coverage: Eliminate more counters by giving them to unreachable nodes

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);

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 {

tests/coverage/assert_not.cov-map

@@ -1,13 +1,13 @@
 Function name: assert_not::main
-Raw bytes (29): 0x[01, 01, 00, 05, 01, 06, 01, 01, 12, 05, 02, 05, 00, 14, 09, 01, 05, 00, 14, 0d, 01, 05, 00, 16, 0d, 01, 01, 00, 02]
+Raw bytes (29): 0x[01, 01, 00, 05, 01, 06, 01, 01, 12, 01, 02, 05, 00, 14, 01, 01, 05, 00, 14, 01, 01, 05, 00, 16, 01, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 6, 1) to (start + 1, 18)
-- Code(Counter(1)) at (prev + 2, 5) to (start + 0, 20)
-- Code(Counter(2)) at (prev + 1, 5) to (start + 0, 20)
-- Code(Counter(3)) at (prev + 1, 5) to (start + 0, 22)
-- Code(Counter(3)) at (prev + 1, 1) to (start + 0, 2)
-Highest counter ID seen: c3
+- Code(Counter(0)) at (prev + 2, 5) to (start + 0, 20)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 20)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 22)
+- Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
+Highest counter ID seen: c0
 

tests/coverage/async2.cov-map

@@ -8,16 +8,16 @@ Number of file 0 mappings: 1
 Highest counter ID seen: c0
 
 Function name: async2::async_func::{closure#0}
-Raw bytes (24): 0x[01, 01, 00, 04, 01, 0f, 17, 03, 09, 05, 03, 0a, 02, 06, 00, 02, 05, 00, 06, 01, 01, 01, 00, 02]
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 0f, 17, 03, 09, 01, 03, 0a, 02, 06, 00, 02, 05, 00, 06, 01, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 4
 - Code(Counter(0)) at (prev + 15, 23) to (start + 3, 9)
-- Code(Counter(1)) at (prev + 3, 10) to (start + 2, 6)
+- Code(Counter(0)) at (prev + 3, 10) to (start + 2, 6)
 - Code(Zero) at (prev + 2, 5) to (start + 0, 6)
 - Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
-Highest counter ID seen: c1
+Highest counter ID seen: c0
 
 Function name: async2::async_func_just_println
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 17, 01, 00, 24]
@@ -47,14 +47,14 @@ Number of file 0 mappings: 1
 Highest counter ID seen: c0
 
 Function name: async2::non_async_func
-Raw bytes (24): 0x[01, 01, 00, 04, 01, 07, 01, 03, 09, 05, 03, 0a, 02, 06, 00, 02, 05, 00, 06, 01, 01, 01, 00, 02]
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 07, 01, 03, 09, 01, 03, 0a, 02, 06, 00, 02, 05, 00, 06, 01, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 4
 - Code(Counter(0)) at (prev + 7, 1) to (start + 3, 9)
-- Code(Counter(1)) at (prev + 3, 10) to (start + 2, 6)
+- Code(Counter(0)) at (prev + 3, 10) to (start + 2, 6)
 - Code(Zero) at (prev + 2, 5) to (start + 0, 6)
 - Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
-Highest counter ID seen: c1
+Highest counter ID seen: c0
 

tests/coverage/bad_counter_ids.cov-map

@@ -20,25 +20,25 @@ Number of file 0 mappings: 3
 Highest counter ID seen: c0
 
 Function name: bad_counter_ids::eq_good
-Raw bytes (14): 0x[01, 01, 00, 02, 01, 10, 01, 02, 1f, 05, 03, 01, 00, 02]
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 10, 01, 02, 1f, 01, 03, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 16, 1) to (start + 2, 31)
-- Code(Counter(1)) at (prev + 3, 1) to (start + 0, 2)
-Highest counter ID seen: c1
+- Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
+Highest counter ID seen: c0
 
 Function name: bad_counter_ids::eq_good_message
-Raw bytes (19): 0x[01, 01, 00, 03, 01, 15, 01, 02, 0f, 00, 02, 20, 00, 2b, 05, 01, 01, 00, 02]
+Raw bytes (19): 0x[01, 01, 00, 03, 01, 15, 01, 02, 0f, 00, 02, 20, 00, 2b, 01, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 3
 - Code(Counter(0)) at (prev + 21, 1) to (start + 2, 15)
 - Code(Zero) at (prev + 2, 32) to (start + 0, 43)
-- Code(Counter(1)) at (prev + 1, 1) to (start + 0, 2)
-Highest counter ID seen: c1
+- Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
+Highest counter ID seen: c0
 
 Function name: bad_counter_ids::ne_bad
 Raw bytes (14): 0x[01, 01, 00, 02, 01, 2e, 01, 02, 1f, 00, 03, 01, 00, 02]
@@ -51,15 +51,15 @@ Number of file 0 mappings: 2
 Highest counter ID seen: c0
 
 Function name: bad_counter_ids::ne_bad_message
-Raw bytes (19): 0x[01, 01, 00, 03, 01, 33, 01, 02, 0f, 05, 02, 20, 00, 2b, 00, 01, 01, 00, 02]
+Raw bytes (19): 0x[01, 01, 00, 03, 01, 33, 01, 02, 0f, 01, 02, 20, 00, 2b, 00, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 3
 - Code(Counter(0)) at (prev + 51, 1) to (start + 2, 15)
-- Code(Counter(1)) at (prev + 2, 32) to (start + 0, 43)
+- Code(Counter(0)) at (prev + 2, 32) to (start + 0, 43)
 - Code(Zero) at (prev + 1, 1) to (start + 0, 2)
-Highest counter ID seen: c1
+Highest counter ID seen: c0
 
 Function name: bad_counter_ids::ne_good
 Raw bytes (14): 0x[01, 01, 00, 02, 01, 1a, 01, 02, 1f, 01, 03, 01, 00, 02]