[llvm] Implement MCDCTVIdxBuilder (LLVM side) (PR #80676)

[Alan Phipps via llvm-commits]

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@@ -223,9 +223,127 @@ Expected<int64_t> CounterMappingContext::evaluate(const Counter &C) const {
   return LastPoppedValue;
 }
 
+MCDCTVIdxBuilder::MCDCTVIdxBuilder(
+    std::function<NodeIDs(bool TellSize)> Fetcher) {
+  // Build Nodes and set up each InCount
+  int MaxID = -1;
+  Nodes.resize(std::get<0>(Fetcher(true)));
+  while (true) {
+    auto [ID1, FalseID1, TrueID1] = Fetcher(false);
+    if (ID1 == 0)
+      break;
+    int ID = ID1 - 1;
+    MaxID = std::max(MaxID, ID);
+    auto &Node = Nodes[ID];
+    Node.Conds[0].ID = FalseID1 - 1;
+    Node.Conds[1].ID = TrueID1 - 1;
+    for (unsigned I = 0; I < 2; ++I) {
+#ifndef NDEBUG
+      Node.Conds[I].Idx = INT_MIN;
+#endif
+      int NextID = Node.Conds[I].ID;
+      if (NextID >= 0)
+        ++Nodes[NextID].InCount;
+    }
+  }
+
+  if (MaxID < 0)
+    return;
+
+  // Sort key ordered by <-Width, Ord>
+  SmallVector<std::tuple<int,      /// -Width
+                         unsigned, /// Ord
+                         int,      /// ID
+                         unsigned  /// Cond (0 or 1)
+                         >>
+      Decisions;
+
+  // Traverse Nodes to assign Idx
+  SmallVector<int> Q;
+  assert(Nodes[0].InCount == 0);
+  Nodes[0].Width = 1;
+  Q.push_back(0);
+
+  unsigned Ord = 0;
----------------
evodius96 wrote:

Can you add a bit more commentary on the overall process of the algorithm (and MCDCTVIdxBuilder)?  What are the expected inputs and the expected output, generally?  I think it would help with the readability a bit more.

https://github.com/llvm/llvm-project/pull/80676