[MC/DC] Refactor: Isolate the final result out of TestVector (#82282)

To reduce conditional judges in the loop in `findIndependencePairs()`, I
have tried a couple of tweaks.

* Isolate the final result in `TestVectors`

`using TestVectors = llvm::SmallVector<std::pair<TestVector,
CondState>>;`
The final result was just piggybacked on `TestVector`, so it has been
isolated.

* Filter out and sort `ExecVectors` by the final result

It will cost more in constructing `ExecVectors`, but it can reduce at
least one conditional judgement in the loop.

Author: chapuni

llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h

@@ -385,7 +385,7 @@ struct MCDCRecord {
   enum CondState { MCDC_DontCare = -1, MCDC_False = 0, MCDC_True = 1 };
 
   using TestVector = llvm::SmallVector<CondState>;
-  using TestVectors = llvm::SmallVector<TestVector>;
+  using TestVectors = llvm::SmallVector<std::pair<TestVector, CondState>>;
   using BoolVector = llvm::SmallVector<bool>;
   using TVRowPair = std::pair<unsigned, unsigned>;
   using TVPairMap = llvm::DenseMap<unsigned, TVRowPair>;
@@ -426,13 +426,13 @@ struct MCDCRecord {
   /// accessing conditions in the TestVectors requires a translation from a
   /// ordinal position to actual condition ID. This is done via PosToID[].
   CondState getTVCondition(unsigned TestVectorIndex, unsigned Condition) {
-    return TV[TestVectorIndex][PosToID[Condition]];
+    return TV[TestVectorIndex].first[PosToID[Condition]];
   }
 
   /// Return the Result evaluation for an executed test vector.
   /// See MCDCRecordProcessor::RecordTestVector().
   CondState getTVResult(unsigned TestVectorIndex) {
-    return TV[TestVectorIndex][getNumConditions()];
+    return TV[TestVectorIndex].second;
   }
 
   /// Determine whether a given condition (indicated by Condition) is covered

llvm/lib/ProfileData/Coverage/CoverageMapping.cpp

@@ -370,9 +370,16 @@ class MCDCRecordProcessor : NextIDsBuilder, mcdc::TVIdxBuilder {
   /// Mapping of calculated MC/DC Independence Pairs for each condition.
   MCDCRecord::TVPairMap IndependencePairs;
 
+  /// Storage for ExecVectors
+  /// ExecVectors is the alias of its 0th element.
+  std::array<MCDCRecord::TestVectors, 2> ExecVectorsByCond;
+
   /// Actual executed Test Vectors for the boolean expression, based on
   /// ExecutedTestVectorBitmap.
-  MCDCRecord::TestVectors ExecVectors;
+  MCDCRecord::TestVectors &ExecVectors;
+
+  /// Number of False items in ExecVectors
+  unsigned NumExecVectorsF;
 
 #ifndef NDEBUG
   DenseSet<unsigned> TVIdxs;
@@ -385,7 +392,8 @@ class MCDCRecordProcessor : NextIDsBuilder, mcdc::TVIdxBuilder {
       : NextIDsBuilder(Branches), TVIdxBuilder(this->NextIDs), Bitmap(Bitmap),
         Region(Region), DecisionParams(Region.getDecisionParams()),
         Branches(Branches), NumConditions(DecisionParams.NumConditions),
-        Folded(NumConditions, false), IndependencePairs(NumConditions) {}
+        Folded(NumConditions, false), IndependencePairs(NumConditions),
+        ExecVectors(ExecVectorsByCond[false]) {}
 
 private:
   // Walk the binary decision diagram and try assigning both false and true to
@@ -415,11 +423,9 @@ class MCDCRecordProcessor : NextIDsBuilder, mcdc::TVIdxBuilder {
         continue;
 
       // Copy the completed test vector to the vector of testvectors.
-      ExecVectors.push_back(TV);
-
       // The final value (T,F) is equal to the last non-dontcare state on the
       // path (in a short-circuiting system).
-      ExecVectors.back().push_back(MCDCCond);
+      ExecVectorsByCond[MCDCCond].push_back({TV, MCDCCond});
     }
 
     // Reset back to DontCare.
@@ -437,6 +443,14 @@ class MCDCRecordProcessor : NextIDsBuilder, mcdc::TVIdxBuilder {
     buildTestVector(TV, 0, 0, 0);
     assert(TVIdxs.size() == unsigned(NumTestVectors) &&
            "TVIdxs wasn't fulfilled");
+
+    // Fill ExecVectors order by False items and True items.
+    // ExecVectors is the alias of ExecVectorsByCond[false], so
+    // Append ExecVectorsByCond[true] on it.
+    NumExecVectorsF = ExecVectors.size();
+    auto &ExecVectorsT = ExecVectorsByCond[true];
+    ExecVectors.append(std::make_move_iterator(ExecVectorsT.begin()),
+                       std::make_move_iterator(ExecVectorsT.end()));
   }
 
   // Find an independence pair for each condition:
@@ -445,13 +459,12 @@ class MCDCRecordProcessor : NextIDsBuilder, mcdc::TVIdxBuilder {
   // - All other conditions' values must be equal or marked as "don't care".
   void findIndependencePairs() {
     unsigned NumTVs = ExecVectors.size();
-    for (unsigned I = 1; I < NumTVs; ++I) {
-      const MCDCRecord::TestVector &A = ExecVectors[I];
-      for (unsigned J = 0; J < I; ++J) {
-        const MCDCRecord::TestVector &B = ExecVectors[J];
-        // Enumerate two execution vectors whose outcomes are different.
-        if (A[NumConditions] == B[NumConditions])
-          continue;
+    for (unsigned I = NumExecVectorsF; I < NumTVs; ++I) {
+      const auto &[A, ACond] = ExecVectors[I];
+      assert(ACond == MCDCRecord::MCDC_True);
+      for (unsigned J = 0; J < NumExecVectorsF; ++J) {
+        const auto &[B, BCond] = ExecVectors[J];
+        assert(BCond == MCDCRecord::MCDC_False);
         unsigned Flip = NumConditions, Idx;
         for (Idx = 0; Idx < NumConditions; ++Idx) {
           MCDCRecord::CondState ACond = A[Idx], BCond = B[Idx];