2 files changed, 16 insertions, 11 deletions
diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs
index 6f9984d5d0a..039f346495b 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters.rs
@@ -114,12 +114,8 @@ pub(crate) fn transcribe_counters(
         let mut new_counters_for_sites = |sites: Vec<BasicCoverageBlock>| {
             sites.into_iter().map(|node| new.ensure_phys_counter(node)).collect::<Vec<_>>()
         };
-        let mut pos = new_counters_for_sites(pos);
-        let mut neg = new_counters_for_sites(neg);
-
-        // These sorts are also not strictly necessary; see above.
-        pos.sort();
-        neg.sort();
+        let pos = new_counters_for_sites(pos);
+        let neg = new_counters_for_sites(neg);
 
         let pos_counter = new.make_sum(&pos).unwrap_or(CovTerm::Zero);
         let new_counter = new.make_subtracted_sum(pos_counter, &neg);
diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index cd89fbe772d..ef86358b205 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -123,11 +123,20 @@ fn coverage_ids_info<'tcx>(
         }
     }
 
-    // FIXME(Zalathar): It should be possible to sort `priority_list[1..]` by
-    // `!bcbs_seen.contains(bcb)` to simplify the mappings even further, at the
-    // expense of some churn in the tests. When doing so, also consider removing
-    // the sorts in `transcribe_counters`.
-    let node_counters = make_node_counters(&fn_cov_info.node_flow_data, &fn_cov_info.priority_list);
+    // Clone the priority list so that we can re-sort it.
+    let mut priority_list = fn_cov_info.priority_list.clone();
+    // The first ID in the priority list represents the synthetic "sink" node,
+    // and must remain first so that it _never_ gets a physical counter.
+    debug_assert_eq!(priority_list[0], priority_list.iter().copied().max().unwrap());
+    assert!(!bcbs_seen.contains(priority_list[0]));
+    // Partition the priority list, so that unreachable nodes (removed by MIR opts)
+    // are sorted later and therefore are _more_ likely to get a physical counter.
+    // This is counter-intuitive, but it means that `transcribe_counters` can
+    // easily skip those unused physical counters and replace them with zero.
+    // (The original ordering remains in effect within both partitions.)
+    priority_list[1..].sort_by_key(|&bcb| !bcbs_seen.contains(bcb));
+
+    let node_counters = make_node_counters(&fn_cov_info.node_flow_data, &priority_list);
     let coverage_counters = transcribe_counters(&node_counters, &bcb_needs_counter, &bcbs_seen);
 
     let CoverageCounters {