coverage: Simplify logic for chaining multiple blocks into one BCB

We only need to take action when the next block cannot be added to the current
chain, but the logic is much simpler if we express it in terms of when the
block _can_ be added.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -127,10 +127,10 @@ impl CoverageGraph {
         let mut bcbs = IndexVec::<BasicCoverageBlock, _>::with_capacity(num_basic_blocks);
         let mut bb_to_bcb = IndexVec::from_elem_n(None, num_basic_blocks);
 
-        let mut add_basic_coverage_block = |basic_blocks: &mut Vec<BasicBlock>| {
+        let mut flush_chain_into_new_bcb = |current_chain: &mut Vec<BasicBlock>| {
             // Take the accumulated list of blocks, leaving the vector empty
             // to be used by subsequent BCBs.
-            let basic_blocks = std::mem::take(basic_blocks);
+            let basic_blocks = std::mem::take(current_chain);
 
             let bcb = bcbs.next_index();
             for &bb in basic_blocks.iter() {
@@ -141,7 +141,7 @@ impl CoverageGraph {
                 bcb_filtered_successors(mir_body[bb].terminator()).is_out_summable()
             });
             let bcb_data = BasicCoverageBlockData { basic_blocks, is_out_summable };
-            debug!("adding bcb{}: {:?}", bcb.index(), bcb_data);
+            debug!("adding {bcb:?}: {bcb_data:?}");
             bcbs.push(bcb_data);
         };
 
@@ -151,37 +151,31 @@ impl CoverageGraph {
         // together, they will be adjacent in the traversal order.
 
         // Accumulates a chain of blocks that will be combined into one BCB.
-        let mut basic_blocks = Vec::new();
+        let mut current_chain = vec![];
 
-        let filtered_successors = |bb| bcb_filtered_successors(mir_body[bb].terminator());
         let subgraph = CoverageRelevantSubgraph::new(&mir_body.basic_blocks);
         for bb in graph::depth_first_search(subgraph, mir::START_BLOCK)
             .filter(|&bb| mir_body[bb].terminator().kind != TerminatorKind::Unreachable)
         {
-            // If the previous block can't be chained into `bb`, flush the accumulated
-            // blocks into a new BCB, then start building the next chain.
-            if let Some(&prev) = basic_blocks.last()
-                && (!filtered_successors(prev).is_chainable() || {
-                    // If `bb` has multiple predecessor blocks, or `prev` isn't
-                    // one of its predecessors, we can't chain and must flush.
-                    let predecessors = &mir_body.basic_blocks.predecessors()[bb];
-                    predecessors.len() > 1 || !predecessors.contains(&prev)
-                })
-            {
-                debug!(
-                    terminator_kind = ?mir_body[prev].terminator().kind,
-                    predecessors = ?&mir_body.basic_blocks.predecessors()[bb],
-                    "can't chain from {prev:?} to {bb:?}"
-                );
-                add_basic_coverage_block(&mut basic_blocks);
+            if let Some(&prev) = current_chain.last() {
+                // Adding a block to a non-empty chain is allowed if the
+                // previous block permits chaining, and the current block has
+                // `prev` as its sole predecessor.
+                let can_chain = subgraph.coverage_successors(prev).is_out_chainable()
+                    && mir_body.basic_blocks.predecessors()[bb].as_slice() == &[prev];
+                if !can_chain {
+                    // The current block can't be added to the existing chain, so
+                    // flush that chain into a new BCB, and start a new chain.
+                    flush_chain_into_new_bcb(&mut current_chain);
+                }
             }
 
-            basic_blocks.push(bb);
+            current_chain.push(bb);
         }
 
-        if !basic_blocks.is_empty() {
+        if !current_chain.is_empty() {
             debug!("flushing accumulated blocks into one last BCB");
-            add_basic_coverage_block(&mut basic_blocks);
+            flush_chain_into_new_bcb(&mut current_chain);
         }
 
         (bcbs, bb_to_bcb)
@@ -398,7 +392,9 @@ struct CoverageSuccessors<'a> {
 }
 
 impl CoverageSuccessors<'_> {
-    fn is_chainable(&self) -> bool {
+    /// If `false`, this terminator cannot be chained into another block when
+    /// building the coverage graph.
+    fn is_out_chainable(&self) -> bool {
         // If a terminator is out-summable and has exactly one out-edge, then
         // it is eligible to be chained into its successor block.
         self.is_out_summable() && self.targets.len() == 1