llvm · kasuga-fj · May 13, 2025 · May 9, 2025 · May 12, 2025 · May 12, 2025
diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -231,18 +231,26 @@ static void interChangeDependencies(CharMatrix &DepMatrix, unsigned FromIndx,
    std::swap(DepMatrix[I][ToIndx], DepMatrix[I][FromIndx]);
 }

-// After interchanging, check if the direction vector is valid.
+// Check if a direction vector is lexicographically positive. Return true if it
+// is positive, nullopt if it is "zero", otherwise false.
 // [Theorem] A permutation of the loops in a perfect nest is legal if and only
 // if the direction matrix, after the same permutation is applied to its
 // columns, has no ">" direction as the leftmost non-"=" direction in any row.
-static bool isLexicographicallyPositive(std::vector<char> &DV) {
-  for (unsigned char Direction : DV) {
+static std::optional<bool> isLexicographicallyPositive(std::vector<char> &DV,
+                                                       unsigned Begin,
+                                                       unsigned End) {
+  ArrayRef<char> DVRef(DV);
+  for (unsigned char Direction : DVRef.slice(Begin, End - Begin)) {
    if (Direction == '<')
      return true;
    if (Direction == '>' || Direction == '*')
      return false;
  }
-  return true;
+  return std::nullopt;
+}
+
+static std::optional<bool> isLexicographicallyPositive(std::vector<char> &DV) {
+  return isLexicographicallyPositive(DV, 0, DV.size());
 }

 // Checks if it is legal to interchange 2 loops.
@@ -256,10 +264,19 @@ static bool isLegalToInterChangeLoops(CharMatrix &DepMatrix,
    // Create temporary DepVector check its lexicographical order
    // before and after swapping OuterLoop vs InnerLoop
    Cur = DepMatrix[Row];
-    if (!isLexicographicallyPositive(Cur))
+
+    // If the surrounding loops already ensure that the direction vector is
+    // lexicographically positive, nothing within the loop will be able to break
+    // the dependence. In such a case we can skip the subsequent check.
+    if (isLexicographicallyPositive(Cur, 0, OuterLoopId) == true)
+      continue;
+
+    // Check if the direction vector is lexicographically positive (or zero)
+    // for both before/after exchanged.
+    if (isLexicographicallyPositive(Cur) == false)
      return false;
    std::swap(Cur[InnerLoopId], Cur[OuterLoopId]);
-    if (!isLexicographicallyPositive(Cur))
+    if (isLexicographicallyPositive(Cur) == false)
      return false;
  }
  return true;