[llvm] [llvm-cov] Simplify and optimize MC/DC computation (PR #79727)

[Fangrui Song via llvm-commits]

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@@ -353,71 +331,30 @@ class MCDCRecordProcessor {
     }
   }
 
-  /// For a given condition and two executed Test Vectors, A and B, see if the
-  /// two test vectors match forming an Independence Pair for the condition.
-  /// For two test vectors to match, the following must be satisfied:
-  /// - The condition's value in each test vector must be opposite.
-  /// - The result's value in each test vector must be opposite.
-  /// - All other conditions' values must be equal or marked as "don't care".
-  bool matchTestVectors(unsigned Aidx, unsigned Bidx, unsigned ConditionIdx) {
-    const MCDCRecord::TestVector &A = ExecVectors[Aidx];
-    const MCDCRecord::TestVector &B = ExecVectors[Bidx];
-
-    // If condition values in both A and B aren't opposites, no match.
-    // Because a value can be 0 (false), 1 (true), or -1 (DontCare), a check
-    // that "XOR != 1" will ensure that the values are opposites and that
-    // neither of them is a DontCare.
-    //  1 XOR  0 ==  1 | 0 XOR  0 ==  0 | -1 XOR  0 == -1
-    //  1 XOR  1 ==  0 | 0 XOR  1 ==  1 | -1 XOR  1 == -2
-    //  1 XOR -1 == -2 | 0 XOR -1 == -1 | -1 XOR -1 ==  0
-    if ((A[ConditionIdx] ^ B[ConditionIdx]) != 1)
-      return false;
-
-    // If the results of both A and B aren't opposites, no match.
-    if ((A[NumConditions] ^ B[NumConditions]) != 1)
-      return false;
-
-    for (unsigned Idx = 0; Idx < NumConditions; ++Idx) {
-      // Look for other conditions that don't match. Skip over the given
-      // Condition as well as any conditions marked as "don't care".
-      const auto ARecordTyForCond = A[Idx];
-      const auto BRecordTyForCond = B[Idx];
-      if (Idx == ConditionIdx ||
-          ARecordTyForCond == MCDCRecord::MCDC_DontCare ||
-          BRecordTyForCond == MCDCRecord::MCDC_DontCare)
-        continue;
-
-      // If there is a condition mismatch with any of the other conditions,
-      // there is no match for the test vectors.
-      if (ARecordTyForCond != BRecordTyForCond)
-        return false;
-    }
-
-    // Otherwise, match.
-    return true;
-  }
-
-  /// Find all possible Independence Pairs for a boolean expression given its
-  /// executed Test Vectors.  This process involves looking at each condition
-  /// and attempting to find two Test Vectors that "match", giving us a pair.
+  // Find an independence pair for each condition.
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MaskRay wrote:

The two comments in the function body complement this one, but I can place the three conditions together...

https://github.com/llvm/llvm-project/pull/79727