[LV] Explicitly disallow interleaved access requiring gap mask for scalable VFs. nfc (#154122)

Currently, VPInterleaveRecipe::execute does not support generating LLVM
IR for interleaved accesses that require a gap mask for scalable VFs.
It would be better to detect and prevent such groups from being
vectorized as interleaved accesses in
LoopVectorizationCostModel::interleavedAccessCanBeWidened, rather than
relying on the TTI function getInterleavedMemoryOpCost to return an
invalid cost.

Author: Mel-Chen

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -3095,6 +3095,12 @@ bool LoopVectorizationCostModel::interleavedAccessCanBeWidened(
   if (Group->isReverse())
     return false;
 
+  // TODO: Support interleaved access that requires a gap mask for scalable VFs.
+  bool NeedsMaskForGaps = LoadAccessWithGapsRequiresEpilogMasking ||
+                          StoreAccessWithGapsRequiresMasking;
+  if (VF.isScalable() && NeedsMaskForGaps)
+    return false;
+
   auto *Ty = getLoadStoreType(I);
   const Align Alignment = getLoadStoreAlignment(I);
   unsigned AS = getLoadStoreAddressSpace(I);

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3496,6 +3496,8 @@ static Value *interleaveVectors(IRBuilderBase &Builder, ArrayRef<Value *> Vals,
 //   store <12 x i32> %interleaved.vec              ; Write 4 tuples of R,G,B
 void VPInterleaveRecipe::execute(VPTransformState &State) {
   assert(!State.Lane && "Interleave group being replicated.");
+  assert((!NeedsMaskForGaps || !State.VF.isScalable()) &&
+         "Masking gaps for scalable vectors is not yet supported.");
   const InterleaveGroup<Instruction> *Group = IG;
   Instruction *Instr = Group->getInsertPos();
 
@@ -3613,8 +3615,6 @@ void VPInterleaveRecipe::execute(VPTransformState &State) {
       createBitMaskForGaps(State.Builder, State.VF.getKnownMinValue(), *Group);
   assert(((MaskForGaps != nullptr) == NeedsMaskForGaps) &&
          "Mismatch between NeedsMaskForGaps and MaskForGaps");
-  assert((!MaskForGaps || !State.VF.isScalable()) &&
-         "masking gaps for scalable vectors is not yet supported.");
   ArrayRef<VPValue *> StoredValues = getStoredValues();
   // Collect the stored vector from each member.
   SmallVector<Value *, 4> StoredVecs;