[LV][NFC]Preselect folding style while chosing the max VF, NFC.

Selects the tail-folding style while choosing the max vector
factor and storing it in the data member rather than calculating it each
time upon getTailFoldingStyle call.

Part of #76172

Reviewers: ayalz, fhahn

Reviewed By: fhahn

Pull Request: #81885

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1510,19 +1510,36 @@ class LoopVectorizationCostModel {
   }
 
   /// Returns the TailFoldingStyle that is best for the current loop.
-  TailFoldingStyle
-  getTailFoldingStyle(bool IVUpdateMayOverflow = true) const {
-    if (!CanFoldTailByMasking)
-      return TailFoldingStyle::None;
+  TailFoldingStyle getTailFoldingStyle(bool IVUpdateMayOverflow = true) const {
+    return IVUpdateMayOverflow ? ChosenTailFoldingStyle.first
+                               : ChosenTailFoldingStyle.second;
+  }
+
+  /// Selects and saves TailFoldingStyle for 2 options - if IV update may
+  /// overflow or not.
+  void setTailFoldinStyles() {
+    assert(ChosenTailFoldingStyle.first == TailFoldingStyle::None &&
+           ChosenTailFoldingStyle.second == TailFoldingStyle::None &&
+           "Tail folding must not be selected yet.");
+    if (!Legal->prepareToFoldTailByMasking())
+      return;
 
-    if (ForceTailFoldingStyle.getNumOccurrences())
-      return ForceTailFoldingStyle;
+    if (ForceTailFoldingStyle.getNumOccurrences()) {
+      ChosenTailFoldingStyle.first = ChosenTailFoldingStyle.second =
+          ForceTailFoldingStyle;
+      return;
+    }
 
-    return TTI.getPreferredTailFoldingStyle(IVUpdateMayOverflow);
+    ChosenTailFoldingStyle.first =
+        TTI.getPreferredTailFoldingStyle(/*IVUpdateMayOverflow=*/true);
+    ChosenTailFoldingStyle.second =
+        TTI.getPreferredTailFoldingStyle(/*IVUpdateMayOverflow=*/false);
   }
 
   /// Returns true if all loop blocks should be masked to fold tail loop.
   bool foldTailByMasking() const {
+    // TODO: check if it is possible to check for None style independent of
+    // IVUpdateMayOverflow flag in getTailFoldingStyle.
     return getTailFoldingStyle() != TailFoldingStyle::None;
   }
 
@@ -1675,8 +1692,10 @@ class LoopVectorizationCostModel {
   /// iterations to execute in the scalar loop.
   ScalarEpilogueLowering ScalarEpilogueStatus = CM_ScalarEpilogueAllowed;
 
-  /// All blocks of loop are to be masked to fold tail of scalar iterations.
-  bool CanFoldTailByMasking = false;
+  /// Control finally chosen tail folding style. The first element is used if
+  /// the IV update may overflow, the second element - if it does not.
+  std::pair<TailFoldingStyle, TailFoldingStyle> ChosenTailFoldingStyle =
+      std::make_pair(TailFoldingStyle::None, TailFoldingStyle::None);
 
   /// A map holding scalar costs for different vectorization factors. The
   /// presence of a cost for an instruction in the mapping indicates that the
@@ -4633,10 +4652,9 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
   // found modulo the vectorization factor is not zero, try to fold the tail
   // by masking.
   // FIXME: look for a smaller MaxVF that does divide TC rather than masking.
-  if (Legal->prepareToFoldTailByMasking()) {
-    CanFoldTailByMasking = true;
+  setTailFoldinStyles();
+  if (foldTailByMasking())
     return MaxFactors;
-  }
 
   // If there was a tail-folding hint/switch, but we can't fold the tail by
   // masking, fallback to a vectorization with a scalar epilogue.