_cutoff);
             debug!("  prev overlaps a closure. Adding span for pre_closure={:?}", pre_closure);
-
-            for mut dup in self.pending_dups.iter().map(DuplicateCovspan::refined_copy) {
-                dup.span = dup.span.with_hi(left_cutoff);
-                debug!("    ...and at least one pre_closure dup={:?}", dup);
-                self.refined_spans.push(dup);
-            }
-
             self.refined_spans.push(pre_closure);
         }
 
@@ -448,58 +353,9 @@ impl<'a> SpansRefiner<'a> {
             self.prev_mut().span = self.prev().span.with_lo(right_cutoff);
             debug!("  Mutated prev.span to start after the closure. prev={:?}", self.prev());
 
-            for dup in &mut self.pending_dups {
-                debug!("    ...and at least one overlapping dup={:?}", dup);
-                dup.span = dup.span.with_lo(right_cutoff);
-            }
-
             // Prevent this curr from becoming prev.
             let closure_covspan = self.take_curr().into_refined();
             self.refined_spans.push(closure_covspan); // since self.prev() was already updated
-        } else {
-            self.pending_dups.clear();
-        }
-    }
-
-    /// Called if `curr.span` equals `prev.original_span` (and potentially equal to all
-    /// `pending_dups` spans, if any). Keep in mind, `prev.span()` may have been changed.
-    /// If prev.span() was merged into other spans (with matching BCB, for instance),
-    /// `prev.span.hi()` will be greater than (further right of) `prev.original_span.hi()`.
-    /// If prev.span() was split off to the right of a closure, prev.span().lo() will be
-    /// greater than prev.original_span.lo(). The actual span of `prev.original_span` is
-    /// not as important as knowing that `prev()` **used to have the same span** as `curr()`,
-    /// which means their sort order is still meaningful for determining the dominator
-    /// relationship.
-    ///
-    /// When two coverage spans have the same `Span`, dominated spans can be discarded; but if
-    /// neither coverage span dominates the other, both (or possibly more than two) are held,
-    /// until their disposition is determined. In this latter case, the `prev` dup is moved into
-    /// `pending_dups` so the new `curr` dup can be moved to `prev` for the next iteration.
-    fn update_pending_dups(&mut self) {
-        let prev_bcb = self.prev().bcb;
-        let curr_bcb = self.curr().bcb;
-
-        // Equal coverage spans are ordered by dominators before dominated (if any), so it should be
-        // impossible for `curr` to dominate any previous coverage span.
-        debug_assert!(!self.basic_coverage_blocks.dominates(curr_bcb, prev_bcb));
-
-        // `prev` is a duplicate of `curr`, so add it to the list of pending dups.
-        // If it dominates `curr`, it will be removed by the subsequent discard step.
-        let prev = self.take_prev().into_dup();
-        debug!(?prev, "adding prev to pending dups");
-        self.pending_dups.push(prev);
-
-        let initial_pending_count = self.pending_dups.len();
-        if initial_pending_count > 0 {
-            self.pending_dups
-                .retain(|dup| !self.basic_coverage_blocks.dominates(dup.bcb, curr_bcb));
-
-            let n_discarded = initial_pending_count - self.pending_dups.len();
-            if n_discarded > 0 {
-                debug!(
-                    "  discarded {n_discarded} of {initial_pending_count} pending_dups that dominated curr",
-                );
-            }
         }
     }
 
@@ -516,19 +372,15 @@ impl<'a> SpansRefiner<'a> {
             if it has statements that end before curr; prev={:?}",
             self.prev()
         );
-        if self.pending_dups.is_empty() {
-            let curr_span = self.curr().span;
-            self.prev_mut().cutoff_statements_at(curr_span.lo());
-            if self.prev().merged_spans.is_empty() {
-                debug!("  ... no non-overlapping statements to add");
-            } else {
-                debug!("  ... adding modified prev={:?}", self.prev());
-                let prev = self.take_prev().into_refined();
-                self.refined_spans.push(prev);
-            }
+
+        let curr_span = self.curr().span;
+        self.prev_mut().cutoff_statements_at(curr_span.lo());
+        if self.prev().merged_spans.is_empty() {
+            debug!("  ... no non-overlapping statements to add");
         } else {
-            // with `pending_dups`, `prev` cannot have any statements that don't overlap
-            self.pending_dups.clear();
+            debug!("  ... adding modified prev={:?}", self.prev());
+            let prev = self.take_prev().into_refined();
+            self.refined_spans.push(prev);
         }
     }
 }