[llvm] [LoopInterchange] Improve profitability check for vectorization (PR #133672)

[Ryotaro Kasuga via llvm-commits]

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@@ -1222,21 +1267,33 @@ LoopInterchangeProfitability::isProfitablePerInstrOrderCost() {
 static bool canVectorize(const CharMatrix &DepMatrix, unsigned LoopId) {
   for (unsigned I = 0; I != DepMatrix.size(); I++) {
     char Dir = DepMatrix[I][LoopId];
-    if (Dir != 'I' && Dir != '=')
-      return false;
+    char DepType = DepMatrix[I].back();
+    assert((DepType == '<' || DepType == '*') &&
+           "Unexpected element in dependency vector");
+
+    // There are no loop-carried dependencies.
+    if (Dir == '=' || Dir == 'I')
+      continue;
+
+    // If both Dir and DepType are '<', it means that the all dependencies are
+    // lexically forward. Such dependencies don't prevent vectorization.
+    if (Dir == '<' && DepType == '<')
+      continue;
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kasuga-fj wrote:

A similar fact holds when `Dir` is `>` and all dependencies are lexically backward? (even if this is true, I don't intend to address it in this PR).

https://github.com/llvm/llvm-project/pull/133672