[llvm] [Coverage] Improve performance of propagating Counter of Expansions (PR #122589)

[Jessica Paquette via llvm-commits]

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@@ -440,26 +441,83 @@ Error RawCoverageMappingReader::read() {
   // Set the counters for the expansion regions.
   // i.e. Counter of expansion region = counter of the first region
   // from the expanded file.
-  // Perform multiple passes to correctly propagate the counters through
-  // all the nested expansion regions.
-  SmallVector<CounterMappingRegion *, 8> FileIDExpansionRegionMapping;
-  FileIDExpansionRegionMapping.resize(VirtualFileMapping.size(), nullptr);
-  for (unsigned Pass = 1, S = VirtualFileMapping.size(); Pass < S; ++Pass) {
-    for (auto &R : MappingRegions) {
-      if (R.Kind != CounterMappingRegion::ExpansionRegion)
-        continue;
-      assert(!FileIDExpansionRegionMapping[R.ExpandedFileID]);
-      FileIDExpansionRegionMapping[R.ExpandedFileID] = &R;
+  // This assumes:
+  //   - Records are partitioned by FileID.
+  //   - The Count in Expansion is propagated from 1st Record in
+  //     ExpandedFileID.
+  // Therefore another fact below can be assumed.
+  //   - Propagation chain consists of Expansions at each 1st Record.
+
+  /// Node per File.
+  struct FileNode {
+    /// 1st Record in the File.
+    CounterMappingRegion *FirstR = nullptr;
+    /// The Expansion Record out of this File.
+    CounterMappingRegion *ExpanderR = nullptr;
+    /// Count hasn't been propagated yet (for Expansion)
+    bool ExpansionPending = false;
+  };
+  SmallVector<FileNode> FileIDExpansionRegionMapping(VirtualFileMapping.size());
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ornata wrote:

Also what will the edges in the tree be? That's not clear to me right now.

https://github.com/llvm/llvm-project/pull/122589