[llvm] Introduce mcdc::TVIdxBuilder (LLVM side, NFC) (PR #80676)

[via llvm-commits]

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@@ -223,9 +223,130 @@ Expected<int64_t> CounterMappingContext::evaluate(const Counter &C) const {
   return LastPoppedValue;
 }
 
+mcdc::TVIdxBuilder::TVIdxBuilder(const SmallVectorImpl<ConditionIDs> &NextIDs,
+                                 int Offset)
+    : Indices(NextIDs.size()) {
+  // Construct Nodes and set up each InCount
+  auto N = NextIDs.size();
+  SmallVector<MCDCNode> Nodes(N);
+  for (unsigned ID = 0; ID < N; ++ID) {
+    for (unsigned C = 0; C < 2; ++C) {
+#ifndef NDEBUG
+      Indices[ID][C] = INT_MIN;
+#endif
+      auto NextID = NextIDs[ID][C];
+      Nodes[ID].NextIDs[C] = NextID;
+      if (NextID >= 0)
+        ++Nodes[NextID].InCount;
+    }
+  }
+
+  // Sort key ordered by <-Width, Ord>
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ZhuUx wrote:

I see, if we did not sort them, we can not reduce size of bitmaps effectively since there might be other conditions with `width>1` getting large index.

I'm trying to remove the sort because I find it breaks some nature if the topological relationship of conditions is abnormal (e.g conditions from pattern matching in rust) and generates troubled index maps. I would investigate it more. Thanks!

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