[LoopInterchange] Skip legality check if surrounding loops already guarantee dependency (NFC)

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -231,18 +231,22 @@ 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", othrewise 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;
 }
 
 // Checks if it is legal to interchange 2 loops.
@@ -256,10 +260,19 @@ 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 direction vector is lexicographically positive due to an element
+    // to the left of OuterLoopId, it is still positive after exchanging the two
+    // loops. 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, 0, Cur.size()) == false)
       return false;
     std::swap(Cur[InnerLoopId], Cur[OuterLoopId]);
-    if (!isLexicographicallyPositive(Cur))
+    if (isLexicographicallyPositive(Cur, 0, Cur.size()) == false)
       return false;
   }
   return true;