[llvm] [RISCV][TTI] Recognize CONCAT_VECTORS if a shufflevector mask is multiple insert subvector. (PR #110457)

[Luke Lau via llvm-commits]

================
@@ -343,6 +343,49 @@ RISCVTTIImpl::getConstantPoolLoadCost(Type *Ty,  TTI::TargetCostKind CostKind) {
                              /*AddressSpace=*/0, CostKind);
 }
 
+InstructionCost
+RISCVTTIImpl::isMultipleInsertSubvector(VectorType *Tp, ArrayRef<int> Mask,
+                                        TTI::TargetCostKind CostKind) {
+  if (!isa<FixedVectorType>(Tp))
+    return InstructionCost::getInvalid();
+  std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(Tp);
+  if (LT.second.getScalarSizeInBits() == 1)
+    return InstructionCost::getInvalid();
+  // Try to guess SubTp.
+  for (unsigned SubVecSize = 1, E = Mask.size(); SubVecSize < E;
+       SubVecSize <<= 1) {
+    if (E % SubVecSize != 0)
+      continue;
+    SmallVector<int> RepeatedPattern(createSequentialMask(0, SubVecSize, 0));
+    bool Skip = false;
+    for (unsigned I = 0; I != E; I += SubVecSize)
+      if (!Mask.slice(I, SubVecSize).equals(RepeatedPattern)) {
+        Skip = true;
+        break;
+      }
+    if (Skip)
+      continue;
----------------
lukel97 wrote:

I think we can work out the subvector size in one iteration over the mask rather than with a nested loop, if we iterate over each index and mark when it goes back to zero, then check the rest of the mask is `mask[i] == i % subvecsize`

https://github.com/llvm/llvm-project/pull/110457