[llvm] [DWARFLinker] Walk each shared subtree's dependencies once (PR #218072)

[Alexey Lapshin via llvm-commits]

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@@ -284,40 +294,121 @@ bool DependencyTracker::markCollectedLiveRootsAsKept(
   return Res;
 }
 
+void DependencyTracker::recordSubtreeDependencies(
+    LiveRootWorklistActionTy Action, const UnitEntryPairTy &RootEntry,
+    const UnitEntryPairTy &Entry) {
+  SubtreeDependencyRefs.push_back({Entry, Action, RootEntry});
+}
+
+void DependencyTracker::materializeSubtreeSummaries() {
+  // Walking a subtree appends the dependencies that belong to a subprogram
+  // nested inside it, so all walking has to finish before the dependency list
+  // is traversed.
+  for (size_t Idx = MaterializedRefs; Idx != SubtreeDependencyRefs.size();
+       ++Idx) {
+    // Copied rather than referenced so that the loop does not depend on the
+    // walk below leaving the vector alone.
+    const SubtreeDependencyRefTy Ref = SubtreeDependencyRefs[Idx];
+    SubtreeDependenciesKeyTy Key{Ref.Subtree.CU, Ref.Subtree.DieEntry,
+                                 Ref.Action};
+    if (SubtreeSummaries.contains(Key))
+      continue;
+
+    // Collected separately so that growing the map cannot invalidate the sink.
+    SubtreeDependenciesTy SubtreeDeps;
+    {
+      SaveAndRestore<SubtreeDependenciesTy *> CollectInto(CollectedSubtreeDeps,
+                                                          &SubtreeDeps);
+
+      // A walk that only records dependencies neither marks nor follows
+      // references, so it cannot discover a new interconnection and cannot
+      // fail.
+      std::atomic<bool> HasNewInterconnectedCUs = false;
+      bool Res = markDIEEntryAsKeptRec(Ref.Action, Ref.ReferencedBy,
+                                       Ref.Subtree, InterCUProcessingWasStarted,
+                                       HasNewInterconnectedCUs,
+                                       TreeWalkKindTy::RecordSubtreeDeps);
+      assert(Res && !HasNewInterconnectedCUs && "record-deps-only walk failed");
+      (void)Res;
----------------
avl-llvm wrote:

probably mark it with [[maybe_unused]]?

[[maybe_unused]] bool Res ...

https://github.com/llvm/llvm-project/pull/218072