[llvm] [LoadStoreVectorizer] Fill gaps in load/store chains to enable vectorization (PR #159388)

[Matt Arsenault via llvm-commits]

================
@@ -344,6 +348,28 @@ class Vectorizer {
   /// Postcondition: For all i, ret[i][0].second == 0, because the first instr
   /// in the chain is the leader, and an instr touches distance 0 from itself.
   std::vector<Chain> gatherChains(ArrayRef<Instruction *> Instrs);
+
+  /// Is a load/store with this alignment allowed by TTI and at least as fast
+  /// as an unvectorized load/store.
+  bool accessIsAllowedAndFast(unsigned SizeBytes, unsigned AS, Align Alignment,
+                              unsigned VecElemBits) const;
+
+  /// Before attempting to fill gaps, check if the chain is a candidate for
+  /// a masked store, to save compile time if it is not possible for the address
+  /// space and element type.
+  bool shouldAttemptMaskedStore(const ArrayRef<ChainElem> C) const;
+
+  /// Create a new GEP and a new Load/Store instruction such that the GEP
+  /// is pointing at PrevElem + Offset. In the case of stores, store poison.
+  /// Extra elements will either be combined into a vector/masked store or
+  /// deleted before the end of the pass.
+  ChainElem createExtraElementAfter(const ChainElem &PrevElem, APInt Offset,
+                                    StringRef Prefix,
+                                    Align Alignment = Align(1));
----------------
arsenm wrote:

```suggestion
                                    Align Alignment = Align());
```

https://github.com/llvm/llvm-project/pull/159388