[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
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ornata wrote:

e.g.

nth record's count == n-1th count + n-2th count ... 0th count?

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