[llvm] [SampleFDO] Improve stale profile matching by diff algorithm (PR #87375)

[via llvm-commits]

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@@ -122,15 +122,153 @@ void SampleProfileMatcher::findProfileAnchors(
   }
 }
 
+MyersDiff::DiffResult
+MyersDiff::longestCommonSequence(const std::vector<Anchor> &A,
+                                 const std::vector<Anchor> &B) const {
+  int32_t N = A.size(), M = B.size(), Max = N + M;
+  auto Index = [&](int32_t I) { return I + Max; };
+
+  DiffResult Diff;
+  if (Max == 0)
+    return Diff;
+
+  // Backtrack the SES result.
+  auto Backtrack = [&](const std::vector<std::vector<int32_t>> &Trace,
+                       const std::vector<Anchor> &A,
+                       const std::vector<Anchor> &B) {
+    int32_t X = N, Y = M;
+    for (int32_t D = Trace.size() - 1; X > 0 || Y > 0; D--) {
+      const auto &P = Trace[D];
+      int32_t K = X - Y;
+      int32_t PrevK = K;
+      if (K == -D || (K != D && P[Index(K - 1)] < P[Index(K + 1)]))
+        PrevK = K + 1;
+      else
+        PrevK = K - 1;
+
+      int32_t PrevX = P[Index(PrevK)];
+      int32_t PrevY = PrevX - PrevK;
+      while (X > PrevX && Y > PrevY) {
+        X--;
+        Y--;
+        Diff.addEqualLocations(A[X].Loc, B[Y].Loc);
+      }
+
+      if (D == 0)
+        break;
+
+      if (Y == PrevY) {
+        X--;
+#ifndef NDEBUG
+        Diff.addInsertion(A[X].Loc);
+#endif
+      } else if (X == PrevX) {
+        Y--;
+#ifndef NDEBUG
+        Diff.addDeletion(B[Y].Loc);
+#endif
+      }
+      X = PrevX;
+      Y = PrevY;
+    }
+  };
+
+  // The greedy LCS/SES algorithm.
+  std::vector<int32_t> V(2 * Max + 1, -1);
+  V[Index(1)] = 0;
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WenleiHe wrote:

Could use a comment for V and Trace

https://github.com/llvm/llvm-project/pull/87375